syscall - indirect system call
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/syscall.h> /* For SYS_xxx definitions */
int syscall(int number, ...);
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
code returned by the system call in errno(3) if an error occurs.
Symbolic constants for system call numbers can be found in the header
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 code is stored in errno.
syscall() first appeared in 4BSD.
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
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() system call would be invoked as follows on the ARM
architecture with the EABI:
syscall(SYS_readahead, fd, 0,
(unsigned int) (offset >> 32),
(unsigned int) (offset & 0xFFFFFFFF),
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).
Similar issues can occur on MIPS with the O32 ABI, on PowerPC with the
32-bit ABI, and on Xtensa.
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).
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, and the register used to return the
system call result.
arch/ABI instruction syscall # retval Notes
arm/OABI swi NR - a1 NR is syscall #
arm/EABI swi 0x0 r7 r0
blackfin excpt 0x0 P0 R0
i386 int $0x80 eax eax
ia64 break 0x100000 r15 r10/r8 bool error/
parisc ble 0x100(%sr2, %r0) r20 r28
s390 svc 0 r1 r2 See below
s390x svc 0 r1 r2 See below
sparc/32 t 0x10 g1 o0
sparc/64 t 0x6d g1 o0
x86_64 syscall rax rax
For s390 and s390x, NR (the system call number) may be passed directly
with "svc NR" if it is less than 256.
The second table shows the registers used to pass the system call
arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7
arm/OABI a1 a2 a3 a4 v1 v2 v3
arm/EABI r0 r1 r2 r3 r4 r5 r6
blackfin R0 R1 R2 R3 R4 R5 -
i386 ebx ecx edx esi edi ebp -
ia64 out0 out1 out2 out3 out4 out5 -
parisc r26 r25 r24 r23 r22 r21 -
s390 r2 r3 r4 r5 r6 r7 -
s390x r2 r3 r4 r5 r6 r7 -
sparc/32 o0 o1 o2 o3 o4 o5 -
sparc/64 o0 o1 o2 o3 o4 o5 -
x86_64 rdi rsi rdx r10 r8 r9 -
Note that these tables don't cover the entire calling convention—some
architectures may indiscriminately clobber other registers not listed
main(int argc, char *argv)
tid = syscall(SYS_gettid);
tid = syscall(SYS_tgkill, getpid(), tid, SIGHUP);
_syscall(2), intro(2), syscalls(2), vdso(7)
This page is part of release 3.65 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.