intro - 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).
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.
A _syscall macro
desired system call
The important thing to know about a system call is its pro-
totype. 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
where X is 05, 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 argu-
ments you specify. Once you include the _syscall() in your
source file, you call the system call by name.
#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;
error = sysinfo(&s_info);
printf("code error = %d\n", error);
printf("Uptime = %ds\nLoad: 1 min %d / 5 min %d / 15 min %d\n"
"RAM: total %d / free %d / shared %d\n"
"Memory in buffers = %d\nSwap: total %d / free %d\n"
"Number of processes = %d\n",
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() mac-
ros 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.
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).
Certain codes are used to indicate Unix variants and
standards to which calls in the section conform. These are:
SVr4 System V Release 4 Unix, as described in the
"Programmer's Reference Manual: Operating System API
(Intel processors)" (Prentice-Hall 1992, ISBN 0-13-
SVID System V Interface Definition, as described in "The
System V Interface Definition, Fourth Edition", avail-
able at ftp://ftp.fpk.novell.com/pub/unix-
standards/svid in Postscript files.
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,
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).
The 4.3 and 4.4 distributions of Berkeley Unix. 4.4BSD
was upward-compatible from 4.3.
V7 Version 7, the ancestral Unix from Bell Labs.
Look at the source header of the manual page for the
author(s) and copyright conditions. Note that these can be
different from page to page!