INTRO(2) Linux Programmer's Manual INTRO(2)
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 arguments 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() 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).
Certain codes are used to indicate Unix variants and standards to which calls in the sec-
tion conform. These are:
SVr4 System V Release 4 Unix, as described in the "Programmer's Reference Manual: Oper-
ating System API (Intel processors)" (Prentice-Hall 1992, ISBN 0-13-951294-2)
SVID System V Interface Definition, as described in "The System V Interface Definition,
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 por-
table 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
The 4.3 and 4.4 distributions of Berkeley Unix. 4.4BSD was upward-compatible from
V7 Version 7, the ancestral Unix from Bell Labs.
Linux 1.2.13 1996-05-22 INTRO(2)