1INTRO(2) Linux Programmer's Manual INTRO(2)
2
6 intro - introduction to system calls
7
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 In many cases, the C library wrapper function does nothing more than:
18 * copying arguments and the unique system call number to the registers
20 where the kernel expects them;
21 * trapping to kernel mode, at which point the kernel does the real
23 work of the system call;
24 * setting errno if the system call returns an error number when the
26 kernel returns the CPU to user mode.
27 However, in a few cases, a wrapper function may do rather more than
29 this, for example, performing some preprocessing of the arguments be‐
30 fore trapping to kernel mode, or postprocessing of values returned by
31 the system call. Where this is the case, the manual pages in Section 2
32 generally try to note the details of both the (usually GNU) C library
33 API interface and the raw system call. Most commonly, the main DE‐
34 SCRIPTION will focus on the C library interface, and differences for
35 the system call are covered in the NOTES section.
36 For a list of the Linux system calls, see syscalls(2).
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40 On error, most system calls return a negative error number (i.e., the
41 negated value of one of the constants described in errno(3)). The C
42 library wrapper hides this detail from the caller: when a system call
43 returns a negative value, the wrapper copies the absolute value into
44 the errno variable, and returns -1 as the return value of the wrapper.
45 The value returned by a successful system call depends on the call.
47 Many system calls return 0 on success, but some can return nonzero val‐
48 ues from a successful call. The details are described in the individ‐
49 ual manual pages.
50 In some cases, the programmer must define a feature test macro in order
52 to obtain the declaration of a system call from the header file speci‐
53 fied in the man page SYNOPSIS section. (Where required, these feature
54 test macros must be defined before including any header files.) In
55 such cases, the required macro is described in the man page. For fur‐
56 ther information on feature test macros, see feature_test_macros(7).
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59 Certain terms and abbreviations are used to indicate UNIX variants and
60 standards to which calls in this section conform. See standards(7).
61
63 Calling directly
64 In most cases, it is unnecessary to invoke a system call directly, but
65 there are times when the Standard C library does not implement a nice
66 wrapper function for you. In this case, the programmer must manually
67 invoke the system call using syscall(2). Historically, this was also
68 possible using one of the _syscall macros described in _syscall(2).
69 Authors and copyright conditions
71 Look at the header of the manual page source for the author(s) and
72 copyright conditions. Note that these can be different from page to
73 page!
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76 _syscall(2), syscall(2), syscalls(2), errno(3), intro(3),
77 capabilities(7), credentials(7), feature_test_macros(7),
78 mq_overview(7), path_resolution(7), pipe(7), pty(7), sem_overview(7),
79 shm_overview(7), signal(7), socket(7), standards(7), symlink(7),
80 system_data_types(7), sysvipc(7), time(7)
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83 This page is part of release 5.13 of the Linux man-pages project. A
84 description of the project, information about reporting bugs, and the
85 latest version of this page, can be found at
86 https://www.kernel.org/doc/man-pages/.
87Linux 2021-08-27 INTRO(2)