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 func-
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. (Where required, these feature test macros must be defined before including any header files.)
In such cases, the required macro is described in the man page. For further information on feature test macros, see fea-
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).
Authors and Copyright Conditions
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
_syscall(2), syscall(2), errno(3), feature_test_macros(7), standards(7)
This page is part of release 3.27 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 2010-09-10 INTRO(2)