How does fread know when the file is over in C?

Question

So I'm not entirely sure how to use fread. I have a binary file in little-endian that I need to convert to big-endian, and I don't know how to read the file. Here is what I have so far:

FILE *in_file=fopen(filename, "rb");
char buffer[4];
while(in_file!=EOF){
    fread(buffer, 4, 1, in_file);
    //convert to big-endian.
    //write to output file.
}

I haven't written anything else yet, but I'm just not sure how to get fread to 'progress', so to speak. Any help would be appreciated.

Solution

That's not how you properly read from a file in C.

fread returns a size_t representing the number of elements read successfully.

FILE* file = fopen(filename, "rb");
char buffer[4];

if (file) {
    /* File was opened successfully. */
    
    /* Attempt to read */
    while (fread(buffer, sizeof *buffer, 4, file) == 4) {
        /* byte swap here */
    }

    fclose(file);
}

As you can see, the above code would stop reading as soon as fread extracts anything other than 4 elements.

Comments

[autistic]

I suggest while (fread(buffer, 1, 4, file) == 4) { ... } in order to ensure that 4 bytes are read and avoid the undefined behaviour of using uninitialised values.

[user123]

Thanks for bringing that up. I just realized that I should be using 1 for the size and 4 for the count. Also, awesome name bro!

[autistic]

Indeed. I hadn't noticed that, however. My comment was in regards to explicitly comparing the return value to 4, rather than 0, because if fread were to return 3, 2 or 1 then there would be uninitialised bytes in buffer, which would result in undefined behaviour if those values were used.

[space_mill]

but what if the file has say 4 characters?