C program forcing me to have return type but I don't need it.!

Question

I have a TCP/IP application in C. I have 1 header file for handling TCP/IP related things & 1 main file to call all functions. Since problem is happening on client side, I am posting my client side code only.

Here is program.c

#include "tcpHeader.h"
#include <pthread.h>
#include <stdio.h>

int main(int argc, char **argv){

    // all code regarding connecting to server on TCP on another thread.
    // Join here after completing the thread execution
    // It works completely fine upto the next line.
    testFunctionFromMyHeader();

    // after executing below line, program do not continue with 'void' return type of the function. 
    // But it works if I change return type to 'int'
    waitForAMessageFromServerAndSendConfirmation(12345678901LL);

    // program expects me to return an integer on last function call 
    // & all other calls from now.
    // even if I don't need integer return type.
    testFunctionFromMyHeader();

    return 0;
}

Here is tcpHeader.h

#ifndef CLIENT_TCP
#define CLIENT_TCP

// all required headers.
// all global variables

void testFunctionFromMyHeader(){
    printf("Test Function");
}

void waitForAMessageFromServerAndSendConfirmation(unsigned long long args){

    // wait until receiving data from server in recv() function.
    // received 'args' is used in processing the data.
    // process the data received
    // send the confirmation message to server in send() function.
    // Server also received this confirmation without any problem on server side.

    // all code in this function also worked properly.
    // Even the next printf line is also executed.
    printf("All operation completed.");

    // with 'void' return type, next line does not make any difference.
    // But if I return an 'int' then it works.
    return NULL;
}

#endif

It stops on waitForAMessageFromServerAndSendConfirmation(unsigned long long) in program.c

but If I do like this in program.c, then it works.(Return type on functions from tcpHeader.h are also changed to appropriate types)

int confirmation = waitForAMessageFromServerAndSendConfirmation(12345678901LL);
if( confirmation == 0 ){ // 0 or any int value
    int testFunctionReturnedValue = testFunctionFromMyHeader();
        .
        .
        .
        // keep going like this with int return type FOR ALL FUNCTIONS
}

Help me to identify the problem. I don't want to use int return types everywhere.

- Why do it force me to have 'int' return type on waitForAMessageFromServerAndSendConfirmation(unsigned long long) function in my header file

- Why do it force me to have 'int' return types on all functions called after that one.

Answer 1

When a function is declared as void it means the function will not return anything.

If you use a return statement with a value in such a function, e.g. return 1;, you attempt to have it return something. But it is defined as not to return something.

Declaring a funcion as void has two intentions:

1. Tell the user of the function not to expect a return value.

2. Tell the author of the function not to return anything. If the author wants to communicate something, he should do that using other means (e.g. have a pointer argument to a variable of the caller in which to place a result value, or adapt the function signature to return something other than void).

For example:

void myFunc1(int*result) {
    *result= 1;  // put value 1 in callers variable
    // there is no return statement that returns a value
}
void myFunc2(int*result) {
    *result= 1;  // put value 1 in callers variable
     return;     // leave the function. There is no value to return.
}

or:

int myFunc(int in) {
    return 2*in;  // leave the function and return a value.
}

---

To get back to your problem:

int confirmation = waitForAMessageFromServerAndSendConfirmation(12345678901LL);

The above says to expect the function to return something and to assign that to variable conformation. However,

void waitForAMessageFromServerAndSendConfirmation(unsigned long long args) {
    //....
    return NULL;

says the function will not return anything, but you try to let it return something.

because of these contradictions, your compiler complains.