_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:



  • 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.




The use of these macros is Linux-specific, and deprecated.


Starting around kernel 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);

/* Note: if you copy directly from the nroff source, remember to
REMOVE the extra backslashes in the printf statement. */

    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,

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 release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright (c) 1993 Michael Haardt (,
  Fri Apr  2 11:32:09 MET DST 1993

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Tue Jul  6 12:42:46 MDT 1993 <>
Added "Calling Directly" and supporting paragraphs

Modified Sat Jul 24 15:19:12 1993 by Rik Faith <>

Modified 21 Aug 1994 by Michael Chastain <>:
  Added explanation of arg stacking when 6 or more args.

Modified 10 June 1995 by Andries Brouwer <>

2007-10-23 mtk: created as a new page, by taking the content
specific to the _syscall() macros from intro(2).