Unusual select / sock stream behavior behavior

Question

I'm writing a small program to request a chunk of a file, and then have another program return that specific chunk of the file. I can get this to work using files up to about 555000 bytes, but on anything lager than that, I get unusual behavior.

In my loop, I check a progress buffer, which is an array of integers, to see whether or not I have a specific chunk of the file. If I do, then I don't send a request for that chunk, but if I don't, then I request the chunk from a peer. I have a linked list that I move over, where each peer in the list has a sockfd associated with that. So, in a sense, I "round robin" the requests out. Once I send them out, I wait for messages to come in. I don't know if this is the best approach, but it seemed to be the most natural.

However, for files that are larger, it hangs at the select call. I'm not entirely sure why. If anyone can shed some light on this, or suggest a better approach, I'd love to hear it.

Here is my code. I didn't include the code for the functions that I call, but I don't think its necessary in this instance (they are modified versions of Beej's sendall and recvall functions). It is also worth mentioning that this is in a multithread application (using pthreads), but I'm not using any shared variables. Thanks for taking the time to read this!

total = (number of chunks in the file)
my_peers = (linked list of peer structs containing sockfds, etc)
int i;
progress_buf = (array of ints, each representing a chunk in a file, 0 means we don't have the chunk)

while (1) {

    /* Send out the chunk requests */
    for(z= 0; z < total; z++) {

        /* Circles through the peer list */
        if (temp -> next != NULL) {
            temp_next = temp->next;
        } else {
            temp_next = my_peers;
        }

        if (progress_buf[z] == 0) { 
            /* Request_blocks performs a send */ 

            /* Need to deal with the "hanging bytes" chunk */       
            if (((z + 1) == total) && (remainder_chunk == 1)) {           
                check = request_blocks(temp, remainder, remainder_chunk, z);
            } else {
                check = request_blocks(temp, remainder, 0, z);
            }

            /* Bad send, remove peer from file descriptors and list */
            if (check != 0 ) {          
                FD_CLR(check, &masterr);
                remove_peer(&my_peers, temp->socket);
            }
        }     
        temp = temp_next;
    }

    read_fdss = masterr; 

    /* HANGS RIGHT HERE */
    if (select(fdmax+1, &read_fdss, NULL, NULL, NULL) < 0) {
        perror("select"); 
    }

    read_fdss = masterr;

    int got_block;
    /* Means we've received a block */
    for(i = 4; i <= fdmax; i++) {
        got_block = -1;
        if (FD_ISSET(i, &read_fdss)) {

            /* Performs a recv */
            got_block = receive_block(i, filename_copy, progress_buf);

            /* Update the progress buffer */
            if (got_block > -1)  {
                remaining_blocks++;
                if (remaining_blocks == total) goto finished;
                    /* Failure- remove the peer */  
                } else if (got_block == -2) {
                    close(i);
                    FD_CLR(i, &masterr);
                    remove_peer(&my_peers, i);
                }
            }
        }
    }
}