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 sys-
tem 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 dec-
laration 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 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).
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 page!
_syscall(2), syscall(2), syscalls(2), errno(3), intro(3), capabilities(7), credentials(7),
feature_test_macros(7), mq_overview(7), path_resolution(7), pipe(7), pty(7),
sem_overview(7), shm_overview(7), signal(7), socket(7), standards(7), svipc(7),
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2010-11-11 INTRO(2)