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NAME

       sockatmark - determine whether socket is at out-of-band mark

SYNOPSIS

       #include <sys/socket.h>

       int sockatmark(int fd);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       sockatmark(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600

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.

VERSIONS

       sockatmark() was added to glibc in version 2.2.4.

CONFORMING TO

       POSIX.1-2001

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(2) operation.

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)

COLOPHON

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