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

c file-io fread
user202925 picture user202925 · Mar 29, 2013 · Viewed 62.8k times · Source

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.

Answer

user123 picture user123 · Mar 29, 2013

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, 1, 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.

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