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

       intro - introduction to system calls

       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.

       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

       *  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 postpro-
       cessing 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 in-
       terface 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).

       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 re-
       turn 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 dec-
       laration 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 in-
       formation on feature test macros, see feature_test_macros(7).

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

   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!

       _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),
       sysvipc(7), time(7)

       This page is part of release 5.05 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of this page, can be
       found at

Linux                                       2019-08-02                                   INTRO(2)

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