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 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 expects 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, performing 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).

       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

       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).

       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

       _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)

       This page is part of release 5.13 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                              2021-08-27                           INTRO(2)