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NAME
syscall - indirect system call
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <unistd.h>
#include <sys/syscall.h> /* For SYS_xxx definitions */
long syscall(long number, ...);
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 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 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 code 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() 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),
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).
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
arm64 svc #0 x8 x0
blackfin excpt 0x0 P0 R0
i386 int $0x80 eax eax
ia64 break 0x100000 r15 r8 See below
mips syscall v0 v0 See below
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 See below
x32 syscall rax rax See below
For s390 and s390x, NR (the system call number) may be passed directly with "svc NR" if it is less than
256.
The x32 ABI uses the same instruction as the x86_64 ABI and is used on the same processors. To
differentiate between them, the bit mask __X32_SYSCALL_BIT is bitwise-ORed into the system call number
for system calls under the x32 ABI.
On a few architectures, a register is used to indicate simple boolean failure of the system call: ia64
uses r10 for this purpose, and mips uses a3.
The second table shows the registers used to pass the system call arguments.
arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes
──────────────────────────────────────────────────────────────────
arm/OABI a1 a2 a3 a4 v1 v2 v3
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 -
mips/o32 a0 a1 a2 a3 - - - See below
mips/n32,64 a0 a1 a2 a3 a4 a5 -
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 -
x32 rdi rsi rdx r10 r8 r9 -
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.
EXAMPLE
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <signal.h>
int
main(int argc, char *argv[])
{
pid_t tid;
tid = syscall(SYS_gettid);
tid = syscall(SYS_tgkill, getpid(), tid, SIGHUP);
}
SEE ALSO
_syscall(2), intro(2), syscalls(2), errno(3), vdso(7)
COLOPHON
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