A few doubts on my C program to check prime [duplicate] - c

Question

This question already has answers here:
Reaching end of function without return statement
(2 answers)
Closed 4 years ago.
This is a simple function to check whether the given number is prime or not and works for the most part except for 2 and 3 (as it doesn't enter the for loop) but they are already prime so no checking is required and the flag should remain untouched.But notice how I put the return statement inside the for loop(This was by mistake) so for integers 2 and 3 as they do not enter the loop the function should return 0(or so I assumed) but they always return 1.why? Is it because the program terminated incorrectly? then why always 1? it can be any non-zero integer right?and are there cases where the main() itself returns a 1?.Please clarify my doubts.Dev-C++ is the IDE used and it uses tdm-gcc 4.9.2 compiler.
int checkPrime(int n)
{
int i, isPrime = 0;
for(i = 2; i <= n/2; ++i) {
if(n % i == 0) {
isPrime = 1;
break;
}
return isPrime;
}
}

Answer 1

Your return statement is in wrong place. You should put it out of for loop. And also in programming 1 stands for true and 0 for false, so you are asking if n is prime and for 17 your function returns 0 (false), but it is prime:
int checkPrime(int n)
{
int i, isPrime = 1;
for(i = 2; i <= n/2; ++i) {
if(n % i == 0) {
isPrime = 0;
break;
}
}
return isPrime;
}

Answer 2

On x86 architecture, the return value of function is in %eax register.
Value present there would be regarded as the return value of the function checkPrime.
And in the %eax register, the value present would be '1', so '1' was treated as the return value.
Proof (Passing '2' to checkPrime() and at the end eax holds 1):-
Dump of assembler code for function checkPrime:
0x0000000000400526 <+0>: push %rbp
0x0000000000400527 <+1>: mov %rsp,%rbp
0x000000000040052a <+4>: mov %edi,-0x14(%rbp)
0x000000000040052d <+7>: movl $0x0,-0x8(%rbp)
0x0000000000400534 <+14>: movl $0x2,-0x4(%rbp)
0x000000000040053b <+21>: nop
0x000000000040053c <+22>: mov -0x14(%rbp),%eax
0x000000000040053f <+25>: mov %eax,%edx
0x0000000000400541 <+27>: shr $0x1f,%edx
0x0000000000400544 <+30>: add %edx,%eax
0x0000000000400546 <+32>: sar %eax
0x0000000000400548 <+34>: cmp -0x4(%rbp),%eax
0x000000000040054b <+37>: jl 0x400568 <checkPrime+66>
0x000000000040054d <+39>: mov -0x14(%rbp),%eax
0x0000000000400550 <+42>: cltd
0x0000000000400551 <+43>: idivl -0x4(%rbp)
0x0000000000400554 <+46>: mov %edx,%eax
0x0000000000400556 <+48>: test %eax,%eax
0x0000000000400558 <+50>: jne 0x400563 <checkPrime+61>
0x000000000040055a <+52>: movl $0x1,-0x8(%rbp)
0x0000000000400561 <+59>: jmp 0x400568 <checkPrime+66>
0x0000000000400563 <+61>: mov -0x8(%rbp),%eax
0x0000000000400566 <+64>: jmp 0x400568 <checkPrime+66>
0x0000000000400568 <+66>: pop %rbp
0x0000000000400569 <+67>: retq
(gdb) break *0x0000000000400569
Breakpoint 1 at 0x400569: file ./test.c, line 19.
(gdb) r
Starting program: /home/syed/Desktop/a.out
Breakpoint 1, 0x0000000000400569 in checkPrime (n=2) at ./test.c:19
19 }
(gdb) info registers eax
eax 0x1 1