How to check if there is a `\0` character in a filename using C?

Question

I'd like to write a function like this:

int validate_file_name(char *filename)
{
    //...
}

which will:

• return 1 if there was no \0 character in the filename,
• 0 otherwise.

I thought it may be achieved using a simple for(size_t i = 0; i < strlen(filename); i++), but I don't know how to determine how much characters I've got to check?

I can't use strlen() because it will terminate on the first occurrence of a \0 character.

How should I approach this problem?

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Clarification:

I am trying to apply these guidelines to a filename I receive. If you should avoid putting a \0 in a filename, how could you validate this if you've got no size parameter.

Moreover, there are strings with multiple \0 characters, like here: http://www.gnu.org/software/libc/manual/html_mono/libc.html#Argz-and-Envz-Vectors. Still, I had no idea that it is impossible to determine their length if it is not explicitly provided.

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Conclusion:

There is no way you can determine the length of string which is not NULL-terminated. Unless you know the length of course or you deploy some dirty hacks: Checking if a pointer is allocated memory or not.

Answer 1

You are trying to solve a problem that does not need to be solved.

A file name is a string. In C, a "string" is by definition "a contiguous sequence of characters terminated by and including the first null character".

It is impossible to have a string or a file name with a null character embedded in it.

It's possible to have a sequence of characters with an embedded null character. For example:

char buf[] = "foo\0bar.txt";

buf is an array of 12 characters; the characters at positions 3 and 11 are both null characters. If you treat buf as a string, for example by calling

fopen(buf, "r")

it will be treated as a string with a length of 3 (the length of a string does not include the terminating null character).

If you're working with character arrays that may or may not contain strings, then it makes sense to do what you're asking. You would need to keep track of the size of the buffer separately from the address of the initial character, either by passing an additional argument or by wrapping the pointer and the length in a structure.

But if you're dealing with file names, it's almost certainly best just to deal with strings and assume that whatever char* value is passed to your function points to a valid string. If it doesn't (if there is no null character anywhere in the array), that's the caller's fault, and not something you can reasonably check.

(Incidentally, Unix/Linux file systems explicitly forbid null characters in file names. The / character is also forbidden, because it's used as a directory name delimiter. Windows file systems have even stricter rules.)

One last point: NULL is (a macro that expands to) a null pointer constant. Please don't use the term NULL to refer to the null character '\0'.

Answer 2

The answer is that you can't write a function that does that if you don't know the length of the string.

To determine the length of the string strlen() searches for the '\0' character which if is not present will cause undefined behavior.

If you knew the length of the string then,

for (int i = 0 ; i < length ; ++i)
 {
    if (string[i] != '\0')
        continue;
    return 1;
 }
return 0;

would work, if you don't know the length of the string then the condition would be

for (int i = 0 ; string[i] != '\0' ; ++i)

which obviously means that then searching for the '\0' makes no sense because it's presence is what makes all other string related functions to work properly.

Answer 3

If the string is not NULL-terminated, what else it is terminated by? And if you don't know that, what is it length? If you know the answer to these problems, you know the answer to your question.