Question

I’m using C and I want to read from a binaryFile.

I know that it is contain strings in the following way: Length of a string, the string itself, the length of a string, string itself, and so on…

I want to count the number of times which the string Str appears in the binary file.

So I want to do something like this:

int N;
while (!feof(file)){
    if (fread(&N, sizeof(int), 1, file)==1)
        ...

Now I need to get the string itself. I know it’s length. Should I do a ‘for’
loop and get with fgetc char by char? I know I’m not allowed to use fscanf since
it’s not a text file, but can I use fgetc? And would I get what I’m expecting for
my string? (To use dynamic allocation for char* for it with the size of the length
and use strcpy to add it to the current string?)

Answer 1

You should probably loop on:

while (fread(&N, sizeof(int), 1, file) == 1)
{
    // Check N for sanity
    char *buffer = malloc(N+1);
    // Check malloc succeeded
    if (fread(buffer, N, 1, file) != 1)
        ...process error...
    buffer[N] = '\0';  // Null terminate for sanity's sake
    ...store buffer (the pointer) for later processing so you aren't leaking...
    ...or free it if you won't need it later...
}

You could use getc() or fgetc() in a loop; that would work. However, the direct fread() is much simpler (and is coded as if it uses getc() in a loop).

You might want to do some sanity checking on N before blindly using it with malloc(). In particular, negative values are likely to lead to much unhappiness.

The file format as written is tied to one class of machine — either big-endian or little-endian, and with the fixed size of int (probably 32-bits). Writing more portable data is slightly fiddlier, but eminently doable — but probably not relevant to you just yet.

Using feof() is seldom the correct way to test for whether to continue with a loop. Indeed, there is not often a need to use feof() in code. When it is used, it is because an I/O operation ‘failed’ and you need to disambiguate between ‘it was not an error — just EOF’ and ‘there was some sort of error on the device’.