syscall
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syscall(2) — Linux manual page
SYSCALL(2) Linux Programmer's Manual SYSCALL(2)
NAME top
syscall - indirect system call
SYNOPSIS top
#include <unistd.h>
#include <sys/syscall.h> /* For SYS_xxx definitions */
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 top
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 top
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 top
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
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 trap #0x17 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:
[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. On powerpc64, the summary overflow bit (SO) in field 0
of the condition register (cr0) is used.
[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 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.
[6] 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 -
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:
[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.
EXAMPLES top
#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 top
_syscall(2), intro(2), syscalls(2), errno(3), vdso(7)
COLOPHON top
This page is part of release 5.09 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
https://www.kernel.org/doc/man-pages/.
Linux 2020-06-09 SYSCALL(2)
Pages that refer to this page: arm_fadvise(2), arm_fadvise64_64(2), arm_sync_file_range(2), clone2(2), __clone2(2), clone(2), clone3(2), create_module(2), delete_module(2), fadvise64(2), fadvise64_64(2), finit_module(2), ftruncate(2), ftruncate64(2), futex(2), getcpu(2), getdents(2), getdents64(2), getegid(2), getegid32(2), geteuid(2), geteuid32(2), getgid(2), getgid32(2), get_kernel_syms(2), getpid(2), getppid(2), get_robust_list(2), get_thread_area(2), gettid(2), getuid(2), getuid32(2), getunwind(2), init_module(2), intro(2), io_cancel(2), io_destroy(2), io_getevents(2), ioprio_get(2), ioprio_set(2), io_setup(2), io_submit(2), kcmp(2), kexec_file_load(2), kexec_load(2), llseek(2), _llseek(2), membarrier(2), modify_ldt(2), mq_getsetattr(2), openat2(2), perf_event_open(2), perfmonctl(2), pidfd_getfd(2), pidfd_open(2), pidfd_send_signal(2), pipe2(2), pipe(2), pivot_root(2), posix_fadvise(2), pread(2), pread64(2), pwrite(2), pwrite64(2), query_module(2), readahead(2), readdir(2), rt_sigqueueinfo(2), rt_tgsigqueueinfo(2), s390_guarded_storage(2), s390_pci_mmio_read(2), s390_pci_mmio_write(2), s390_runtime_instr(2), s390_sthyi(2), set_robust_list(2), set_thread_area(2), set_tid_address(2), sgetmask(2), spu_create(2), spu_run(2), ssetmask(2), subpage_prot(2), sync_file_range2(2), sync_file_range(2), _syscall(2), syscalls(2), sysctl(2), _sysctl(2), tgkill(2), tkill(2), truncate(2), truncate64(2), uselib(2), userfaultfd(2), vdso(7)
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