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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 */
int syscall(int 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 the
VDSO), 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
blackfin excpt 0x0 P0 R0
i386 int $0x80 eax eax
ia64 break 0x100000 r15 r10/r8
parisc ble 0x100(%sr2, %r0) r20 r28
s390 svc 0 r1 r2 NR may be passed directly with
s390x svc 0 r1 r2 "svc NR" if NR is less than 256
sparc/32 t 0x10 g1 o0
sparc/64 t 0x6d g1 o0
x86_64 syscall rax rax
The second table shows the registers used to pass the system call arguments.
arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7
──────────────────────────────────────────────────────────
arm/OABI a1 a2 a3 a4 v1 v2 v3
arm/EABI r0 r1 r2 r3 r4 r5 r6
blackfin R0 R1 R2 R3 R4 R5 -
i386 ebx ecx edx esi edi ebp -
ia64 r11 r9 r10 r14 r15 r13 -
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 -
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)
COLOPHON
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