1 .\" Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
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23 .\" 2007-10-23 mtk: moved the _syscallN specific material to the
24 .\" new _syscall(2) page, and substantially enhanced and rewrote
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27 .TH INTRO 2 2010-11-11 "Linux" "Linux Programmer's Manual"
29 intro \- Introduction to system calls
31 Section 2 of the manual describes the Linux system calls.
32 A system call is an entry point into the Linux kernel.
33 Usually, system calls are not invoked directly:
34 instead, most system calls have corresponding C library
35 wrapper functions which perform the steps required
36 (e.g., trapping to kernel mode) in order to invoke
38 Thus, making a system call looks the same as invoking a normal
41 For a list of the Linux system calls, see
44 On error, most system calls return a negative error number
45 (i.e., the negated value of one of the constants described in
47 The C library wrapper hides this detail from the caller: when a
48 system call returns a negative value, the wrapper copies the
49 absolute value into the
51 variable, and returns \-1 as the return value of the wrapper.
53 The value returned by a successful system call depends on the call.
54 Many system calls return 0 on success, but some can return nonzero
55 values from a successful call.
56 The details are described in the individual manual pages.
59 the programmer must define a feature test macro in order to obtain
60 the declaration of a system call from the header file specified
61 in the man page SYNOPSIS section.
62 (Where required, these feature test macros must be defined before including
65 In such cases, the required macro is described in the man page.
66 For further information on feature test macros, see
67 .BR feature_test_macros (7).
69 Certain terms and abbreviations are used to indicate UNIX variants
70 and standards to which calls in this section conform.
74 .SS "Calling Directly"
75 In most cases, it is unnecessary to invoke a system call directly,
76 but there are times when the Standard C library does not implement
77 a nice wrapper function for you.
78 In this case, the programmer must manually invoke the system call using
80 Historically, this was also possible using one of the _syscall macros
83 .SS Authors and Copyright Conditions
84 Look at the header of the manual page source for the author(s) and copyright
86 Note that these can be different from page to page!
97 .BR feature_test_macros (7),
99 .BR path_resolution (7),
102 .BR sem_overview (7),
103 .BR shm_overview (7),