.\" Copyright, The contributors to the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.TH _syscall 2 2025-05-06 "Linux man-pages 6.14"
.SH NAME
_syscall \- invoking a system call without library support (OBSOLETE)
.SH SYNOPSIS
.nf
.B #include <linux/unistd.h>
.P
A _syscall macro
.P
desired system call
.fi
.SH DESCRIPTION
The important thing to know about a system call is its prototype.
You need to know how many arguments, their types,
and the function return type.
There are seven macros that make the actual call into the system easier.
They have the form:
.P
.in +4n
.EX
.RI _syscall X ( type , name , type1 , arg1 , type2 , arg2 ,...)
.EE
.in
.P
where
.IP
.I X
is 0\[en]6, which are the number of arguments taken by the
system call
.IP
.I type
is the return type of the system call
.IP
.I name
is the name of the system call
.IP
.I typeN
is the Nth argument's type
.IP
.I argN
is the name of the Nth argument
.P
These macros create a function called
.I name
with the arguments you
specify.
Once you include the _syscall() in your source file,
you call the system call by
.IR name .
.SH FILES
.I /usr/include/linux/unistd.h
.SH STANDARDS
Linux.
.SH HISTORY
Starting around Linux 2.6.18, the _syscall macros were removed
from header files supplied to user space.
Use
.BR syscall (2)
instead.
(Some architectures, notably ia64, never provided the _syscall macros;
on those architectures,
.BR syscall (2)
was always required.)
.SH NOTES
The _syscall() macros
.I "do not"
produce a prototype.
You may have to
create one, especially for C++ users.
.P
System calls are not required to return only positive or negative error
codes.
You need to read the source to be sure how it will return errors.
Usually, it is the negative of a standard error code,
for example,
.RI \- EPERM .
The _syscall() macros will return the result
.I r
of the system call
when
.I r
is nonnegative, but will return \-1 and set the variable
.I errno
to
.RI \- r
when
.I r
is negative.
For the error codes, see
.BR errno (3).
.P
When defining a system call, the argument types
.I must
be
passed by-value or by-pointer (for aggregates like structs).
.\" The preferred way to invoke system calls that glibc does not know
.\" about yet is via
.\" .BR syscall (2).
.\" However, this mechanism can be used only if using a libc
.\" (such as glibc) that supports
.\" .BR syscall (2),
.\" and if the
.\" .I <sys/syscall.h>
.\" header file contains the required SYS_foo definition.
.\" Otherwise, the use of a _syscall macro is required.
.\"
.SH EXAMPLES
.\" SRC BEGIN (_syscall.c)
.EX
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <linux/unistd.h>       /* for _syscallX macros/related stuff */
#include <linux/kernel.h>       /* for struct sysinfo */
\&
_syscall1(int, sysinfo, struct sysinfo *, info);
\&
int
main(void)
{
    struct sysinfo s_info;
    int error;
\&
    error = sysinfo(&s_info);
    printf("code error = %d\[rs]n", error);
    printf("Uptime = %lds\[rs]nLoad: 1 min %lu / 5 min %lu / 15 min %lu\[rs]n"
           "RAM: total %lu / free %lu / shared %lu\[rs]n"
           "Memory in buffers = %lu\[rs]nSwap: total %lu / free %lu\[rs]n"
           "Number of processes = %d\[rs]n",
           s_info.uptime, s_info.loads[0],
           s_info.loads[1], s_info.loads[2],
           s_info.totalram, s_info.freeram,
           s_info.sharedram, s_info.bufferram,
           s_info.totalswap, s_info.freeswap,
           s_info.procs);
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SS Sample output
.EX
code error = 0
uptime = 502034s
Load: 1 min 13376 / 5 min 5504 / 15 min 1152
RAM: total 15343616 / free 827392 / shared 8237056
Memory in buffers = 5066752
Swap: total 27881472 / free 24698880
Number of processes = 40
.EE
.SH SEE ALSO
.BR intro (2),
.BR syscall (2),
.BR errno (3)