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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.
In many cases, the C library wrapper function does nothing more than:
* copying arguments and the unique system call number to the
registers where the kernel expects them;
* trapping to kernel mode, at which point the kernel does the real
work of the system call; and
* setting errno if the system call returns an error number when the
kernel returns the CPU to user mode.
However, in a few cases, a wrapper function may do rather more than
this, for example, performing some preprocessing of the arguments of
arguments before trapping to kernel mode, or postprocessing of values
returned by the system call. Where this is the case, the manual
pages in Section 2 generally try to note the details of both the
(usually GNU) C library API interface and the raw system call. Most
commonly, the main DESCRIPTION will focus on the C library interface,
and differences for the system call are covered in the NOTES section.
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 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
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 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!
SEE ALSO
_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),
symlink(7), time(7)
COLOPHON
This page is part of release 3.77 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
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