syscall(2)

NAME
syscall - indirect system call

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>

long syscall(long number, ...);

    Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

syscall():
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Before glibc 2.19:
        _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
syscall() is a small library function that invokes the system call whose assembly language interface has the specified number with the specified arguments. Employing syscall() is useful, for example, when invoking a system call that has no wrapper function in the C library.

syscall() saves CPU registers before making the system call, restores the registers upon return from the system call, and stores any error returned by the system call in errno(3).

Symbolic constants for system call numbers can be found in the header file <sys/syscall.h>.

RETURN VALUE
The return value is defined by the system call being invoked. In general, a 0 return value indicates success. A -1 return value indicates an error, and an error number is stored in errno.

NOTES
syscall() first appeared in 4BSD.

  Architecture-specific requirements
Each architecture ABI has its own requirements on how system call arguments are passed to the kernel. For system calls that have a glibc wrapper (e.g., most system calls), glibc handles the details of copying arguments to the right registers in a manner suitable for the architecture. However, when using syscall() to make a system call, the caller might need to handle architecture-dependent details; this requirement is most commonly encountered on certain 32-bit architectures.

For example, on the ARM architecture Embedded ABI (EABI), a 64-bit value (e.g., long long) must be aligned to an even register pair. Thus, using syscall() instead of the wrapper provided by glibc, the readahead(2) system call would be invoked as follows on the ARM architecture with the EABI in little endian mode:

    syscall(SYS_readahead, fd, 0,
            (unsigned int) (offset & 0xFFFFFFFF),
            (unsigned int) (offset >> 32),
            count);

Since the offset argument is 64 bits, and the first argument (fd) is passed in r0, the caller must manually split and align the 64-bit value so that it is passed in the r2/r3 register pair. That means inserting a dummy value into r1 (the second argument of 0). Care also must be taken so that the split follows endian conventions (according to the C ABI for the platform).

Similar issues can occur on MIPS with the O32 ABI, on PowerPC and parisc with the 32-bit ABI, and on Xtensa.

Note that while the parisc C ABI also uses aligned register pairs, it uses a shim layer to hide the issue from user space.

The affected system calls are fadvise64_64(2), ftruncate64(2), posix_fadvise(2), pread64(2), pwrite64(2), readahead(2), sync_file_range(2), and truncate64(2).

This does not affect syscalls that manually split and assemble 64-bit values such as _llseek(2), preadv(2), preadv2(2), pwritev(2), and pwritev2(2). Welcome to the wonderful world of historical baggage.

  Architecture calling conventions
Every architecture has its own way of invoking and passing arguments to the kernel. The details for various architectures are listed in the two tables below.

The first table lists the instruction used to transition to kernel mode (which might not be the fastest or best way to transition to the kernel, so you might have to refer to vdso(7)), the register used to indicate the system call number, the register(s) used to return the system call result, and the register used to signal an error.
Arch/ABI | Instruction | System | Ret | Ret | Error | Notes
 |  | call # | val | val2
alpha | callsys | v0 | v0 | a4 | a3 | 1, 6
arc | trap0 | r8 | r0 | - | -
arm/OABI | swi NR | - | r0 | - | - | 2
arm/EABI | swi 0x0 | r7 | r0 | r1 | -
arm64 | svc #0 | w8 | x0 | x1 | -
blackfin | excpt 0x0 | P0 | R0 | - | -
i386 | int $0x80 | eax | eax | edx | -
ia64 | break 0x100000 | r15 | r8 | r9 | r10 | 1, 6
loongarch | syscall 0 | a7 | a0 | - | -
m68k | trap #0 | d0 | d0 | - | -
microblaze | brki r14,8 | r12 | r3 | - | -
mips | syscall | v0 | v0 | v1 | a3 | 1, 6
nios2 | trap | r2 | r2 | - | r7
parisc | ble 0x100(%sr2, %r0) | r20 | r28 | - | -
powerpc | sc | r0 | r3 | - | r0 | 1
powerpc64 | sc | r0 | r3 | - | cr0.SO | 1
riscv | ecall | a7 | a0 | a1 | -
s390 | svc 0 | r1 | r2 | r3 | - | 3
s390x | svc 0 | r1 | r2 | r3 | - | 3
superh | trapa #31 | r3 | r0 | r1 | - | 4, 6
sparc/32 | t 0x10 | g1 | o0 | o1 | psr/csr | 1, 6
sparc/64 | t 0x6d | g1 | o0 | o1 | psr/csr | 1, 6
tile | swint1 | R10 | R00 | - | R01 | 1
x86-64 | syscall | rax | rax | rdx | - | 5
x32 | syscall | rax | rax | rdx | - | 5
xtensa | syscall | a2 | a2 | - | -

Notes:

•   On a few architectures, a register is used as a boolean (0 indicating no error, and -1 indicating an error) to signal that the system call failed. The actual error value is still contained in the return register. On sparc, the carry bit (csr) in the processor status register (psr) is used instead of a full register. On powerpc64, the summary overflow bit (SO) in field 0 of the condition register (cr0) is used.

•   NR is the system call number.

•   For s390 and s390x, NR (the system call number) may be passed directly with svc NR if it is less than 256.

•   On SuperH additional trap numbers are supported for historic reasons, but trapa#31 is the recommended "unified" ABI.

•   The x32 ABI shares syscall table with x86-64 ABI, but there are some nuances:

    •   In order to indicate that a system call is called under the x32 ABI, an additional bit, __X32_SYSCALL_BIT, is bitwise-ORed with the system call number. The ABI used by a process affects some process behaviors, including signal handling or system call restarting.

    •   Since x32 has different sizes for long and pointer types, layouts of some (but not all; struct timeval or struct rlimit are 64-bit, for example) structures are different. In order to handle this, additional system calls are added to the system call table, starting from number 512 (without the __X32_SYSCALL_BIT). For example, __NR_readv is defined as 19 for the x86-64 ABI and as __X32_SYSCALL_BIT | 515 for the x32 ABI. Most of these additional system calls are actually identical to the system calls used for providing i386 compat. There are some notable exceptions, however, such as preadv2(2), which uses struct iovec entities with 4-byte pointers and sizes ("compat_iovec" in kernel terms), but passes an 8-byte pos argument in a single register and not two, as is done in every other ABI.

•   Some architectures (namely, Alpha, IA-64, MIPS, SuperH, sparc/32, and sparc/64) use an additional register ("Retval2" in the above table) to pass back a second return value from the pipe(2) system call; Alpha uses this technique in the architecture-specific getxpid(2), getxuid(2), and getxgid(2) system calls as well. Other architectures do not use the second return value register in the system call interface, even if it is defined in the System V ABI.

The second table shows the registers used to pass the system call arguments.
Arch/ABI | arg1 | arg2 | arg3 | arg4 | arg5 | arg6 | arg7 | Notes
alpha | a0 | a1 | a2 | a3 | a4 | a5 | -
arc | r0 | r1 | r2 | r3 | r4 | r5 | -
arm/OABI | r0 | r1 | r2 | r3 | r4 | r5 | r6
arm/EABI | r0 | r1 | r2 | r3 | r4 | r5 | r6
arm64 | x0 | x1 | x2 | x3 | x4 | x5 | -
blackfin | R0 | R1 | R2 | R3 | R4 | R5 | -
i386 | ebx | ecx | edx | esi | edi | ebp | -
ia64 | out0 | out1 | out2 | out3 | out4 | out5 | -
loongarch | a0 | a1 | a2 | a3 | a4 | a5 | a6
m68k | d1 | d2 | d3 | d4 | d5 | a0 | -
microblaze | r5 | r6 | r7 | r8 | r9 | r10 | -
mips/o32 | a0 | a1 | a2 | a3 | - | - | - | 1
mips/n32,64 | a0 | a1 | a2 | a3 | a4 | a5 | -
nios2 | r4 | r5 | r6 | r7 | r8 | r9 | -
parisc | r26 | r25 | r24 | r23 | r22 | r21 | -
powerpc | r3 | r4 | r5 | r6 | r7 | r8 | r9
powerpc64 | r3 | r4 | r5 | r6 | r7 | r8 | -
riscv | a0 | a1 | a2 | a3 | a4 | a5 | -
s390 | r2 | r3 | r4 | r5 | r6 | r7 | -
s390x | r2 | r3 | r4 | r5 | r6 | r7 | -
superh | r4 | r5 | r6 | r7 | r0 | r1 | r2
sparc/32 | o0 | o1 | o2 | o3 | o4 | o5 | -
sparc/64 | o0 | o1 | o2 | o3 | o4 | o5 | -
tile | R00 | R01 | R02 | R03 | R04 | R05 | -
x86-64 | rdi | rsi | rdx | r10 | r8 | r9 | -
x32 | rdi | rsi | rdx | r10 | r8 | r9 | -
xtensa | a6 | a3 | a4 | a5 | a8 | a9 | -

Notes:

•   The mips/o32 system call convention passes arguments 5 through 8 on the user stack.

Note that these tables don't cover the entire calling convention—some architectures may indiscriminately clobber other registers not listed here.

EXAMPLES
    #define _GNU_SOURCE
    #include <signal.h>
    #include <sys/syscall.h>
    #include <unistd.h>
    int
    main(void)
    {
        pid_t tid;
        tid = syscall(SYS_gettid);
        syscall(SYS_tgkill, getpid(), tid, SIGHUP);
    }

SEE ALSO
_syscall(2), intro(2), syscalls(2), errno(3), vdso(7)