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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
       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.

       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 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).

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 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

       _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), svipc(7), standards(7), socket(7),
       symlink(7), time(7)

COLOPHON

       This page is part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.