Question

I am parsing a text (css) file using fscanf. The basic goal is simple; I want to pull out anything that matches this pattern:

@import “some/file/somewhere.css”;

So I’m using fscanf, telling it to read and discard everything up to a ‘@’ character and then store everything until it reaches a ‘;’ character. Here’s the function that does this:

char* readDelimitedSectionAsChar(FILE *file)
{
char buffer[4096];

int charsRead;
do
{
    fscanf(file, "%*[^@] %[^;]", buffer, &charsRead);

} while(charsRead == 4095);

char *ptr = buffer;
return ptr;
}

I’ve created a buffer that should be able to hold 4095 characters, as I understand it. However, I’m discovering that this is not the case. If I have a file that contains a matching string that’s long, like this:

@import “some/really/really/really/long/file/path/to/a/file”;

That gets truncated to 31 characters using a buffer of char[4096]. (If I use printf to check the value of buffer, I find that the string is cut short.)

If I increase the buffer size, more of the string is included. I was under the impression that one character takes one byte (though I am aware this is affected by encoding). I am trying to understand what’s going on here.

Ideally, I’d like to be able to set the buffer as large as it needs to be “on the fly” — that is, have fscanf just create a buffer big enough to store the string. Can this be done? (I know of the %as flag for GNU, but this is a Mac application for OS 10.5/10.6 and I’m unsure if that will work on this platform.)

Answer 1

The main problem you have is that you’re returning a pointer to a local buffer on the stack, which is dangling (and so overwritten by the next call you make). You also have a potential buffer overflow.
You mention the ‘a’ option, which would help a lot, but its unfortunately a GNU extension which isn’t generally available.

Second, you have this extra option to scanf, &charsRead which will never be written to as there’s no % for it in the format string. So charsRead will always be random garbage — which means you loop will (probably) just run once, or (rarely) loop forever. Try something like

char* readDelimitedSectionAsChar(FILE *file)
{
    char buffer[4096], term[2] = "", *rv = 0;
    int len = 0;

    fscanf(file, "%*[^@]");
    while (term[0] != ';' && !feof(file)) {
        if (fscanf(file, "%4095[^;]%1[;]", buffer, term) > 0) {
            int read = strlen(buffer);
            rv = rv ? realloc(rv, len+read+1) : malloc(read+1);
            strcpy(rv+len, buffer);
            len += read;
        }
    }
    return rv;
}

This is still broken in that it will misbehave if you run out of memory (which can easily happen if you feed it a huge malformed file with an @ in the beginning and no ;),