This is libc.info, produced by makeinfo version 4.0 from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.2.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002
Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software Needs Free Documentation" and
"GNU Lesser General Public License", the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled "GNU Free
Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Printf Extension Example,  Next: Predefined Printf Handlers,  Prev: Defining the Output Handler,  Up: Customizing Printf

`printf' Extension Example
--------------------------

   Here is an example showing how to define a `printf' handler function.
This program defines a data structure called a `Widget' and defines the
`%W' conversion to print information about `Widget *' arguments,
including the pointer value and the name stored in the data structure.
The `%W' conversion supports the minimum field width and
left-justification options, but ignores everything else.

     #include <stdio.h>
     #include <stdlib.h>
     #include <printf.h>
     
     typedef struct
     {
       char *name;
     }
     Widget;
     
     int
     print_widget (FILE *stream,
                   const struct printf_info *info,
                   const void *const *args)
     {
       const Widget *w;
       char *buffer;
       int len;
     
       /* Format the output into a string. */
       w = *((const Widget **) (args[0]));
       len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
       if (len == -1)
         return -1;
     
       /* Pad to the minimum field width and print to the stream. */
       len = fprintf (stream, "%*s",
                      (info->left ? -info->width : info->width),
                      buffer);
     
       /* Clean up and return. */
       free (buffer);
       return len;
     }
     
     
     int
     print_widget_arginfo (const struct printf_info *info, size_t n,
                           int *argtypes)
     {
       /* We always take exactly one argument and this is a pointer to the
          structure.. */
       if (n > 0)
         argtypes[0] = PA_POINTER;
       return 1;
     }
     
     
     int
     main (void)
     {
       /* Make a widget to print. */
       Widget mywidget;
       mywidget.name = "mywidget";
     
       /* Register the print function for widgets. */
       register_printf_function ('W', print_widget, print_widget_arginfo);
     
       /* Now print the widget. */
       printf ("|%W|\n", &mywidget);
       printf ("|%35W|\n", &mywidget);
       printf ("|%-35W|\n", &mywidget);
     
       return 0;
     }

   The output produced by this program looks like:

     |<Widget 0xffeffb7c: mywidget>|
     |      <Widget 0xffeffb7c: mywidget>|
     |<Widget 0xffeffb7c: mywidget>      |


File: libc.info,  Node: Predefined Printf Handlers,  Prev: Printf Extension Example,  Up: Customizing Printf

Predefined `printf' Handlers
----------------------------

   The GNU libc also contains a concrete and useful application of the
`printf' handler extension.  There are two functions available which
implement a special way to print floating-point numbers.

 - Function: int printf_size (FILE *FP, const struct printf_info *INFO,
          const void *const *ARGS)
     Print a given floating point number as for the format `%f' except
     that there is a postfix character indicating the divisor for the
     number to make this less than 1000.  There are two possible
     divisors: powers of 1024 or powers of 1000.  Which one is used
     depends on the format character specified while registered this
     handler.  If the character is of lower case, 1024 is used.  For
     upper case characters, 1000 is used.

     The postfix tag corresponds to bytes, kilobytes, megabytes,
     gigabytes, etc.  The full table is:

     +------+--------------+--------+--------+---------------+
     |low|Multiplier|From|Upper|Multiplier|
     +------+--------------+--------+--------+---------------+
     |' '|1||' '|1|
     +------+--------------+--------+--------+---------------+
     |k|2^10 (1024)|kilo|K|10^3 (1000)|
     +------+--------------+--------+--------+---------------+
     |m|2^20|mega|M|10^6|
     +------+--------------+--------+--------+---------------+
     |g|2^30|giga|G|10^9|
     +------+--------------+--------+--------+---------------+
     |t|2^40|tera|T|10^12|
     +------+--------------+--------+--------+---------------+
     |p|2^50|peta|P|10^15|
     +------+--------------+--------+--------+---------------+
     |e|2^60|exa|E|10^18|
     +------+--------------+--------+--------+---------------+
     |z|2^70|zetta|Z|10^21|
     +------+--------------+--------+--------+---------------+
     |y|2^80|yotta|Y|10^24|
     +------+--------------+--------+--------+---------------+

     The default precision is 3, i.e., 1024 is printed with a lower-case
     format character as if it were `%.3fk' and will yield `1.000k'.

   Due to the requirements of `register_printf_function' we must also
provide the function which returns information about the arguments.

 - Function: int printf_size_info (const struct printf_info *INFO,
          size_t N, int *ARGTYPES)
     This function will return in ARGTYPES the information about the
     used parameters in the way the `vfprintf' implementation expects
     it.  The format always takes one argument.

   To use these functions both functions must be registered with a call
like

     register_printf_function ('B', printf_size, printf_size_info);

   Here we register the functions to print numbers as powers of 1000
since the format character `'B'' is an upper-case character.  If we
would additionally use `'b'' in a line like

     register_printf_function ('b', printf_size, printf_size_info);

we could also print using a power of 1024.  Please note that all that is
different in these two lines is the format specifier.  The
`printf_size' function knows about the difference between lower and
upper case format specifiers.

   The use of `'B'' and `'b'' is no coincidence.  Rather it is the
preferred way to use this functionality since it is available on some
other systems which also use format specifiers.


File: libc.info,  Node: Formatted Input,  Next: EOF and Errors,  Prev: Customizing Printf,  Up: I/O on Streams

Formatted Input
===============

   The functions described in this section (`scanf' and related
functions) provide facilities for formatted input analogous to the
formatted output facilities.  These functions provide a mechanism for
reading arbitrary values under the control of a "format string" or
"template string".

* Menu:

* Formatted Input Basics::      Some basics to get you started.
* Input Conversion Syntax::     Syntax of conversion specifications.
* Table of Input Conversions::  Summary of input conversions and what they do.
* Numeric Input Conversions::   Details of conversions for reading numbers.
* String Input Conversions::    Details of conversions for reading strings.
* Dynamic String Input::	String conversions that `malloc' the buffer.
* Other Input Conversions::     Details of miscellaneous other conversions.
* Formatted Input Functions::   Descriptions of the actual functions.
* Variable Arguments Input::    `vscanf' and friends.


File: libc.info,  Node: Formatted Input Basics,  Next: Input Conversion Syntax,  Up: Formatted Input

Formatted Input Basics
----------------------

   Calls to `scanf' are superficially similar to calls to `printf' in
that arbitrary arguments are read under the control of a template
string.  While the syntax of the conversion specifications in the
template is very similar to that for `printf', the interpretation of
the template is oriented more towards free-format input and simple
pattern matching, rather than fixed-field formatting.  For example,
most `scanf' conversions skip over any amount of "white space"
(including spaces, tabs, and newlines) in the input file, and there is
no concept of precision for the numeric input conversions as there is
for the corresponding output conversions.  Ordinarily, non-whitespace
characters in the template are expected to match characters in the
input stream exactly, but a matching failure is distinct from an input
error on the stream.

   Another area of difference between `scanf' and `printf' is that you
must remember to supply pointers rather than immediate values as the
optional arguments to `scanf'; the values that are read are stored in
the objects that the pointers point to.  Even experienced programmers
tend to forget this occasionally, so if your program is getting strange
errors that seem to be related to `scanf', you might want to
double-check this.

   When a "matching failure" occurs, `scanf' returns immediately,
leaving the first non-matching character as the next character to be
read from the stream.  The normal return value from `scanf' is the
number of values that were assigned, so you can use this to determine if
a matching error happened before all the expected values were read.

   The `scanf' function is typically used for things like reading in
the contents of tables.  For example, here is a function that uses
`scanf' to initialize an array of `double':

     void
     readarray (double *array, int n)
     {
       int i;
       for (i=0; i<n; i++)
         if (scanf (" %lf", &(array[i])) != 1)
           invalid_input_error ();
     }

   The formatted input functions are not used as frequently as the
formatted output functions.  Partly, this is because it takes some care
to use them properly.  Another reason is that it is difficult to recover
from a matching error.

   If you are trying to read input that doesn't match a single, fixed
pattern, you may be better off using a tool such as Flex to generate a
lexical scanner, or Bison to generate a parser, rather than using
`scanf'.  For more information about these tools, see *Note Top:
(flex.info)Top, and *Note Top: (bison.info)Top.


File: libc.info,  Node: Input Conversion Syntax,  Next: Table of Input Conversions,  Prev: Formatted Input Basics,  Up: Formatted Input

Input Conversion Syntax
-----------------------

   A `scanf' template string is a string that contains ordinary
multibyte characters interspersed with conversion specifications that
start with `%'.

   Any whitespace character (as defined by the `isspace' function;
*note Classification of Characters::) in the template causes any number
of whitespace characters in the input stream to be read and discarded.
The whitespace characters that are matched need not be exactly the same
whitespace characters that appear in the template string.  For example,
write ` , ' in the template to recognize a comma with optional
whitespace before and after.

   Other characters in the template string that are not part of
conversion specifications must match characters in the input stream
exactly; if this is not the case, a matching failure occurs.

   The conversion specifications in a `scanf' template string have the
general form:

     % FLAGS WIDTH TYPE CONVERSION

   In more detail, an input conversion specification consists of an
initial `%' character followed in sequence by:

   * An optional "flag character" `*', which says to ignore the text
     read for this specification.  When `scanf' finds a conversion
     specification that uses this flag, it reads input as directed by
     the rest of the conversion specification, but it discards this
     input, does not use a pointer argument, and does not increment the
     count of successful assignments.

   * An optional flag character `a' (valid with string conversions only)
     which requests allocation of a buffer long enough to store the
     string in.  (This is a GNU extension.)  *Note Dynamic String
     Input::.

   * An optional decimal integer that specifies the "maximum field
     width".  Reading of characters from the input stream stops either
     when this maximum is reached or when a non-matching character is
     found, whichever happens first.  Most conversions discard initial
     whitespace characters (those that don't are explicitly
     documented), and these discarded characters don't count towards
     the maximum field width.  String input conversions store a null
     character to mark the end of the input; the maximum field width
     does not include this terminator.

   * An optional "type modifier character".  For example, you can
     specify a type modifier of `l' with integer conversions such as
     `%d' to specify that the argument is a pointer to a `long int'
     rather than a pointer to an `int'.

   * A character that specifies the conversion to be applied.

   The exact options that are permitted and how they are interpreted
vary between the different conversion specifiers.  See the descriptions
of the individual conversions for information about the particular
options that they allow.

   With the `-Wformat' option, the GNU C compiler checks calls to
`scanf' and related functions.  It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a `scanf'-style format string.  *Note Declaring Attributes
of Functions: (gcc.info)Function Attributes, for more information.


File: libc.info,  Node: Table of Input Conversions,  Next: Numeric Input Conversions,  Prev: Input Conversion Syntax,  Up: Formatted Input

Table of Input Conversions
--------------------------

   Here is a table that summarizes the various conversion
specifications:

`%d'
     Matches an optionally signed integer written in decimal.  *Note
     Numeric Input Conversions::.

`%i'
     Matches an optionally signed integer in any of the formats that
     the C language defines for specifying an integer constant.  *Note
     Numeric Input Conversions::.

`%o'
     Matches an unsigned integer written in octal radix.  *Note Numeric
     Input Conversions::.

`%u'
     Matches an unsigned integer written in decimal radix.  *Note
     Numeric Input Conversions::.

`%x', `%X'
     Matches an unsigned integer written in hexadecimal radix.  *Note
     Numeric Input Conversions::.

`%e', `%f', `%g', `%E', `%G'
     Matches an optionally signed floating-point number.  *Note Numeric
     Input Conversions::.

`%s'
     Matches a string containing only non-whitespace characters.  *Note
     String Input Conversions::.  The presence of the `l' modifier
     determines whether the output is stored as a wide character string
     or a multibyte string.  If `%s' is used in a wide character
     function the string is converted as with multiple calls to
     `wcrtomb' into a multibyte string.  This means that the buffer
     must provide room for `MB_CUR_MAX' bytes for each wide character
     read.  In case `%ls' is used in a multibyte function the result is
     converted into wide characters as with multiple calls of `mbrtowc'
     before being stored in the user provided buffer.

`%S'
     This is an alias for `%ls' which is supported for compatibility
     with the Unix standard.

`%['
     Matches a string of characters that belong to a specified set.
     *Note String Input Conversions::.  The presence of the `l' modifier
     determines whether the output is stored as a wide character string
     or a multibyte string.  If `%[' is used in a wide character
     function the string is converted as with multiple calls to
     `wcrtomb' into a multibyte string.  This means that the buffer
     must provide room for `MB_CUR_MAX' bytes for each wide character
     read.  In case `%l[' is used in a multibyte function the result is
     converted into wide characters as with multiple calls of `mbrtowc'
     before being stored in the user provided buffer.

`%c'
     Matches a string of one or more characters; the number of
     characters read is controlled by the maximum field width given for
     the conversion.  *Note String Input Conversions::.

     If the `%c' is used in a wide stream function the read value is
     converted from a wide character to the corresponding multibyte
     character before storing it.  Note that this conversion can
     produce more than one byte of output and therefore the provided
     buffer be large enough for up to `MB_CUR_MAX' bytes for each
     character.  If `%lc' is used in a multibyte function the input is
     treated as a multibyte sequence (and not bytes) and the result is
     converted as with calls to `mbrtowc'.

`%C'
     This is an alias for `%lc' which is supported for compatibility
     with the Unix standard.

`%p'
     Matches a pointer value in the same implementation-defined format
     used by the `%p' output conversion for `printf'.  *Note Other
     Input Conversions::.

`%n'
     This conversion doesn't read any characters; it records the number
     of characters read so far by this call.  *Note Other Input
     Conversions::.

`%%'
     This matches a literal `%' character in the input stream.  No
     corresponding argument is used.  *Note Other Input Conversions::.

   If the syntax of a conversion specification is invalid, the behavior
is undefined.  If there aren't enough function arguments provided to
supply addresses for all the conversion specifications in the template
strings that perform assignments, or if the arguments are not of the
correct types, the behavior is also undefined.  On the other hand, extra
arguments are simply ignored.


File: libc.info,  Node: Numeric Input Conversions,  Next: String Input Conversions,  Prev: Table of Input Conversions,  Up: Formatted Input

Numeric Input Conversions
-------------------------

   This section describes the `scanf' conversions for reading numeric
values.

   The `%d' conversion matches an optionally signed integer in decimal
radix.  The syntax that is recognized is the same as that for the
`strtol' function (*note Parsing of Integers::) with the value `10' for
the BASE argument.

   The `%i' conversion matches an optionally signed integer in any of
the formats that the C language defines for specifying an integer
constant.  The syntax that is recognized is the same as that for the
`strtol' function (*note Parsing of Integers::) with the value `0' for
the BASE argument.  (You can print integers in this syntax with
`printf' by using the `#' flag character with the `%x', `%o', or `%d'
conversion.  *Note Integer Conversions::.)

   For example, any of the strings `10', `0xa', or `012' could be read
in as integers under the `%i' conversion.  Each of these specifies a
number with decimal value `10'.

   The `%o', `%u', and `%x' conversions match unsigned integers in
octal, decimal, and hexadecimal radices, respectively.  The syntax that
is recognized is the same as that for the `strtoul' function (*note
Parsing of Integers::) with the appropriate value (`8', `10', or `16')
for the BASE argument.

   The `%X' conversion is identical to the `%x' conversion.  They both
permit either uppercase or lowercase letters to be used as digits.

   The default type of the corresponding argument for the `%d' and `%i'
conversions is `int *', and `unsigned int *' for the other integer
conversions.  You can use the following type modifiers to specify other
sizes of integer:

`hh'
     Specifies that the argument is a `signed char *' or `unsigned char
     *'.

     This modifier was introduced in ISO C99.

`h'
     Specifies that the argument is a `short int *' or `unsigned short
     int *'.

`j'
     Specifies that the argument is a `intmax_t *' or `uintmax_t *'.

     This modifier was introduced in ISO C99.

`l'
     Specifies that the argument is a `long int *' or `unsigned long
     int *'.  Two `l' characters is like the `L' modifier, below.

     If used with `%c' or `%s' the corresponding parameter is
     considered as a pointer to a wide character or wide character
     string respectively.  This use of `l' was introduced in
     Amendment 1 to ISO C90.

`ll'
`L'
`q'
     Specifies that the argument is a `long long int *' or `unsigned
     long long int *'.  (The `long long' type is an extension supported
     by the GNU C compiler.  For systems that don't provide extra-long
     integers, this is the same as `long int'.)

     The `q' modifier is another name for the same thing, which comes
     from 4.4 BSD; a `long long int' is sometimes called a "quad" `int'.

`t'
     Specifies that the argument is a `ptrdiff_t *'.

     This modifier was introduced in ISO C99.

`z'
     Specifies that the argument is a `size_t *'.

     This modifier was introduced in ISO C99.

   All of the `%e', `%f', `%g', `%E', and `%G' input conversions are
interchangeable.  They all match an optionally signed floating point
number, in the same syntax as for the `strtod' function (*note Parsing
of Floats::).

   For the floating-point input conversions, the default argument type
is `float *'.  (This is different from the corresponding output
conversions, where the default type is `double'; remember that `float'
arguments to `printf' are converted to `double' by the default argument
promotions, but `float *' arguments are not promoted to `double *'.)
You can specify other sizes of float using these type modifiers:

`l'
     Specifies that the argument is of type `double *'.

`L'
     Specifies that the argument is of type `long double *'.

   For all the above number parsing formats there is an additional
optional flag `''.  When this flag is given the `scanf' function
expects the number represented in the input string to be formatted
according to the grouping rules of the currently selected locale (*note
General Numeric::).

   If the `"C"' or `"POSIX"' locale is selected there is no difference.
But for a locale which specifies values for the appropriate fields in
the locale the input must have the correct form in the input.
Otherwise the longest prefix with a correct form is processed.


File: libc.info,  Node: String Input Conversions,  Next: Dynamic String Input,  Prev: Numeric Input Conversions,  Up: Formatted Input

String Input Conversions
------------------------

   This section describes the `scanf' input conversions for reading
string and character values: `%s', `%S', `%[', `%c', and `%C'.

   You have two options for how to receive the input from these
conversions:

   * Provide a buffer to store it in.  This is the default.  You should
     provide an argument of type `char *' or `wchar_t *' (the latter of
     the `l' modifier is present).

     *Warning:* To make a robust program, you must make sure that the
     input (plus its terminating null) cannot possibly exceed the size
     of the buffer you provide.  In general, the only way to do this is
     to specify a maximum field width one less than the buffer size.
     *If you provide the buffer, always specify a maximum field width
     to prevent overflow.*

   * Ask `scanf' to allocate a big enough buffer, by specifying the `a'
     flag character.  This is a GNU extension.  You should provide an
     argument of type `char **' for the buffer address to be stored in.
     *Note Dynamic String Input::.

   The `%c' conversion is the simplest: it matches a fixed number of
characters, always.  The maximum field width says how many characters to
read; if you don't specify the maximum, the default is 1.  This
conversion doesn't append a null character to the end of the text it
reads.  It also does not skip over initial whitespace characters.  It
reads precisely the next N characters, and fails if it cannot get that
many.  Since there is always a maximum field width with `%c' (whether
specified, or 1 by default), you can always prevent overflow by making
the buffer long enough.

   If the format is `%lc' or `%C' the function stores wide characters
which are converted using the conversion determined at the time the
stream was opened from the external byte stream.  The number of bytes
read from the medium is limited by `MB_CUR_LEN * N' but at most N wide
character get stored in the output string.

   The `%s' conversion matches a string of non-whitespace characters.
It skips and discards initial whitespace, but stops when it encounters
more whitespace after having read something.  It stores a null character
at the end of the text that it reads.

   For example, reading the input:

      hello, world

with the conversion `%10c' produces `" hello, wo"', but reading the
same input with the conversion `%10s' produces `"hello,"'.

   *Warning:* If you do not specify a field width for `%s', then the
number of characters read is limited only by where the next whitespace
character appears.  This almost certainly means that invalid input can
make your program crash--which is a bug.

   The `%ls' and `%S' format are handled just like `%s' except that the
external byte sequence is converted using the conversion associated
with the stream to wide characters with their own encoding.  A width or
precision specified with the format do not directly determine how many
bytes are read from the stream since they measure wide characters.  But
an upper limit can be computed by multiplying the value of the width or
precision by `MB_CUR_MAX'.

   To read in characters that belong to an arbitrary set of your choice,
use the `%[' conversion.  You specify the set between the `[' character
and a following `]' character, using the same syntax used in regular
expressions.  As special cases:

   * A literal `]' character can be specified as the first character of
     the set.

   * An embedded `-' character (that is, one that is not the first or
     last character of the set) is used to specify a range of
     characters.

   * If a caret character `^' immediately follows the initial `[', then
     the set of allowed input characters is the everything _except_ the
     characters listed.

   The `%[' conversion does not skip over initial whitespace characters.

   Here are some examples of `%[' conversions and what they mean:

`%25[1234567890]'
     Matches a string of up to 25 digits.

`%25[][]'
     Matches a string of up to 25 square brackets.

`%25[^ \f\n\r\t\v]'
     Matches a string up to 25 characters long that doesn't contain any
     of the standard whitespace characters.  This is slightly different
     from `%s', because if the input begins with a whitespace character,
     `%[' reports a matching failure while `%s' simply discards the
     initial whitespace.

`%25[a-z]'
     Matches up to 25 lowercase characters.

   As for `%c' and `%s' the `%[' format is also modified to produce
wide characters if the `l' modifier is present.  All what is said about
`%ls' above is true for `%l['.

   One more reminder: the `%s' and `%[' conversions are *dangerous* if
you don't specify a maximum width or use the `a' flag, because input
too long would overflow whatever buffer you have provided for it.  No
matter how long your buffer is, a user could supply input that is
longer.  A well-written program reports invalid input with a
comprehensible error message, not with a crash.


File: libc.info,  Node: Dynamic String Input,  Next: Other Input Conversions,  Prev: String Input Conversions,  Up: Formatted Input

Dynamically Allocating String Conversions
-----------------------------------------

   A GNU extension to formatted input lets you safely read a string
with no maximum size.  Using this feature, you don't supply a buffer;
instead, `scanf' allocates a buffer big enough to hold the data and
gives you its address.  To use this feature, write `a' as a flag
character, as in `%as' or `%a[0-9a-z]'.

   The pointer argument you supply for where to store the input should
have type `char **'.  The `scanf' function allocates a buffer and
stores its address in the word that the argument points to.  You should
free the buffer with `free' when you no longer need it.

   Here is an example of using the `a' flag with the `%[...]'
conversion specification to read a "variable assignment" of the form
`VARIABLE = VALUE'.

     {
       char *variable, *value;
     
       if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
                      &variable, &value))
         {
           invalid_input_error ();
           return 0;
         }
     
       ...
     }


File: libc.info,  Node: Other Input Conversions,  Next: Formatted Input Functions,  Prev: Dynamic String Input,  Up: Formatted Input

Other Input Conversions
-----------------------

   This section describes the miscellaneous input conversions.

   The `%p' conversion is used to read a pointer value.  It recognizes
the same syntax used by the `%p' output conversion for `printf' (*note
Other Output Conversions::); that is, a hexadecimal number just as the
`%x' conversion accepts.  The corresponding argument should be of type
`void **'; that is, the address of a place to store a pointer.

   The resulting pointer value is not guaranteed to be valid if it was
not originally written during the same program execution that reads it
in.

   The `%n' conversion produces the number of characters read so far by
this call.  The corresponding argument should be of type `int *'.  This
conversion works in the same way as the `%n' conversion for `printf';
see *Note Other Output Conversions::, for an example.

   The `%n' conversion is the only mechanism for determining the
success of literal matches or conversions with suppressed assignments.
If the `%n' follows the locus of a matching failure, then no value is
stored for it since `scanf' returns before processing the `%n'.  If you
store `-1' in that argument slot before calling `scanf', the presence
of `-1' after `scanf' indicates an error occurred before the `%n' was
reached.

   Finally, the `%%' conversion matches a literal `%' character in the
input stream, without using an argument.  This conversion does not
permit any flags, field width, or type modifier to be specified.


File: libc.info,  Node: Formatted Input Functions,  Next: Variable Arguments Input,  Prev: Other Input Conversions,  Up: Formatted Input

Formatted Input Functions
-------------------------

   Here are the descriptions of the functions for performing formatted
input.  Prototypes for these functions are in the header file `stdio.h'.

 - Function: int scanf (const char *TEMPLATE, ...)
     The `scanf' function reads formatted input from the stream `stdin'
     under the control of the template string TEMPLATE.  The optional
     arguments are pointers to the places which receive the resulting
     values.

     The return value is normally the number of successful assignments.
     If an end-of-file condition is detected before any matches are
     performed, including matches against whitespace and literal
     characters in the template, then `EOF' is returned.

 - Function: int wscanf (const wchar_t *TEMPLATE, ...)
     The `wscanf' function reads formatted input from the stream
     `stdin' under the control of the template string TEMPLATE.  The
     optional arguments are pointers to the places which receive the
     resulting values.

     The return value is normally the number of successful assignments.
     If an end-of-file condition is detected before any matches are
     performed, including matches against whitespace and literal
     characters in the template, then `WEOF' is returned.

 - Function: int fscanf (FILE *STREAM, const char *TEMPLATE, ...)
     This function is just like `scanf', except that the input is read
     from the stream STREAM instead of `stdin'.

 - Function: int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...)
     This function is just like `wscanf', except that the input is read
     from the stream STREAM instead of `stdin'.

 - Function: int sscanf (const char *S, const char *TEMPLATE, ...)
     This is like `scanf', except that the characters are taken from the
     null-terminated string S instead of from a stream.  Reaching the
     end of the string is treated as an end-of-file condition.

     The behavior of this function is undefined if copying takes place
     between objects that overlap--for example, if S is also given as
     an argument to receive a string read under control of the `%s',
     `%S', or `%[' conversion.

 - Function: int swscanf (const wchar_t *WS, const char *TEMPLATE, ...)
     This is like `wscanf', except that the characters are taken from
     the null-terminated string WS instead of from a stream.  Reaching
     the end of the string is treated as an end-of-file condition.

     The behavior of this function is undefined if copying takes place
     between objects that overlap--for example, if WS is also given as
     an argument to receive a string read under control of the `%s',
     `%S', or `%[' conversion.


File: libc.info,  Node: Variable Arguments Input,  Prev: Formatted Input Functions,  Up: Formatted Input

Variable Arguments Input Functions
----------------------------------

   The functions `vscanf' and friends are provided so that you can
define your own variadic `scanf'-like functions that make use of the
same internals as the built-in formatted output functions.  These
functions are analogous to the `vprintf' series of output functions.
*Note Variable Arguments Output::, for important information on how to
use them.

   *Portability Note:* The functions listed in this section were
introduced in ISO C99 and were before available as GNU extensions.

 - Function: int vscanf (const char *TEMPLATE, va_list AP)
     This function is similar to `scanf', but instead of taking a
     variable number of arguments directly, it takes an argument list
     pointer AP of type `va_list' (*note Variadic Functions::).

 - Function: int vwscanf (const wchar_t *TEMPLATE, va_list AP)
     This function is similar to `wscanf', but instead of taking a
     variable number of arguments directly, it takes an argument list
     pointer AP of type `va_list' (*note Variadic Functions::).

 - Function: int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list
          AP)
     This is the equivalent of `fscanf' with the variable argument list
     specified directly as for `vscanf'.

 - Function: int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE,
          va_list AP)
     This is the equivalent of `fwscanf' with the variable argument list
     specified directly as for `vwscanf'.

 - Function: int vsscanf (const char *S, const char *TEMPLATE, va_list
          AP)
     This is the equivalent of `sscanf' with the variable argument list
     specified directly as for `vscanf'.

 - Function: int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE,
          va_list AP)
     This is the equivalent of `swscanf' with the variable argument list
     specified directly as for `vwscanf'.

   In GNU C, there is a special construct you can use to let the
compiler know that a function uses a `scanf'-style format string.  Then
it can check the number and types of arguments in each call to the
function, and warn you when they do not match the format string.  For
details, *Note Declaring Attributes of Functions: (gcc.info)Function
Attributes.


File: libc.info,  Node: EOF and Errors,  Next: Error Recovery,  Prev: Formatted Input,  Up: I/O on Streams

End-Of-File and Errors
======================

   Many of the functions described in this chapter return the value of
the macro `EOF' to indicate unsuccessful completion of the operation.
Since `EOF' is used to report both end of file and random errors, it's
often better to use the `feof' function to check explicitly for end of
file and `ferror' to check for errors.  These functions check
indicators that are part of the internal state of the stream object,
indicators set if the appropriate condition was detected by a previous
I/O operation on that stream.

 - Macro: int EOF
     This macro is an integer value that is returned by a number of
     narrow stream functions to indicate an end-of-file condition, or
     some other error situation.  With the GNU library, `EOF' is `-1'.
     In other libraries, its value may be some other negative number.

     This symbol is declared in `stdio.h'.

 - Macro: int WEOF
     This macro is an integer value that is returned by a number of wide
     stream functions to indicate an end-of-file condition, or some
     other error situation.  With the GNU library, `WEOF' is `-1'.  In
     other libraries, its value may be some other negative number.

     This symbol is declared in `wchar.h'.

 - Function: int feof (FILE *STREAM)
     The `feof' function returns nonzero if and only if the end-of-file
     indicator for the stream STREAM is set.

     This symbol is declared in `stdio.h'.

 - Function: int feof_unlocked (FILE *STREAM)
     The `feof_unlocked' function is equivalent to the `feof' function
     except that it does not implicitly lock the stream.

     This function is a GNU extension.

     This symbol is declared in `stdio.h'.

 - Function: int ferror (FILE *STREAM)
     The `ferror' function returns nonzero if and only if the error
     indicator for the stream STREAM is set, indicating that an error
     has occurred on a previous operation on the stream.

     This symbol is declared in `stdio.h'.

 - Function: int ferror_unlocked (FILE *STREAM)
     The `ferror_unlocked' function is equivalent to the `ferror'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

     This symbol is declared in `stdio.h'.

   In addition to setting the error indicator associated with the
stream, the functions that operate on streams also set `errno' in the
same way as the corresponding low-level functions that operate on file
descriptors.  For example, all of the functions that perform output to a
stream--such as `fputc', `printf', and `fflush'--are implemented in
terms of `write', and all of the `errno' error conditions defined for
`write' are meaningful for these functions.  For more information about
the descriptor-level I/O functions, see *Note Low-Level I/O::.


File: libc.info,  Node: Error Recovery,  Next: Binary Streams,  Prev: EOF and Errors,  Up: I/O on Streams

Recovering from errors
======================

   You may explicitly clear the error and EOF flags with the `clearerr'
function.

 - Function: void clearerr (FILE *STREAM)
     This function clears the end-of-file and error indicators for the
     stream STREAM.

     The file positioning functions (*note File Positioning::) also
     clear the end-of-file indicator for the stream.

 - Function: void clearerr_unlocked (FILE *STREAM)
     The `clearerr_unlocked' function is equivalent to the `clearerr'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

   Note that it is _not_ correct to just clear the error flag and retry
a failed stream operation.  After a failed write, any number of
characters since the last buffer flush may have been committed to the
file, while some buffered data may have been discarded.  Merely retrying
can thus cause lost or repeated data.

   A failed read may leave the file pointer in an inappropriate
position for a second try.  In both cases, you should seek to a known
position before retrying.

   Most errors that can happen are not recoverable -- a second try will
always fail again in the same way.  So usually it is best to give up and
report the error to the user, rather than install complicated recovery
logic.

   One important exception is `EINTR' (*note Interrupted Primitives::).
Many stream I/O implementations will treat it as an ordinary error,
which can be quite inconvenient.  You can avoid this hassle by
installing all signals with the `SA_RESTART' flag.

   For similar reasons, setting nonblocking I/O on a stream's file
descriptor is not usually advisable.


File: libc.info,  Node: Binary Streams,  Next: File Positioning,  Prev: Error Recovery,  Up: I/O on Streams

Text and Binary Streams
=======================

   The GNU system and other POSIX-compatible operating systems organize
all files as uniform sequences of characters.  However, some other
systems make a distinction between files containing text and files
containing binary data, and the input and output facilities of ISO C
provide for this distinction.  This section tells you how to write
programs portable to such systems.

   When you open a stream, you can specify either a "text stream" or a
"binary stream".  You indicate that you want a binary stream by
specifying the `b' modifier in the OPENTYPE argument to `fopen'; see
*Note Opening Streams::.  Without this option, `fopen' opens the file
as a text stream.

   Text and binary streams differ in several ways:

   * The data read from a text stream is divided into "lines" which are
     terminated by newline (`'\n'') characters, while a binary stream is
     simply a long series of characters.  A text stream might on some
     systems fail to handle lines more than 254 characters long
     (including the terminating newline character).

   * On some systems, text files can contain only printing characters,
     horizontal tab characters, and newlines, and so text streams may
     not support other characters.  However, binary streams can handle
     any character value.

   * Space characters that are written immediately preceding a newline
     character in a text stream may disappear when the file is read in
     again.

   * More generally, there need not be a one-to-one mapping between
     characters that are read from or written to a text stream, and the
     characters in the actual file.

   Since a binary stream is always more capable and more predictable
than a text stream, you might wonder what purpose text streams serve.
Why not simply always use binary streams?  The answer is that on these
operating systems, text and binary streams use different file formats,
and the only way to read or write "an ordinary file of text" that can
work with other text-oriented programs is through a text stream.

   In the GNU library, and on all POSIX systems, there is no difference
between text streams and binary streams.  When you open a stream, you
get the same kind of stream regardless of whether you ask for binary.
This stream can handle any file content, and has none of the
restrictions that text streams sometimes have.


File: libc.info,  Node: File Positioning,  Next: Portable Positioning,  Prev: Binary Streams,  Up: I/O on Streams

File Positioning
================

   The "file position" of a stream describes where in the file the
stream is currently reading or writing.  I/O on the stream advances the
file position through the file.  In the GNU system, the file position is
represented as an integer, which counts the number of bytes from the
beginning of the file.  *Note File Position::.

   During I/O to an ordinary disk file, you can change the file position
whenever you wish, so as to read or write any portion of the file.  Some
other kinds of files may also permit this.  Files which support changing
the file position are sometimes referred to as "random-access" files.

   You can use the functions in this section to examine or modify the
file position indicator associated with a stream.  The symbols listed
below are declared in the header file `stdio.h'.

 - Function: long int ftell (FILE *STREAM)
     This function returns the current file position of the stream
     STREAM.

     This function can fail if the stream doesn't support file
     positioning, or if the file position can't be represented in a
     `long int', and possibly for other reasons as well.  If a failure
     occurs, a value of `-1' is returned.

 - Function: off_t ftello (FILE *STREAM)
     The `ftello' function is similar to `ftell', except that it
     returns a value of type `off_t'.  Systems which support this type
     use it to describe all file positions, unlike the POSIX
     specification which uses a long int.  The two are not necessarily
     the same size.  Therefore, using ftell can lead to problems if the
     implementation is written on top of a POSIX compliant low-level
     I/O implementation, and using `ftello' is preferable whenever it
     is available.

     If this function fails it returns `(off_t) -1'.  This can happen
     due to missing support for file positioning or internal errors.
     Otherwise the return value is the current file position.

     The function is an extension defined in the Unix Single
     Specification version 2.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system this function is in fact `ftello64'.  I.e., the LFS
     interface transparently replaces the old interface.

 - Function: off64_t ftello64 (FILE *STREAM)
     This function is similar to `ftello' with the only difference that
     the return value is of type `off64_t'.  This also requires that the
     stream STREAM was opened using either `fopen64', `freopen64', or
     `tmpfile64' since otherwise the underlying file operations to
     position the file pointer beyond the 2^31 bytes limit might fail.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `ftello'
     and so transparently replaces the old interface.

 - Function: int fseek (FILE *STREAM, long int OFFSET, int WHENCE)
     The `fseek' function is used to change the file position of the
     stream STREAM.  The value of WHENCE must be one of the constants
     `SEEK_SET', `SEEK_CUR', or `SEEK_END', to indicate whether the
     OFFSET is relative to the beginning of the file, the current file
     position, or the end of the file, respectively.

     This function returns a value of zero if the operation was
     successful, and a nonzero value to indicate failure.  A successful
     call also clears the end-of-file indicator of STREAM and discards
     any characters that were "pushed back" by the use of `ungetc'.

     `fseek' either flushes any buffered output before setting the file
     position or else remembers it so it will be written later in its
     proper place in the file.

 - Function: int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE)
     This function is similar to `fseek' but it corrects a problem with
     `fseek' in a system with POSIX types.  Using a value of type `long
     int' for the offset is not compatible with POSIX.  `fseeko' uses
     the correct type `off_t' for the OFFSET parameter.

     For this reason it is a good idea to prefer `ftello' whenever it is
     available since its functionality is (if different at all) closer
     the underlying definition.

     The functionality and return value is the same as for `fseek'.

     The function is an extension defined in the Unix Single
     Specification version 2.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system this function is in fact `fseeko64'.  I.e., the LFS
     interface transparently replaces the old interface.

 - Function: int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE)
     This function is similar to `fseeko' with the only difference that
     the OFFSET parameter is of type `off64_t'.  This also requires
     that the stream STREAM was opened using either `fopen64',
     `freopen64', or `tmpfile64' since otherwise the underlying file
     operations to position the file pointer beyond the 2^31 bytes
     limit might fail.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `fseeko'
     and so transparently replaces the old interface.

   *Portability Note:* In non-POSIX systems, `ftell', `ftello', `fseek'
and `fseeko' might work reliably only on binary streams.  *Note Binary
Streams::.

   The following symbolic constants are defined for use as the WHENCE
argument to `fseek'.  They are also used with the `lseek' function
(*note I/O Primitives::) and to specify offsets for file locks (*note
Control Operations::).

 - Macro: int SEEK_SET
     This is an integer constant which, when used as the WHENCE
     argument to the `fseek' or `fseeko' function, specifies that the
     offset provided is relative to the beginning of the file.

 - Macro: int SEEK_CUR
     This is an integer constant which, when used as the WHENCE
     argument to the `fseek' or `fseeko' function, specifies that the
     offset provided is relative to the current file position.

 - Macro: int SEEK_END
     This is an integer constant which, when used as the WHENCE
     argument to the `fseek' or `fseeko' function, specifies that the
     offset provided is relative to the end of the file.

 - Function: void rewind (FILE *STREAM)
     The `rewind' function positions the stream STREAM at the beginning
     of the file.  It is equivalent to calling `fseek' or `fseeko' on
     the STREAM with an OFFSET argument of `0L' and a WHENCE argument
     of `SEEK_SET', except that the return value is discarded and the
     error indicator for the stream is reset.

   These three aliases for the `SEEK_...' constants exist for the sake
of compatibility with older BSD systems.  They are defined in two
different header files: `fcntl.h' and `sys/file.h'.

`L_SET'
     An alias for `SEEK_SET'.

`L_INCR'
     An alias for `SEEK_CUR'.

`L_XTND'
     An alias for `SEEK_END'.

