sockatmark(3)

NAME

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

SYNOPSIS

   #include <sys/socket.h>

   int sockatmark(int sockfd);

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

   sockatmark(): _POSIX_C_SOURCE >= 200112L

DESCRIPTION

   sockatmark()  returns  a  value  indicating  whether  or not the socket
   referred to by the file descriptor sockfd 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  sockfd is not a valid file descriptor.

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

VERSIONS

   sockatmark() was added to glibc in version 2.2.4.

ATTRIBUTES

   For  an  explanation  of  the  terms  used   in   this   section,   see
   attributes(7).

   
   Interface     Attribute      Value   
   
   sockatmark()  Thread safety  MT-Safe 
   

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008.

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 supported only 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(sockfd);
           if (atmark == -1) {
               perror("sockatmark");
               break;
           }

           if (atmark)
               break;

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

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

SEE ALSO

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

COLOPHON

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