intro — Introduction to system calls
Section 2 of the manual describes the Linux system calls. A system call is an entry point into the Linux kernel. Usually, system calls are not invoked directly: instead, most system calls have corresponding C library wrapper functions which perform the steps required (e.g., trapping to kernel mode) in order to invoke the system call. Thus, making a system call looks the same as invoking a normal library function.
For a list of the Linux system calls, see syscalls(2).
On error, most system calls return a negative error number
(i.e., the negated value of one of the constants described in
errno(3)). The C library
wrapper hides this detail from the caller: when a system call
returns a negative value, the wrapper copies the absolute
value into the errno variable,
and returns −1 as the return value of the wrapper.
The value returned by a successful system call depends on the call. Many system calls return 0 on success, but some can return nonzero values from a successful call. The details are described in the individual manual pages.
In some cases, the programmer must define a feature test macro in order to obtain the declaration of a system call from the header file specified in the man page SYNOPSIS section. In such cases, the required macro is described in the man page. For further information on feature test macros, see feature_test_macros(7).
Certain terms and abbreviations are used to indicate Unix variants and standards to which calls in this section conform. See standards(7).
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 wrapper function for you. In this case, the programmer must manually invoke the system call using syscall(2). Historically, this was also possible using one of the _syscall macros described in _syscall(2).
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Copyright (C) 2007 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. 2007-10-23 mtk: moved the _syscallN specific material to the new _syscall(2) page, and substantially enhanced and rewrote the remaining material on this page. |