INTRO(2) manual page
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intro - introduction to system calls
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.
In many cases, the C library wrapper
function does nothing more than:
- *
- copying arguments and the unique system
call number to the registers where the kernel expects them;
- *
- trapping to
kernel mode, at which point the kernel does the real work of the system
call; and
- *
- setting errno if the system call returns an error number when
the kernel returns the CPU to user mode.
However, in a few cases, a wrapper
function may do rather more than this, for example, performing some preprocessing
of the arguments of arguments before trapping to kernel mode, or postprocessing
of values returned by the system call. Where this is the case, the manual
pages in Section 2 generally try to note the details of both the (usually
GNU) C library API interface and the raw system call. Most commonly, the
main DESCRIPTION will focus on the C library interface, and differences
for the system call are covered in the NOTES section.
For a list of the
Linux system calls, see syscalls(2)
.
On error, most system calls
return a negative error number (i.e., the negated value of one of the constants
described in 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 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 any header files.)
In such cases, the required macro is described in the man page. For further
information on feature test macros, see feature_test_macros(7)
.
Certain terms and abbreviations are used to indicate UNIX variants and
standards to which calls in this section conform. See standards(7)
.
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 syscall(2)
. Historically, this was also possible using
one of the _syscall macros described in _syscall(2).
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!
_syscall(2), syscall(2)
, syscalls(2)
, errno(3)
, intro(3)
,
capabilities(7)
, credentials(7)
, feature_test_macros(7)
, mq_overview(7)
,
path_resolution(7)
, pipe(7)
, pty(7)
, sem_overview(7)
, shm_overview(7)
,
signal(7)
, socket(7)
, standards(7)
, svipc(7)
, symlink(7)
, time(7)
This
page is part of release 3.78 of the Linux man-pages project. A description
of the project, information about reporting bugs, and the latest version
of this page, can be found at http://www.kernel.org/doc/man-pages/.
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