This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       None.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical  and  Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE  and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                    GETSOCKOPT(3POSIX)

       The getsockopt() function manipulates options associated with a socket.

       The  getsockopt()  function shall retrieve the value for the option specified by the option_name argument
       for the socket specified by the socket argument. If  the  size  of  the  option  value  is  greater  than
       option_len,  the  value  stored  in  the object pointed to by the option_value argument shall be silently
       truncated. Otherwise, the object pointed to by the option_len argument shall be modified to indicate  the
       actual length of the value.

       The  level  argument specifies the protocol level at which the option resides. To retrieve options at the
       socket level, specify the level argument as SOL_SOCKET. To retrieve options at other levels,  supply  the
       appropriate  level  identifier for the protocol controlling the option.  For example, to indicate that an
       option is interpreted by the TCP (Transmission Control Protocol), set level to IPPROTO_TCP as defined  in
       the <netinet/in.h> header.

       The  socket  in  use  may  require  the  process  to  have appropriate privileges to use the getsockopt()
       function.

       The option_name argument specifies a single option to be retrieved. It can be  one  of  the  socket-level
       options defined in <sys_socket.h> and described in Section2.10.16, UseofOptions.

       The getsockopt() function shall fail if:

       EBADF  The socket argument is not a valid file descriptor.

       EINVAL The specified option is invalid at the specified socket level.

       ENOPROTOOPT
              The option is not supported by the protocol.

       ENOTSOCK
              The socket argument does not refer to a socket.

       The getsockopt() function may fail if:

       EACCES The calling process does not have appropriate privileges.

       EINVAL The socket has been shut down.

       ENOBUFS
              Insufficient resources are available in the system to complete the function.

       Thefollowingsectionsareinformative.

       None.

       None.

       getsockopt — get the socket options

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       None.

       Upon successful completion, getsockopt() shall return 0; otherwise, -1 shall be returned and errno set to
       indicate the error.

Section2.10.16, UseofOptions, bind(), close(), endprotoent(), setsockopt(), socket()

       The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>, <netinet_in.h>

       #include <sys/socket.h>

       int getsockopt(int socket, int level, int option_name,
           void *restrict option_value, socklen_t *restrict option_len);