syscall (2)

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(The comments found at the beginning of the groff file "man2/syscall.2".)

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

This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at www.kernel.org/doc/man-pages