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INTRO(2)                                                                Linux Programmer's 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 di‐
       rectly: 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).

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

Linux                                                                          2021-08-27                                                                       INTRO(2)