intro(2) -- Linux man page



intro, _syscall - Introduction to system calls  


This chapter describes the Linux system calls. For a list of the 164 syscalls present in Linux 2.0, see syscalls(2).  

Calling Directly

In most cases, it is unnecessary to invoke a system call directly, but there are times when the Standard C library does not implement a nice function call for you.  


#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 six macros that make the actual call into the system easier. They have the form:

where X is 0-5, 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.  


#include <stdio.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. */

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,

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


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, e.g., -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).

Some system calls, such as mmap, require more than five arguments. These are handled by pushing the arguments on the stack and passing a pointer to the block of arguments.

When defining a system call, the argument types 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 syscall(2).  


Certain codes are used to indicate Unix variants and standards to which calls in the section conform. These are:
System V Release 4 Unix, as described in the "Programmer's Reference Manual: Operating System API (Intel processors)" (Prentice-Hall 1992, ISBN 0-13-951294-2)
System V Interface Definition, as described in "The System V Interface Definition, Fourth Edition".
IEEE 1003.1-1990 part 1, aka ISO/IEC 9945-1:1990s, aka "IEEE Portable Operating System Interface for Computing Environments", as elucidated in Donald Lewine's "POSIX Programmer's Guide" (O'Reilly & Associates, Inc., 1991, ISBN 0-937175-73-0.
IEEE Std 1003.1b-1993 (POSIX.1b standard) describing real-time facilities for portable operating systems, aka ISO/IEC 9945-1:1996, as elucidated in "Programming for the real world - POSIX.4" by Bill O. Gallmeister (O'Reilly & Associates, Inc. ISBN 1-56592-074-0).
Single Unix Specification. (Developed by X/Open and The Open Group. See also .)
The 4.3 and 4.4 distributions of Berkeley Unix. 4.4BSD was upward-compatible from 4.3.
Version 7, the ancestral Unix from Bell Labs.




syscall(2), errno(3)