intro(2) — Linux manual page


intro(2)                   System Calls Manual                  intro(2)

NAME         top

       intro - introduction to system calls

DESCRIPTION         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

       •  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 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).

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)).
       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).

STANDARDS         top

       Certain terms and abbreviations are used to indicate UNIX
       variants and standards to which calls in this section conform.
       See standards(7).

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), 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), symlink(7), system_data_types(7), sysvipc(7),

Linux man-pages (unreleased)     (date)                         intro(2)

Pages that refer to this page: syscall(2)_syscall(2)syscalls(2)intro(3)man-pages(7)