Question

I am just learning socket programming and I am trying to write an echo client that reads from stdin and writes to a socket and then reads the server response from the socket to stdout. The problem is that I don’t know how long stdin will be or how long the server’s response will be. The code I am trying to use is as follows (creating the socket and connecting to the server are left out):

length = BUF_SIZE;
while (length == BUF_SIZE) {  // length will equal BUF_SIZE if buf is full, when length < BUF_SIZE we have reached an EOF                                 
   // Reads from STDIN to buf                                                                                                                              
   if ((length = read(STDIN_FILENO, buf, BUF_SIZE)) < 0){
     fprintf(stderr, "Error in reading from STDIN");
     return 4;
   }
   // Writes from buf to the socket                                                                                                                        
   if ((write(sock, buf, BUF_SIZE)) < 0){
     fprintf(stderr, "Error writing to socket");
     return 5;
   }
 }

if ((status = shutdown(sock, 1)) < 0){ // Shuts down socket from doing more receives                                                                      
   fprintf(stderr, "Error shutting down socket for writing");
   return 6;
}

length = BUF_SIZE; 
while (length == BUF_SIZE){
   // Read from socket to buf                                                                                                                              
   if ((length = read(sock, buf, BUF_SIZE)) < 0){
     fprintf(stderr, "Error reading from socket");
     return 7;
   }
   // Write from buf to STDOUT                                                                                                                             
   if ((write(STDOUT_FILENO, buf, BUF_SIZE)) < 0){
     fprintf(stderr, "Error writing to STDOUT");
     return 8;
   }
}

close(sock);
exit(0);

BUF_SIZE is defined as 100. When I run my program the program typically connects to the server and sends the proper message, but what it writes to stdout is either nothing or gibberish.

What am I doing wrong?

Answer 1

Your while loop is only going to work the first time through. read()/write() are only going to return the amount that they actually read/write which may well not be equal to BUF_SIZE. Let’s say you read ten bytes from the socket and then you write a hundred to stdout – the last 90 are going to be garbage.

Something along these lines will get you somewhat closer to what you want.

while (1)
{
    if ((length = read(STDIN_FILENO, buf, BUF_SIZE)) < 0)
    {
        fprintf(stderr, "Error in reading from STDIN");
        return 4;
    }

    if ((write(sock, buf, length)) < 0)
    {
        fprintf(stderr, "Error writing to socket");
        return 5;
    }

    if ((length = read(sock, buf, BUF_SIZE)) < 0)
    {
        fprintf(stderr, "Error reading from socket");
        return 7;
    }

    if ((write(STDOUT_FILENO, buf, length)) < 0)
    {
        fprintf(stderr, "Error writing to STDOUT");
        return 8;
    }
}