How to fopen() a .IMA file?

Question

I'm working in C and trying to read a file system image (eg: name.IMA) and I need it in binary form because I need to read it byte by byte. What is the correct way to read these type of files in C? The below does not work:

FILE *filePointer = fopen("name.IMA","rb");
fread(buffer, fileLength, 1, filePointer);

MAIN PROBLEM: The fread() is actually opening and reading as expected. The confusion is because of the return value given by fread() is for some reason equal to 1 (even though way more than 1 byte was read). What's the issue here?

What is `fileLength` initialized to? How did you choose that value? Did you test the value returned by `fopen()`? The one returned by `fread()`? — Jonathan Leffler, Nov 17 '15 at 06:24

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

2

If your fread line is not doing what you expected, I'd be looking at whatever fileLength is set to. If it's more bytes than can be read, then the fread will return zero.

One thing you need to keep in mind is that fread() returns the number of items read, not the number of bytes (my emphasis):

size_t fread (void *ptr, size_t size, size_t nmemb, FILE *stream);

On success, fread() return the number of items read. This number equals the number of bytes transferred only when size is 1.

So there is a difference between these two:

size_t n = fread (buffer, s, 1, fp);
size_t n = fread (buffer, 1, s, fp);

The first is reading up to one element of size s so will only ever return zero or one. The second is reading up to s elements of size one so can return a value of 0..s inclusive.

The latter is the one you want to use if you want to read as many bytes at a time as you can fit in a buffer:

char buff[128];
size_t sz;
FILE *fp = fopen ("name.ima", "rb");
if (fp != NULL) {
    while ((sz = fread (buff, 1, sizeof (buff), fp)) > 0)
        doSomethingWith (buffer, sz);
    fclose (fp);
}

edited Jun 20 '20 at 09:12

Community

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answered Nov 17 '15 at 06:23

paxdiablo

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Okay, so here's the situation. The fread() is actually opening and reading as expected. The confusion is because of the return value given by fread() is for some reason 1 (even though way more than 1 byte was read). What's the issue here? – Vimzy Nov 17 '15 at 06:31
1

@Vimzy, you have specified to read *one* element of size `fileLength`. `fread()` returns the number of *elements* read, not the number of bytes. If you swap the arguments thus `fread(buffer, 1, fileLength, filePointer)`, you are asking to read `fileLength` elements of size 1, and you'll get the number of bytes read. – paxdiablo Nov 17 '15 at 06:34
One last clarification, does the fread() insert a null terminator at the end of the buffer? The buffer is a pointer of ints (made using malloc)? – Vimzy Nov 17 '15 at 07:28
@Vimzy, no, it reads the data from the file exactly as is, adding nothing to the end. – paxdiablo Nov 17 '15 at 07:40

score 0 · Answer 2 · answered Nov 17 '15 at 06:38

0

Your fread returns 1 because it is asked to read only one element

size_t fread ( void * ptr, size_t size, size_t count, FILE * stream );

In your case, you are setting the count to be 1. That's why its reading only one element i.e. one byte

Moreover you are setting the size to be fileLength, which is wrong. size represents the size of each element.

answered Nov 17 '15 at 06:38

veda

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Just one nitpick: an element is only a byte when the size is 1 which, as you state in the following text, is not the case. Otherwise it will be multiple bytes long. Actually two nitpicks since it's perfectly acceptable to have an element the same size as the file if you want to just atomically load the entire file. There are cases where you should load elements individually but I don't think this is one of them. – paxdiablo Nov 17 '15 at 06:50

How to fopen() a .IMA file?

2 Answers2