intro – introduction to system calls


       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 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).


       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

       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


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



       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


       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