intro(2) - Linux manual page (original) (raw)
intro(2) System Calls Manual intro(2)
NAME top
intro - introduction to system callsDESCRIPTION top
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;
• setting _[errno](../man3/errno.3.html)_ 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 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)](../man2/syscalls.2.html).RETURN VALUE top
On error, most system calls return a negative error number (i.e.,
the negated value of one of the constants described in [errno(3)](../man3/errno.3.html)).
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](../man3/errno.3.html)_ 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)](../man7/feature%5Ftest%5Fmacros.7.html).STANDARDS top
Certain terms and abbreviations are used to indicate UNIX variants
and standards to which calls in this section conform. See
[standards(7)](../man7/standards.7.html).NOTES top
Calling directly 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).
Authors and copyright conditions 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!
SEE ALSO top
[_syscall(2)](../man2/%5Fsyscall.2.html), [syscall(2)](../man2/syscall.2.html), [syscalls(2)](../man2/syscalls.2.html), [errno(3)](../man3/errno.3.html), [intro(3)](../man3/intro.3.html),
[capabilities(7)](../man7/capabilities.7.html), [credentials(7)](../man7/credentials.7.html), [feature_test_macros(7)](../man7/feature%5Ftest%5Fmacros.7.html),
[mq_overview(7)](../man7/mq%5Foverview.7.html), [path_resolution(7)](../man7/path%5Fresolution.7.html), [pipe(7)](../man7/pipe.7.html), [pty(7)](../man7/pty.7.html),
[sem_overview(7)](../man7/sem%5Foverview.7.html), [shm_overview(7)](../man7/shm%5Foverview.7.html), [signal(7)](../man7/signal.7.html), [socket(7)](../man7/socket.7.html),
[standards(7)](../man7/standards.7.html), [symlink(7)](../man7/symlink.7.html), [system_data_types(7)](../man7/system%5Fdata%5Ftypes.7.html), [sysvipc(7)](../man7/sysvipc.7.html),
[time(7)](../man7/time.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-05-02 intro(2)
Pages that refer to this page:syscall(2), _syscall(2), syscalls(2), intro(3), man-pages(7)