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.

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);
    syscall(SYS_tgkill, getpid(), tid, SIGHUP);
}

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 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 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 userspace.

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 used to return the system call result, and the register used to signal an error.

arch/ABI instruction syscall # retval error Notes
alpha callsys v0 a0 a3 [1]
arc trap0 r8 r0 -
arm/OABI swi NR - a1 - [2]
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 r10 [1]
m68k trap #0 d0 d0 -
microblaze brki r14,8 r12 r3 -
mips syscall v0 v0 a3 [1]
nios2 trap r2 r2 r7
parisc ble 0x100(%sr2, %r0) r20 r28 -
powerpc sc r0 r3 r0 [1]
s390 svc 0 r1 r2 - [3]
s390x svc 0 r1 r2 - [3]
superh trap #0x17 r3 r0 - [4]
sparc/32 t 0x10 g1 o0 psr/csr [1]
sparc/64 t 0x6d g1 o0 psr/csr [1]
tile swint1 R10 R00 R01 [1]
x86-64 syscall rax rax - [5]
x32 syscall rax rax - [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.

  • 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, the trap number controls the maximum number of arguments passed. A trap #0x10 can be used with only 0-argument system calls, a trap #0x11 can be used with 0- or 1-argument system calls, and so on up to trap #0x17 for 7-argument system calls.

  • 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. Both system call tables are available though, so setting the bit is not a hard requirement.

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 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 -
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
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.

SEE ALSO

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

COLOPHON

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