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


   sockatmark() was added to glibc in version 2.2.4.


   For  an  explanation  of  the  terms  used   in   this   section,   see

   Interface     Attribute      Value   
   sockatmark()  Thread safety  MT-Safe 


   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) {

           if (atmark)

           s = read(sockfd, buf, BUF_LEN) <= 0);
           if (s == -1)
           if (s <= 0)

       if (atmark == 1) {
           if (recv(sockfd, &oobdata, 1, MSG_OOB) == -1) {


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


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   description  of  the project, information about reporting bugs, and the
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