Peeking at Data

The recv , recvfrom , and rcvmsg calls allow the user to look at received data without consuming it (the data will still be available for the next receive-type call). This is handy should the receiving process need to examine a message to, say, perhaps act upon it rather than pass it on to another process. To implement a nonconsumptive receive, the user must set the integer flags argument for the receive call to the defined constant MSG_PEEK. A modified Internet domain server program (Program 10.6) shows how this can be done. The processing loop of the program (where a child process is generated to handle the connection from the client) is modified to include a peek at the incoming message. These modifications are shown in Program 10.14.

Program 10.14 Internet domain connection-oriented server using MSG_PEEK.

File : p10.14.cxx
 /*
 Internet domain, connection-oriented SERVER - MSG_PEEK
 */
 .
 . // Same as Program p10.6.cxx
 .
 if ( fork ( ) ==0 ) { // Generate a CHILD
 while ( (len==recv(new_sock, buf, BUFSIZ, MSG_PEEK)) > 0){
 write( fileno(stdout), "Peeked and found: ",19);
 50 write( fileno(stdout), buf, len); // show peeked msg
 if ( !strncmp(buf, ".done", len-1) ) break;
 len=recv(new_sock, buf, BUFSIZ, 0 ); // retrieve same msg
 write( fileno(stdout), "Re-read buffer : ",19);
 write( fileno(stdout), buf, len);
 + }
 write( fileno(stdout),"Leaving child process
",22);
 close(new_sock); // In CHILD process
 return 0;
 } else close(new_sock); // In PARENT process
 60 } while( true ); // FOREVER
 .
 . // Same as Program p10.6.cxx
 .

The modifications to the client program (Program 10.7) are shown in the partial listing in Program 10.15.

Program 10.15 Internet domain connection-oriented client using MSG_PEEK.

File : p10.15.cxx
 /*
 Internet domain, connection-oriented CLIENT
 */
 .
 . // Same as Program p10.7.cxx
 .
 + do { // Process
 write(fileno(stdout),"> ", 3);
 if ((len=read(fileno(stdin), buf, BUFSIZ)) > 0) {
 write(fileno(stdout), "Sending ", 9);
 write(fileno(stdout), buf, len);
 40 send(orig_sock, buf, len, 0);
 }
 } while( strncmp(buf, ".done", len-1) ); // until end of input
 .
 . // Same as Program p10.7.cxx
 .

When these modified programs are compiled and run (as shown in Figure 10.18), it is easy to see that the server process can peek at the received data by specifying MSG_PEEK. When the second receive call is made, the peeked-at data is received again.

Figure 10.18 Peeking at messagesserver and client running on separate hosts .

linux$ g++ p10.14.cxx -o server
linux$ g++ p10.15.cxx -o client perseus$ client linux
linux$ server > George Orwell was an optimist.
Peeked and found: George Orwell was Sending George Orwell was an
an optimist. optimist.
Re-read buffer : George Orwell was > Life is like a simile.
an optimist. Sending Life is like a simile.
Peeked and found: Life is like a > .done
simile. Sending .done
Re-read buffer : Life is like a perseus$
simile.
Peeked and found: .done
Leaving child process