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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 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);
             if (s == -1)
                 perror("read");
             if (s <= 0)
                 break;
         }

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