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INTRO(2) Linux Programmer's Manual INTRO(2)
NAME
intro - Introduction to system calls
DESCRIPTION
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).
RETURN VALUE
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).
CONFORMING TO
Certain terms and abbreviations are used to indicate Unix variants and standards to
which calls in this section conform. See standards(7).
NOTES
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 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 Terms
Look at the header of the manual page source for the author(s) and copyright condi-
tions. Note that these can be different from page to page!
SEE ALSO
_syscall(2), syscall(2), errno(3), feature_test_macros(7), standards(7)
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of
the project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2007-10-23 INTRO(2)
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