syscall man page

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:

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 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/ABIinstructionsyscall #retvalerrorNotes
alphacallsysv0a0a3[1]
arctrap0r8r0-
arm/OABIswi NR-a1-[2]
arm/EABIswi 0x0r7r0-
arm64svc #0x8x0-
blackfinexcpt 0x0P0R0-
i386int $0x80eaxeax-
ia64break 0x100000r15r8r10[1]
m68ktrap #0d0d0-
microblazebrki r14,8r12r3-
mipssyscallv0v0a3[1]
nios2trapr2r2r7
pariscble 0x100(%sr2, %r0)r20r28-
powerpcscr0r3r0[1]
s390svc 0r1r2-[3]
s390xsvc 0r1r2-[3]
superhtrap #0x17r3r0-[4]
sparc/32t 0x10g1o0psr/csr[1]
sparc/64t 0x6dg1o0psr/csr[1]
tileswint1R10R00R01[1]
x86_64syscallraxrax-[5]
x32syscallraxrax-[5]
xtensasyscalla2a2-

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/ABIarg1arg2arg3arg4arg5arg6arg7Notes
alphaa0a1a2a3a4a5-
arcr0r1r2r3r4r5-
arm/OABIa1a2a3a4v1v2v3
arm/EABIr0r1r2r3r4r5r6
arm64x0x1x2x3x4x5-
blackfinR0R1R2R3R4R5-
i386ebxecxedxesiediebp-
ia64out0out1out2out3out4out5-
m68kd1d2d3d4d5a0-
microblazer5r6r7r8r9r10-
mips/o32a0a1a2a3---[1]
mips/n32,64a0a1a2a3a4a5-
nios2r4r5r6r7r8r9-
pariscr26r25r24r23r22r21-
powerpcr3r4r5r6r7r8r9
s390r2r3r4r5r6r7-
s390xr2r3r4r5r6r7-
superhr4r5r6r7r0r1r2
sparc/32o0o1o2o3o4o5-
sparc/64o0o1o2o3o4o5-
tileR00R01R02R03R04R05-
x86_64rdirsirdxr10r8r9-
x32rdirsirdxr10r8r9-
xtensaa6a3a4a5a8a9-

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

See Also

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

Colophon

This page is part of release 4.08 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/.

Referenced By

create_module(2), delete_module(2), futex(2), getcpu(2), getdents(2), get_kernel_syms(2), getpid(2), get_robust_list(2), gettid(2), getunwind(2), init_module(2), intro(2), io_cancel(2), io_destroy(2), io_getevents(2), ioprio_set(2), io_setup(2), io_submit(2), kcmp(2), kexec_load(2), llseek(2), memfd_create(2), mlock(2), modify_ldt(2), mq_getsetattr(2), perf_event_open(2), perfmonctl(2), pivot_root(2), posix_fadvise(2), pread(2), pth(3), pthsem(3), query_module(2), readahead(2), readdir(2), rt_sigqueueinfo(2), s390_pci_mmio_write(2), s390_runtime_instr(2), set_thread_area(2), sgetmask(2), spu_create(2), spu_run(2), subpage_prot(2), sync_file_range(2), _syscall(2), syscalls(2), sysctl(2), tkill(2), truncate(2), uselib(2), vdso(7).

2016-10-08 Linux Linux Programmer's Manual