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
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 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 nega
tive 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
Certain codes are used to indicate Unix variants and stan
dards to which calls in the section conform. These are:
grammer's Reference Manual: Operating System API
(Intel processors)" (Prentice-Hall 1992, ISBN
SVID 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) describ
ing 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
Single Unix Specification. (Developed by X/Open
and The Open Group. See also http://www.UNIX-sys
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.