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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.

EXAMPLES
       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");
                   ...
               }
           }

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

Linux                                                                          2021-03-22                                                                  SOCKATMARK(3)