mirror of https://github.com/mkerrisk/man-pages
136 lines
4.9 KiB
Groff
136 lines
4.9 KiB
Groff
.\" Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" 2007-10-23 mtk: moved the _syscallN specific material to the
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.\" new _syscall(2) page, and substantially enhanced and rewrote
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.\" the remaining material on this page.
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.\"
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.TH INTRO 2 2021-08-27 "Linux" "Linux Programmer's Manual"
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.SH NAME
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intro \- introduction to system calls
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.SH DESCRIPTION
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Section 2 of the manual describes the Linux system calls.
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A system call is an entry point into the Linux kernel.
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Usually, system calls are not invoked directly:
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instead, most system calls have corresponding C library
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wrapper functions which perform the steps required
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(e.g., trapping to kernel mode) in order to invoke
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the system call.
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Thus, making a system call looks the same as invoking a normal
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library function.
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.PP
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In many cases, the C library wrapper function does nothing more than:
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.IP * 3
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copying arguments and the unique system call number to the
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registers where the kernel expects them;
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.IP *
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trapping to kernel mode,
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at which point the kernel does the real work of the system call;
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.IP *
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setting
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.I errno
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if the system call returns an error number when the kernel returns the
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CPU to user mode.
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.PP
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However, in a few cases, a wrapper function may do rather more than this,
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for example, performing some preprocessing
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of the arguments before trapping to kernel mode,
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or postprocessing of values returned by the system call.
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Where this is the case, the manual pages in Section 2 generally
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try to note the details of both the (usually GNU) C library API
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interface and the raw system call.
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Most commonly, the main DESCRIPTION will focus on the C library interface,
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and differences for the system call are covered in the NOTES section.
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.PP
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For a list of the Linux system calls, see
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.BR syscalls (2).
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.SH RETURN VALUE
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On error, most system calls return a negative error number
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(i.e., the negated value of one of the constants described in
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.BR errno (3)).
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The C library wrapper hides this detail from the caller: when a
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system call returns a negative value, the wrapper copies the
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absolute value into the
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.I errno
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variable, and returns \-1 as the return value of the wrapper.
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.PP
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The value returned by a successful system call depends on the call.
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Many system calls return 0 on success, but some can return nonzero
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values from a successful call.
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The details are described in the individual manual pages.
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.PP
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In some cases,
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the programmer must define a feature test macro in order to obtain
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the declaration of a system call from the header file specified
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in the man page SYNOPSIS section.
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(Where required, these feature test macros must be defined before including
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.I any
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header files.)
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In such cases, the required macro is described in the man page.
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For further information on feature test macros, see
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.BR feature_test_macros (7).
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.SH CONFORMING TO
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Certain terms and abbreviations are used to indicate UNIX variants
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and standards to which calls in this section conform.
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See
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.BR standards (7).
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.SH NOTES
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.SS Calling directly
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In most cases, it is unnecessary to invoke a system call directly,
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but there are times when the Standard C library does not implement
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a nice wrapper function for you.
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In this case, the programmer must manually invoke the system call using
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.BR syscall (2).
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Historically, this was also possible using one of the _syscall macros
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described in
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.BR _syscall (2).
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.SS Authors and copyright conditions
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Look at the header of the manual page source for the author(s) and copyright
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conditions.
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Note that these can be different from page to page!
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.SH SEE ALSO
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.ad l
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.nh
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.BR _syscall (2),
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.BR syscall (2),
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.BR syscalls (2),
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.BR errno (3),
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.BR intro (3),
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.BR capabilities (7),
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.BR credentials (7),
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.BR feature_test_macros (7),
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.BR mq_overview (7),
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.BR path_resolution (7),
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.BR pipe (7),
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.BR pty (7),
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.BR sem_overview (7),
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.BR shm_overview (7),
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.BR signal (7),
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.BR socket (7),
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.BR standards (7),
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.BR symlink (7),
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.BR system_data_types (7),
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.BR sysvipc (7),
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.BR time (7)
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