Function pointer arguments ignored/not needed

Question

I'm currently writing C code for a microprocessor and I came across something I couldn't explain. I have implemented a command line interface using function pointers. To do this I have made a struct holding the name of the command, the pointer to the function to be run and also a help description.

typedef void(*command)(char *);
typedef struct commandStruct {
    char const *name;
    command execute;
    char const *help;
} commandStruct;

const commandStruct commands[] =
{
    {"led", CmdLed, "Turns on or off the LED1"},
    {"AT+START_SIM", start_simulation, "Starts the simulation"},
    {"AT+STOP_SIM", stop_simulation, "Stops the simulation"},
    {"",0,""} //End of table indicator.
};

void exec_command(char *buffer)
{
    uint16 i = 0;
    char *cmd = buffer;
    char *args;
    while (buffer[i])
    {
        if(buffer[i] == '=')
        {
            buffer[i] = 0;
            args = buffer + i + 1;
            break;
        }
        i++;
    }

    uint16 cmdCount = 0;
    while(strcmp(commands[cmdCount].name,""))
    {
        if(!strcmp(commands[cmdCount].name,cmd))
        {
            commands[cmdCount].execute(args);
            break;
        }
        cmdCount++;
    }
}

void start_simulation(void) {run = 1;}
void stop_simulation(void) {run = 0;}
void CmdLed(char *args)
{
    P1DIR |= BIT0;
    if(!strcmp(args,"on")) P1OUT = 1;
    if(!strcmp(args,"off")) P1OUT = 0;
}

I have included above the function exec_command which is where the function pointer is used. At the bottom I have also put the start_simulation and stop_simulation functions, along with CmdLed. I wrote CmdLed at an earlier date and then came back and wrote start_simulation and stop_simulation. I had forgotten that I had defined my function pointer as taking a (char *) as an argument. However, I was surprised to find that everything still compiled and ran absolutely fine. Why is this? It seems that any arguments are just 'dumped' and not used.

Answer 1

This should not compile with modern compilers.

What happens here :

start_simulation will be called with a char* argument, but as start_simulation has no argument the argument is simply ignored (or "dumped" as you write).

Remember, in C function arguments are pushed onto the stack and the caller cleans up the stack after the call. So if you call a function with no argument pretending it has argument(s), then

• the caller pushes the argument(s) onto the stack
• the argumentless function is called by the caller
• the function does it's business ignoring the argument(s) on the stack
• the functions returns to the caller
• the caller cleans up the stack

Have also a look at this SO question.