syscall.2: Add notes that caution users when passing arguments to syscall()

[thirdparty/man-pages.git] / 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 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.

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 (i.g., most system calls) glibc handles the details of copy 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 ARM architecture, a
.I "long long"
argument is considered to be 8-byte aligned and to be split into two 4-byte arguments.

.BR readahead ()
system call could be called like below in ARM architecture.

syscall(__NR_readahead, fd, 
.I 0
, (unsigned int)(
.I offset
>> 32), (unsigned int)(
.I offset
& 0xFFFFFFFF), count)

.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.

.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)