I have a client-server application where a server listens on one machine on a particular port, and a client can connect to that machine via it’s IP address and port.

The client sends data to the server with the command it wants to execute, for instance, cat /etc/passwd. That server uses popen to execute the command and store the results in a buffer, like so:

/* Receive commands from client */
char command_in[256];
char command_out[2048];
char command_out_send[2048];
ssize_t recv_data = recv(client_s, command_in, sizeof(command_in), 0);

// <SNIP>

/* Execute system command and obtain output */
FILE *fp = popen(command_in, "r");
if (fp == NULL) {
    close(client_s);
    close(s);
    return 1;
} else {
    while(fgets(command_out, sizeof(command_out) - 1, fp) != NULL) {
        /* Capture all output and save it to buffer */
        strcat(command_out_send, command_out);
    }
}

/* Send executed command to client*/
if (send(client_s, command_out_send, sizeof(command_out_send), 0) == -1) {
    close(client_s);
    close(s);
    return 1;
}

/* Cleanup */
memset(command_out_send, 0, sizeof(char));
pclose(fp);

The issue is the fact that it creates a buffer to hold the contents.

The client is as follows.

while (1) {
    char command_in[256];
    char command_out[2048];
    printf("~ ");
    fgets(command_in, 256, stdin);

    /* Send command to server */
    if (send(s, command_in, sizeof(command_in), 0) == -1) {
        close(s);
        return 1;
    }

    /* Receive response from server */
    ssize_t recv_data = recv(s, command_out, sizeof(command_out), 0);
    if (recv_data == -1) {
        close(s);
        return 1;
    }

    /* Print out command results */
    fputs(command_out, stdout);
}

Currently, the client retrieves the data and prints the output. Instead, I want to send in chunks while there is data to send on the server side, and for that client to wait until all that data is sent rather than creating a buffer to hold the entire contents of the command..

How can I do this?