Client - Server Socket C Code: Server doesn't execute code after "read" until another line is sent

Question

I am writing some code to create a client - server connection. The client sends strings to the server and the server is supposed to parse and print the string depending on what was sent.

Client code:

// Client side C/C++ program to demonstrate Socket programming
#include <stdio.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <string.h>
#define PORT 8080

int main(int argc, char const *argv[]) {
    struct sockaddr_in address;
    int sock = 0, valread;
    struct sockaddr_in serv_addr;
    size_t len = 0;
    char *buffer = NULL;
    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        printf("\n Socket creation error \n");
        return -1;
    }

    memset(&serv_addr, '0', sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(PORT);

    // Convert IPv4 and IPv6 addresses from text to binary form
    if (inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr) <= 0) {
        printf("\nInvalid address/ Address not supported \n");
        return -1;
    }

    if (connect(sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        printf("\nConnection Failed \n");
        return -1;
    }

    while(1) {
        getline(&buffer, &len, stdin);
        printf("Line sent: %s", buffer);
        send(sock, buffer, strlen(buffer), 0);
    }
}

Server code:

/*Required Headers*/

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include<string.h>

int main()
{

    char str[100];
    int listen_fd, comm_fd;

    struct sockaddr_in servaddr;

    listen_fd = socket(AF_INET, SOCK_STREAM, 0);

    bzero( &servaddr, sizeof(servaddr));

    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htons(INADDR_ANY);
    servaddr.sin_port = htons(8080);

    bind(listen_fd, (struct sockaddr *) &servaddr, sizeof(servaddr));

    listen(listen_fd, 10);

    comm_fd = accept(listen_fd, (struct sockaddr*) NULL, NULL);

    while(1)
    {
        bzero( str, 100);

        read(comm_fd,str,100);

        printf("Received: %s",str);
        if(strncmp(str, "NEW", 3) == 0)
        {
            printf("Client - NEW");
        }
        else if(strncmp(str, "FILE", 4) == 0)
        {
            printf("Client - FILE");
        }
        else if(strncmp(str, "NAME", 4) == 0)
        {
            strtok(str, ": ");
            printf("NAME: %s", str);
        }
        else if(strncmp(str, "DESCRIPTION", 11) == 0)
        {
            strtok(str, ": ");
            printf("DESCRIP: %s", str);
        }
        else if(strncmp(str, "PRINTER", 7) == 0)
        {
            strtok(str, ": ");
            printf("PRINTER: %s", str);
        }

    }
}

What I am expecting to happen is that when I send "NEW" from the client to the server, the server prints "Client - NEW". Then when I send "FILE" the server prints "CLIENT - FILE". What happens is there is a delay of one send. So when I send NEW, the server prints "Received: NEW", but it doesn't go into the if check to print "CLIENT - NEW". Then when I send "FILE", the server prints "Client - NEWRecieved: FILE".

What could I be doing wrong here?

Answer 1

So when I send NEW, the server prints "Received: NEW"

Yes, that's here:

printf("Received: %s",str);

but to be specific, you send "NEW\n", and the server prints "Received: NEW\n". This newline is relevant, because it triggers the output on line buffered terminals

but it doesn't go into the if check to print "CLIENT - NEW".

Yes, it executes the conditional:

printf("Client - NEW");

but the output is buffered until the next newline gets printed.

Then when I send "FILE", the server prints "Client - NEWRecieved: FILE".

Now after you send "FILE", this gets executed:

printf("Received: %s",str);

with str having "FILE\n" as content. This flushes the output buffer, which currently contains "Client - NEWRecieved: FILE\n". Furthermore

printf("Client - FILE");

gets executed (but not output, only buffered) and the process iterates.

So the fix is, to add a newline "\n" at the end of each printf format string (or use puts in the simple cases).

printf("Received: %s\n",str); // Although "str" will usually contain a newline, but better play it safe

puts("Client - NEW");

Furthermore, if you have a line-based protocol, you should switch to buffered input for your socket:

comm_fd = accept(listen_fd, (struct sockaddr*) NULL, NULL);
FILE *sock = fdopen(comm_fd, "r+");

and then read the input line-by-line

while (1) {
    fgets(str, sizeof(str), sock);