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Fread() C function

by Adyssa
Tags: fread, function
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Mar16-12, 06:18 AM
P: 188
I'm reading a binary file with fread, and I'm getting some strange results. This is from the man page:

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

The function fread() reads nmemb elements of data, each size bytes long, from the stream pointed to by stream, storing them at the location given by ptr.

I understand it to read (nmemb * size) bytes from the file and I'm trying to use it like this:

fread(record_array, num_records, sizeof(record), myfile)

and I'm expecting to read (num_records * sizeof(record)) bytes, call it 10 * 50 = 500.

However, when I do this, it only reads sizeof(record) (50) bytes. If I call it like this it works fine (many elements of size 1 byte each):

fread(record_array, 1, (num_records * sizeof(record)), myfile)

so it seems to ignore the 2nd function parameter. Weird? It's not killing me, but I'm not sure why the size_t size variable is redundant?
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Mar16-12, 06:54 AM
P: 15,166
You are calling fread with the size and nmemb arguments reversed. Your call should be fread (record_array, sizeof(record), num_records, myfile).

The return value from fread is the number of records that were read, not the number of bytes.
Mar16-12, 08:03 AM
P: 188
Oooh I missed that, thanks. Also, they were reversed /facepalm. =S

It's been a long day! :)

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