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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)