Question

I’m having trouble comparing strings in C (with which I am fairly new to). I have socket on this server application waiting to accept data from a client. In this particular portion of my program I want to be able to execute a MySQL query based on the data received from the client. I want to be able to know when the received data has the value of “newuser” to initiate a simple registration procedure. Strcmp is returning a positive 1 value where I believe I should be getting a 0 because the values should be equal.

Source Code:

//setup socket
//loop and select structure to handle multiple connections

if ((nbytes = recv(i, buf, sizeof buf, 0)) <= 0) {
// got error or connection closed by client
    if (nbytes == 0) {
        // connection closed
        printf("selectserver: socket %d hung up\n", i);
    } else {
        perror("recv");
    }
    close(i); // bye!
    FD_CLR(i, &master); // remove from master set
} else {

    char check[] = "newuser";
    char fromUser[sizeof check];

    strncpy(fromUser,buf, sizeof check);
    printf("length of fromUser: %d\n", sizeof fromUser);
    printf("length of check: %d\n", sizeof check);
    printf("message from user: %s\n", fromUser);
    printf("check = %s \n", check);
    int diff = strcmp(fromUser, check);
    printf("compare fromUser to check: %d\n", diff);
    if ( strcmp(fromUser, check) == 0) {
        printf("aha! new user");
    }

Output:

length of fromUser: 8
length of check: 8
newuser from user: newuser
check = newuser 
compare fromUser to check:

I have a feeling I’m not handling the incoming buffer correctly or erroneously copying the buffer.

Answer 1

strncpy copies at most – in this case – sizeof check bytes. If the nul byte is not in that range it is not copied. You probably are getting the word “newuser” as part of a longer sentence, like “newuser blah blah” so you need to place that nul yourself

strncpy(fromUser, buf, sizeof check);
fromUser[sizeof check - 1] = '\0';

or use strlcpy, if available.