NAME | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
INTRO(2) Linux Programmer's Manual INTRO(2)
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 non-zero 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).
Look at the header of the manual page source for the author(s) and copyright
conditions. Note that these can be different from page to page!
_syscall(2), syscall(2), errno(3), feature_test_macros(7), standards(7)
This page is part of release 3.21 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/.
Linux 2007-10-23 INTRO(2)