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.\"     @(#)syscall.2	8.1 (Berkeley) 6/16/93
.\"
.\"
.\" 2002-03-20  Christoph Hellwig <hch@infradead.org>
.\"	- adopted for Linux
.\"
.TH SYSCALL 2 2012-08-14 "Linux" "Linux Programmer's Manual"
.SH NAME
syscall \- indirect system call
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <unistd.h>
.BR "#include <sys/syscall.h>   "  "/* For SYS_xxx definitions */"

.BI "int syscall(int " number ", ...);"
.fi
.SH DESCRIPTION
.BR syscall ()
is a small library function that invokes
the system call whose assembly language
interface has the specified
.I number
with the specified arguments.
Employing
.BR syscall ()
is useful, for example,
when invoking a system call that has no wrapper function in the C library.

.BR 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
.BR errno (3)
if an error occurs.

Symbolic constants for system call numbers can be found in the header file
.IR <sys/syscall.h> .
.SH 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
.IR errno .
.SH NOTES
.BR syscall ()
first appeared in
4BSD.
.SH EXAMPLE
.nf
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>

int
main(int argc, char *argv[])
{
    pid_t tid;

    tid = syscall(SYS_gettid);
    tid = syscall(SYS_tgkill, getpid(), tid);
}
.fi
.SH SEE ALSO
.BR _syscall (2),
.BR intro (2),
.BR syscalls (2)
.SH COLOPHON
This page is part of release 3.44 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.