sockatmark() — Determine whether a socket is at the out-of-band mark

Standards

Format

#define _POSIX_C_SOURCE 200112L
#include <sys/socket.h>

int sockatmark(int s);

General description

The sockatmark() function determines whether the socket specified by the descriptor s is at the out-of-band data mark. If the protocol for the socket supports out-of-band data by marking the stream with an out-of-band data mark, the sockatmark() function returns 1 when all data preceding the mark has been read and the out-of-band data mark is the first element in the receive queue. The sockatmark() function does not remove the mark from the stream.

Returned value

Upon successful completion, the sockatmark() function returns a value indicating whether the socket is at an out-of-band data mark. If the protocol has marked the data stream and all data preceding the mark has been read, the return value is 1; if there is no mark, or if data precedes the mark in the receive queue, the sockatmark() function returns 0. Otherwise, it returns a value of -1 and set errno to indicate the error.

Error Code

Description

EBADF

The s argument is not a valid file descriptor.

ENOTTY

The s argument does not specify a descriptor for a socket.

Example

CELEBS74

⁄* CELEBS74

   This example demonstrates the use of the sockatmark() function.

   Expected output:
   C: Sending regular data
   C: Sending OOB data
   S: Received "123a"
   S: At the mark
   S: Received "b"

*⁄

#define _POSIX_C_SOURCE 200112L
#include <sys⁄socket.h>
#include <netinet⁄in.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

int main(int argc, char **argv) {
   struct sockaddr_in saddr;
   socklen_t addr_len = sizeof(saddr);
   int port = 12121, n, ld, connfd, servfd;
   char buffer[25];
   pid_t pid;

   if((ld = socket(AF_INET,SOCK_STREAM,0)) == -1){
      printf("socket error\n");
      return 0;
   }

   saddr.sin_family = AF_INET;
   saddr.sin_port    = 12121;

   if(bind(ld,(struct sockaddr *)&saddr,addr_len) == -1){
      printf("bind error\n");
      return 0;
   }

   if(listen(ld,5) == -1){
      printf("listen error\n");
      return 0;
   }

   pid = fork();
   if(pid==0){
      if((connfd = socket(AF_INET,SOCK_STREAM,0)) == -1){
         printf("socket error\n");
         exit(0);
      }

      if(connect(connfd,(struct sockaddr *)&saddr,addr_len) == -1){
         printf("connect error\n");
         exit(0);
      }

      printf("C: Sending regular data\n");
      send(connfd,"123",3,0);
      printf("C: Sending OOB data\n");
      send(connfd,"ab",2,MSG_OOB);

      close(connfd);
      exit(0);
   }
   else {
      servfd = accept(ld,(struct sockaddr *)&saddr,&addr_len);
      if(servfd == -1) {
         printf("accept error\n");
         exit(0);
      }

      sleep(5);
      memset(buffer,0,sizeof(buffer));
      recv(servfd,&buffer,sizeof(buffer),0);
      printf("S: Received \"%s\"\n",buffer);

      memset(buffer,0,sizeof(buffer));

      n = sockatmark(servfd);
      if(n == 1) printf("S: At the mark\n");
      recv(servfd,&buffer,sizeof(buffer),MSG_OOB);
      printf("S: Received \"%s\"\n",buffer);

      close(servfd);
      close(ld);
   }
}

Related information

• sys/socket.h