Question

I want to use select system call to multiplex STDIN and SOCKFD (connected to a server) such that I must be listening to both STDIN and SOCKFD and depending upon where the data is available for read I must proceed further.

Note: #define STDIN 0

This is what happens when I do the following.

• I connect to the server the usual way [ socket() then connect() ]
• I add STDIN and connection socket descriptor ‘SOCKFD’ to the fd_set (named ‘readset’) meant for reading.
• Then I call select().
• Use FD_ISSET to determine which fd is ready for read.

The problem with this set up is that FD_ISSET is always true for both the fd. Once I get connected to the server I see FD_ISSET(sockfd, &readset) is always true and so is FD_ISSET(STDIN, &readset) whether, the server is sending data or not or if I am entering data from keyboard or not.

What could I be doing wrong?

/* connection successful */

FD_ZERO(&connset);

while(1) {

    FD_SET(sockfd,&connset); /* add sockfd to connset */
    FD_SET(STDIN,&connset); /* add STDIN to connset */
    fdmax=sockfd;

    if(select(fdmax+1,&readset,NULL,NULL,NULL)<0){
        fprintf(stdout, "select() error\n");
        exit(0);
    }       

    /* select returned
    * check which socket is set
    */

    if(FD_ISSET(sockfd,&connset)) {

        /*
         * Server sends msglen 
         * client reads msg of length msglen
         * client prints it to stdout
         * client waits for next activity of its listen sockets
         */
        size=4;
        ptr=(char *)&msglen;
        while(1) {
            if((nread=recv(sockfd,ptr,size,0)),MSG_DONTWAIT) {
                close(sockfd);
                exit(0);
            }
            size-=nread;
            ptr+=nread;
        }
        bytesToRead = ntohl(msglen);
        readbuf = (char *)malloc(sizeof(char)*(bytesToRead+1));
        ptr=readbuf;
        while(1) {
            nread=recv(sockfd,ptr,bytesToRead,MSG_DONTWAIT);
            if(nread<=0) {
                close(sockfd);
                exit(0);
            }
            bytesToRead-=nread;
            ptr+=nread;
        }
        /* msg read successfully */
        *ptr='\0';
        fprintf(stdout, "in: %s\n",readbuf);
        free(readbuf);
    }

    if(FD_ISSET(STDIN,&connset)) {
        /* data at STDIN */
        fgets(buf,2,stdin); /* read the first newline char (< enter >) */
        fprintf(stdout,"Enter msg: ");
        fgets(buf,MAXLEN,stdin); /* read the msg */
        buf[strlen(buf)-1]='\0';
        msglen = htonl((uint32_t)(strlen(buf)-1));
        ptr = (char *)&msglen;
        bytesToSend = sizeof(uint32_t);
        cnt = bytesToSend;
        while(cnt>0) {
            if((nsent=send(sockfd,ptr,cnt,0))>0) {
                cnt-=nsent;
                ptr+=nsent;
            }
            else {
                fprintf(stdout,"send error\n");
                exit(0);
            }
        }
        ptr=buf;
        bytesToSend=(uint32_t)(strlen(buf)-1);
        cnt=bytesToSend;
        while(cnt>0) {
            if((nsent=send(sockfd,ptr,cnt,0))>0) {
                cnt-=nsent;
                ptr+=nsent;
            }
            else {
                fprintf(stdout,"send error\n");
                exit(0);
            }
        }       
    }       
}   
return 0;

}

Answer 1

What is STDIN? There is stdin which is declared in <stdio.h>, is a FILE* and not a file descriptor so can’t be used in an fdset and STDIN_FILENO, which is defined in <unistd.h> and is a file descriptor and thus can be used with fdsets.

BTW, in <stdio.h> there is also a function fileno() which returns the file descriptor of a FILE* (depending on your compilation flags, you may need to define some feature macros to get the declaration).