This is literally the first thing I've ever written in C, so please feel free to point out all it's flaws. :) My issue, however is this: if I write the program the way I feel is cleanest, I get a broken program:

#include <sys/queue.h> 

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

/* Removed prototypes and non related code for brevity */

int
main()
{
    char    *cmd = NULL; 
    unsigned int acct = 0; 
    int amount = 0; 
    int done = 0; 

    while (done==0) {
        scanf ("%s %u %i", cmd, &acct, &amount);

        if (strcmp (cmd, "exit") == 0)
            done = 1;
        else if ((strcmp (cmd, "dep") == 0) || (strcmp (cmd, "deb") == 0))
            debit (acct, amount);
        else if ((strcmp (cmd, "wd") == 0) || (strcmp (cmd, "cred") == 0))
            credit (acct, amount);
        else if (strcmp (cmd, "fee") == 0)
            service_fee(acct, amount);
        else
            printf("Invalid input!\n");
    }
    return(0);
}

void
credit(unsigned int acct, int amount)
{
}

void
debit(unsigned int acct, int amount)
{
}

void
service_fee(unsigned int acct, int amount)
{
}

As it stands, the above generates no errors at compile, but gives me a segfault when ran. I can fix this by changing the program to pass cmd by reference when calling scanf and strcmp. The segfault goes away and is replaced by warnings for each use of strcmp at compile time. Despite the warnings, the affected code works.

warning: passing arg 1 of 'strcmp' from incompatible pointer type

As an added bonus, modifying the scanf and strcmp calls allows the program to progress far enough to execute return(0), at which point the thing crashes with an Abort trap. If I swap out return(0) for exit(0) then everything works as expected.

This leaves me with two questions: why was the original program wrong? How can I fix it better than I have?

The bit about needing to use exit instead of return has me especially baffled.