INTRO

INTRO

NAME DESCRIPTION EXAMPLE Sample Output NOTES CONFORMING TO FILES SEE ALSO

NAME

intro - Introduction to system calls

DESCRIPTION

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.

Synopsis

#include

A _syscall macro

desired system call

Setup

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:

_syscallX(type,name,type1,arg1,type2,arg2,...)

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.

EXAMPLE

1. include

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

NOTES

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).

CONFORMING TO

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

SVID

System V Interface Definition, as described in

POSIX.1

IEEE 1003.1-1990 part 1, aka ISO/IEC 9945-1:1990s, aka

POSIX.1b

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

SUS, SUSv2

Single Unix Specification. (Developed by X/Open and The Open Group. See also http://www.UNIX-systems.org/version2/ .)

4.3BSD/4.4BSD

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.

FILES

/usr/include/linux/unistd.h

SEE ALSO

errno(3)