file mode

if I open a file that does not exist with mode r will the file pointer return as false? Example:

Code:

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FILE *filePtr = fopen(".//file_does_not_exist.dat", "r");

if(!filePtr) {
    printf("File not found.");
} else {
    printf("File opened.");
    //other actions here
}

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Will that return "File not found." or "File opened."?

If a file does not exist fopen will return NULL if using mode "r"

!NULL == true (on most compilers)
NULL == false (again on most compilers)

So it should get you "File not found."

thanx

I agree with you Thantos except for the "on most compilers" part. NULL is a null pointer constant, something defined by C to be a constant expression with the value 0, or such an expression cast to void *. As we know, 0 is always false, therefore !0 is always true. So "on most compilers" should be changed to "on all compilers that claim to implement C". ;)

Thanks for the clarification. I thought I read that NULL wasn't required to equate to 0.

>I thought I read that NULL wasn't required to equate to 0.
It isn't required to be all bits zero, but the value is always equivalent to 0.

So it could equate to negative zero is what you are saying ex

Code:

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0000 0000 == NULL
1000 0000 == NULL

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That is the only way I can see not all bits being zero and it stil equate to zero

10000000 != -0

>10000000 != -0
He's talking about binary representations of signed chars, but still, 1000 000 is actually -128.

I was trying to put up the actual value but I couldn't connect to the server :)

Is that what Prelude means though?

>Is that what Prelude means though?
Something like that. I'll admit I wasn't being very clear. A null pointer can be any absolute invalid address, so it need not be all bits zero, ie. address 0x000000. The macro NULL on the other hand must evaluate to the value 0.

Quote:

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Originally Posted by linuxdude

Is that what Prelude means though?

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Comparing a pointer to NULL and comparing a pointer to zero are equivalent,
so, if fp is a FILE * , then
if(fp==0) and if(fp==NULL) are testing the same thing

if(fp) (which is equivalent to if(fp!=0)) is the same as if(fp != NULL)

From the Bible

Quote:

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An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.55) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer

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So, 0 is a null pointer constant as are 0L, (void *) 0 etc. etc.

Also look up the documentation on the equality operator (==) 6.9.5.5

Quote:

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If one operand is a pointer and the other is a null pointer constant, the null pointer constant is converted to the type of the pointer....

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