man-pages/man2/syscall.2

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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 2013-04-01 "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.

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 copyiing arguments to the right registers
in a manner suitable for the architecture.
However, when using
.BR syscall ()
to make a system call,
the caller may need to handle architecture-dependent details.
For example, on the ARM architecture Embbeded ABI (EABI), a
.I "long long"
argument is considered to be 8-byte aligned and to be split
into two 4-byte arguments.

For example, the
.BR readahead ()
system call would be invoked as follows on the ARM architecture with the EABI:

.in +4n
.nf
syscall(__NR_readahead, fd, 0, (unsigned int)(offset >> 32),
        (unsigned int)(offset & 0xFFFFFFFF), count);
.fi
.in
.PP
.I offset
is 64 bit and should be 8-byte aligned.
Thus, a padding is inserted before
.I offset
and
.I offset
is split into two 32-bit arguments.
Similar issues can occur on MIPS with the O32 ABI and 
on PowerPC with the 32-bit ABI.
The affected system calls are
.BR fadvise64_64 (2)
.BR ftruncate64 (2)
.BR pread64 (2)
.BR pwrite64 (2)
.BR readahead (2)
and
.BR truncate64 (2).
.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)