Manpages - sockatmark.3

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

  #include <sys/socket.h>

  int sockatmark(int sockfd);

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

*sockatmark*():

      _POSIX_C_SOURCE >= 200112L

*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.

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.

EBADF

sockfd is not a valid file descriptor.

EINVAL

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

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

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

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

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.

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

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");
              ...
          }
      }

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

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