1INTRO(2) Linux Programmer's Manual INTRO(2)
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6 intro - Introduction to system calls
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9 Section 2 of the manual describes the Linux system calls. A system
10 call is an entry point into the Linux kernel. Usually, system calls
11 are not invoked directly: instead, most system calls have corresponding
12 C library wrapper functions which perform the steps required (e.g.,
13 trapping to kernel mode) in order to invoke the system call. Thus,
14 making a system call looks the same as invoking a normal library func‐
15 tion.
16 For a list of the Linux system calls, see syscalls(2).
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20 On error, most system calls return a negative error number (i.e., the
21 negated value of one of the constants described in errno(3)). The C
22 library wrapper hides this detail from the caller: when a system call
23 returns a negative value, the wrapper copies the absolute value into
24 the errno variable, and returns -1 as the return value of the wrapper.
25 The value returned by a successful system call depends on the call.
27 Many system calls return 0 on success, but some can return non-zero
28 values from a successful call. The details are described in the indi‐
29 vidual manual pages.
30 In some cases, the programmer must define a feature test macro in order
32 to obtain the declaration of a system call from the header file speci‐
33 fied in the man page SYNOPSIS section. In such cases, the required
34 macro is described in the man page. For further information on feature
35 test macros, see feature_test_macros(7).
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38 Certain terms and abbreviations are used to indicate Unix variants and
39 standards to which calls in this section conform. See standards(7).
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42 Calling Directly
43 In most cases, it is unnecessary to invoke a system call directly, but
44 there are times when the Standard C library does not implement a nice
45 wrapper function for you. In this case, the programmer must manually
46 invoke the system call using syscall(2). Historically, this was also
47 possible using one of the _syscall macros described in _syscall(2).
48 Authors and Copyright Terms
50 Look at the header of the manual page source for the author(s) and
51 copyright conditions. Note that these can be different from page to
52 page!
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55 _syscall(2), syscall(2), errno(3), feature_test_macros(7), standards(7)
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58 This page is part of release 3.22 of the Linux man-pages project. A
59 description of the project, and information about reporting bugs, can
60 be found at http://www.kernel.org/doc/man-pages/.
61Linux 2007-10-23 INTRO(2)