Question

I’ve faced the next one sample:

#include <stdio.h>

// test multiple return                                                                                                                                                             
int foo()
{
    return 1,2,3,4,5,6;
}

// main entry point                                                                                                                                                                 
int main(int argc, char* argv[])
{
    printf("foo returns: %d\n", foo());
    return 0;
}

compile it, then run:

gcc main.cpp -o main
./main

The results are confusing me:

foo returns: 6

The question is: why there is no compile time error?

Answer 1

Because you are using the comma operator: the expression a,b where a and b are arbitrary (usually side-effecting) sub-expressions mean: evaluate the left-hand side a and discard its result (so a is only evaluated for side-effects), then evaluate b and give it as result.

You cannot return several things from a C function. You should return e.g. an aggregate (usually a struct) or a dynamically heap-allocated pointer.

As to the question, why the compiler don’t say anything? Because you didn’t ask it. You really should compile with gcc -Wall (for C code) or g++ -Wall (for C++ code), and then you get warnings:

 egor7.c: In function ‘foo’:
 egor7.c:6:13: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:15: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:17: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:19: warning: left-hand operand of comma expression has no effect [-Wunused-value]
 egor7.c:6:21: warning: left-hand operand of comma expression has no effect [-Wunused-value]