man > INTRO(2)

intro(2)                                 System Calls 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  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 ex‐
          pects 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, per‐
       forming 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
       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 de‐
       scribed in the individual manual pages.

       In some cases, the programmer must define a feature test macro in order to obtain the  decla‐
       ration  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 infor‐
       mation on feature test macros, see feature_test_macros(7).

STANDARDS
       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), socket(7), standards(7), symlink(7), system_data_types(7),
       sysvipc(7), time(7)

Linux man-pages 6.7                          2023-10-31                                     intro(2)