SOCKATMARK(3)              Linux Programmer's Manual             SOCKATMARK(3)
NAME
       sockatmark - determine whether socket is at out-of-band mark

SYNOPSIS
       #include <sys/socket.h>

       int sockatmark(int fd);

DESCRIPTION
       sockatmark()  returns  a  value  indicating  whether  or not the socket
       referred to by the file descriptor fd is at the out-of-band  mark.   If
       the  socket is at the mark, then 1 is returned; if the socket is not at
       the mark, 0 is returned.  This function does not remove the out-of-band
       mark.

RETURN VALUE
       A  successful  call  to  sockatmark() returns 1 if the socket is at the
       out-of-band mark, or 0 if it is not.  On  error,  -1  is  returned  and
       errno is set to indicate the error.

ERRORS
       EBADF  fd is not a valid file descriptor.

       EINVAL fd  is  not  a  file  descriptor  to  which  sockatmark() can be
              applied.

NOTES
       If sockatmark() returns 1, then the out-of-band data can be read  using
       the MSG_OOB flag of recv(2).

       Out-of-band data is only supported on some stream socket protocols.

       sockatmark() can safely be called from a handler for the SIGURG signal.

       sockatmark() is implemented using the SIOCATMARK ioctl() operation.

CONFORMING TO
       POSIX.1-2001

VERSIONS
       sockatmark() was added to glibc in version 2.2.4.

BUGS
       Prior to glibc 2.4, sockatmark() did not work.

EXAMPLE
       The following code can be used after receipt of a SIGURG signal to read
       (and  discard)  all data up to the mark, and then read the byte of data
       at the mark:

           char buf[BUF_LEN];
           char oobdata;
           int atmark, s;

           for (;;) {
               atmark = sockatmark(fd);
               if (atmark == -1) {
                   perror("sockatmark");
                   break;
               }

               if (atmark)
                   break;

               s = read(fd, buf, BUF_LEN) <= 0);
               if (s == -1)
                   perror("read");
               if (s <= 0)
                   break;
           }

           if (atmark == 1) {
               if (recv(fd, &oobdata, 1, MSG_OOB) == -1) {
                   perror("recv");
                   ...
               }
           }

SEE ALSO
       fcntl(2), recv(2), send(2), tcp(7)

Linux                             2006-04-24                     SOCKATMARK(3)