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NAME | SYNOPSIS | DESCRIPTION | FILES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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_syscall(2) System Calls Manual _syscall(2)
_syscall - invoking a system call without library support
(OBSOLETE)
#include <linux/unistd.h>
A _syscall macro
desired system call
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:
_syscallX(type,name,type1,arg1,type2,arg2,...)
where
X is 0–6, which are the number of arguments taken by the
system call
type is the return type of the system call
name is the name of the system call
typeN is the Nth argument's type
argN is the name of the Nth argument
These macros create a function called name with the arguments you
specify. Once you include the _syscall() in your source file, you
call the system call by name.
/usr/include/linux/unistd.h
Linux.
Starting around Linux 2.6.18, the _syscall macros were removed
from header files supplied to user space. Use syscall(2) instead.
(Some architectures, notably ia64, never provided the _syscall
macros; on those architectures, syscall(2) was always required.)
The _syscall() macros do not produce a prototype. You may have to
create one, especially for C++ users.
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, -EPERM. The _syscall() macros will return the
result r of the system call when r is nonnegative, but will return
-1 and set the variable errno to -r when r is negative. For the
error codes, see errno(3).
When defining a system call, the argument types must be passed by-
value or by-pointer (for aggregates like structs).
#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\n", error);
printf("Uptime = %lds\nLoad: 1 min %lu / 5 min %lu / 15 min %lu\n"
"RAM: total %lu / free %lu / shared %lu\n"
"Memory in buffers = %lu\nSwap: total %lu / free %lu\n"
"Number of processes = %d\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);
}
Sample output
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
intro(2), syscall(2), errno(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 _syscall(2)
Pages that refer to this page: intro(2), syscall(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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