Thread: Assignment of variables in if conditions

Which code snippet is more effective.

case 1:

Code:

 char *str;
 str = (char *) malloc(15);
 if(!str)
  return -ENOMEM;

cases 2:

Code:

char *str;
if(!(str = (char *) malloc(15)))
  return -ENOMEM

does it effect execution time?

If so, it's negligible. I'd suggest making the decision based on your style preferences. Two things worth noting about these snippets:

* The cast may be omitted, as void *s may be converted to other object pointer types by the = operator in C.
* It's totally fine to use ! or ==0 to check whether str stores a null pointer, but some prefer using ==NULL in this scenario.

No, no difference at all when tested on MSVC compiler at max optimisation.

Here's the disassembly of your first snippet:

Code:

char* test1(void) {
push        ebp  
mov         ebp,esp  
sub         esp,0CCh  
push        ebx  
push        esi  
push        edi  
lea         edi,[ebp-0CCh]  
mov         ecx,33h  
mov         eax,0CCCCCCCCh  
rep stos    dword ptr es:[edi]  
	char *str;
	str = (char *)malloc(15);
mov         esi,esp  
push        0Fh  
call        dword ptr ds:[0A89114h]  
add         esp,4  
cmp         esi,esp  
call        __RTC_CheckEsp (0A8113Bh)  
mov         dword ptr [str],eax  
	if (!str)
cmp         dword ptr [str],0  
jne         test1+3Eh (0A8140Eh)  
	return str;
mov         eax,dword ptr [str]  
}
pop         edi  
pop         esi  
pop         ebx  
add         esp,0CCh  
cmp         ebp,esp  
call        __RTC_CheckEsp (0A8113Bh)  
mov         esp,ebp  
pop         ebp  
ret

And your second:

Code:

char* test2(void) {
push        ebp  
mov         ebp,esp  
sub         esp,0CCh  
push        ebx  
push        esi  
push        edi  
lea         edi,[ebp-0CCh]  
mov         ecx,33h  
mov         eax,0CCCCCCCCh  
rep stos    dword ptr es:[edi]  
	char *str;
	if (!(str = (char *)malloc(15)))
mov         esi,esp  
push        0Fh  
call        dword ptr ds:[0A89114h]  
add         esp,4  
cmp         esi,esp  
call        __RTC_CheckEsp (0A8113Bh)  
mov         dword ptr [str],eax  
cmp         dword ptr [str],0  
jne         test2+3Eh (0A8147Eh)  
		return str;
mov         eax,dword ptr [str]  
}
pop         edi  
pop         esi  
pop         ebx  
add         esp,0CCh  
cmp         ebp,esp  
call        __RTC_CheckEsp (0A8113Bh)  
mov         esp,ebp  
pop         ebp  
ret

The first is only 1 line longer because of the interleaved source code. Of course, I can't speak for all compilers/instruction sets, but I don't see how a compiler could mess your first example up. And as zyxwvuts said, if there is any difference on any platform it'd be negligible.


On the subject of ! or 0 or NULL.... MISRA insists that you use NULL. In fact, you're meant to use != 0 for testing variables in conditionals, lest some poor soul be confusied..... it's an insane piece of literature. Made me laugh out loud at some points.

I personally find your first snippet clearer. And I have a preference for NULL over 0.