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Fread() C function 
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#1
Mar1612, 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? 


#2
Mar1612, 06:54 AM

Mentor
P: 15,204

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. 


#3
Mar1612, 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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