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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 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:
[1] 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.
[2] NR is the system call number.
[3] For s390 and s390x, NR (the system call number) may be passed directly with svc NR
if it is less than 256.
[4] 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.
[5] 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:
[1] 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);
syscall(SYS_tgkill, getpid(), tid, SIGHUP);
}
SEE ALSO
_syscall(2), intro(2), syscalls(2), errno(3), vdso(7)
COLOPHON
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