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.\" 2007-10-23 mtk: moved the _syscallN specific material to the
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.TH INTRO 2 2010-11-11 "Linux" "Linux Programmer's Manual"
.SH NAME
intro \- introduction to system calls
.SH 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
.BR syscalls (2).
.SH RETURN VALUE
On error, most system calls return a negative error number
(i.e., the negated value of one of the constants described in
.BR 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
.I 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
.I any
header files.)
In such cases, the required macro is described in the man page.
For further information on feature test macros, see
.BR feature_test_macros (7).
.SH "CONFORMING TO"
Certain terms and abbreviations are used to indicate UNIX variants
and standards to which calls in this section conform.
See
.BR standards (7).
.SH NOTES
.SS "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
.BR syscall (2).
Historically, this was also possible using one of the _syscall macros
described in
.BR _syscall (2).
.SS 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!
.SH "SEE ALSO"
.ad l
.nh
.BR _syscall (2),
.BR syscall (2),
.BR syscalls (2),
.BR errno (3),
.BR intro (3),
.BR capabilities (7),
.BR credentials (7),
.BR feature_test_macros (7),
.BR mq_overview (7),
.BR path_resolution (7),
.BR pipe (7),
.BR pty (7),
.BR sem_overview (7),
.BR shm_overview (7),
.BR signal (7),
.BR socket (7),
.BR standards (7),
.BR svipc (7),
.BR symlink (7),
.BR time (7)