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INTRO(2)                            Linux Programmer's Manual                            INTRO(2)



NAME
       intro - Introduction to system calls

DESCRIPTION
       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 sys-
       tem  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.

       For a list of the Linux system calls, see syscalls(2).

RETURN VALUE
       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 non-zero 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 dec-
       laration of a system call from the header file specified in the man page SYNOPSIS section.
       In such cases, the required macro is described in the man page.  For  further  information
       on feature test macros, see feature_test_macros(7).

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

NOTES
   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 Terms
       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
       _syscall(2), syscall(2), errno(3), feature_test_macros(7), standards(7)

COLOPHON
       This page is part of release 3.05 of the Linux man-pages project.  A  description  of  the
       project,   and   information  about  reporting  bugs,  can  be  found  at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                                       2007-10-23                                   INTRO(2)

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