Coding question, function pointer, check if null.

[Taggsladder 2]

Hello

I don't know if this is quite the right place to post this or if my title is describing this correct and maybe this is more of a general C questions, but I hope its OK to post it here. It is on a MSP430 Launchpad anyway.

The code is simplified, there is more parameters passed to the function etc.

// I have a function like this.
void foo (char* bar) {
  
  if (!bar) Serial.print("bar is NULL"); // I want to check when NULL is passed on but this doesn't work?
  // Also tried (bar == NULL) and (bar[0] == NULL)...
  else Serial.print("bar is not NULL");
}

// Usually I call it like this.
foo("Normally text passed here");

// But I would like to call it like this also.
foo(NULL);

Looking forward to any reply.

Best regards

[enl 227]

"if (bar[0]==NULL)" won't look at the pointer value, but instead the memory the pointer references, so that is right out, unless you want to know if the parameter is an empty string (only the null terminator)

"if (bar == NULL)" should work, in particular if you explicitly pass NULL in since you are comparing a value to itself, as should "if (!bar)", since NULL should be 0.

I just checked both (using gcc on a windoze machine) and they work as expected, so I can offer no insight as to why you would have a problem. I would suspect that you are seeing a symptom of some other issue than pointer comparison.

[Taggsladder 2]

22 hours ago, enl said:

"if (bar[0]==NULL)" won't look at the pointer value, but instead the memory the pointer references, so that is right out, unless you want to know if the parameter is an empty string (only the null terminator)

"if (bar == NULL)" should work, in particular if you explicitly pass NULL in since you are comparing a value to itself, as should "if (!bar)", since NULL should be 0.

I just checked both (using gcc on a windoze machine) and they work as expected, so I can offer no insight as to why you would have a problem. I would suspect that you are seeing a symptom of some other issue than pointer comparison.

 

Thank you for your input. I think you are right, I looked at some other of my projects and found that I use similar code there that works. I need to dig into it

 

[chicken 630]

Depending on you environment NULL may not be defined. You can use 0 as alternative. E.g. if ( bar  == 0 )