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clinuxstdio

fread() parameters 2 and 3

What is the difference between:

fread( buf, sizeof buf, 1, file ); // 'file' is valid open 'FILE *'

and

fread( buf, 1, sizeof buf, file );

Parameters 2 and 3 are size_t size, size_t nmemb described as "fread() reads nmemb items of data, each size bytes long". I think the final number of bytes read must be size * nmemb but for some reason only second syntax (with size=1) is working for me.

Solution

  • The difference is in the following two:

    1. The return value is the number of elements which were actually read (zero up to nmemb). This is different in your two statement, if they succeed.

    2. In the first case:

      fread( buf, sizeof(buf), 1, file )

      You will get all or nothing; fread cannot read something that is smaller than sizeof(buf) in case file is smaller.

      In the second case:

      fread( buf, 1, sizeof(buf), file );

      You allow fread to read as much bytes as possible, even if less than sizeof(buf) are available in the file. The number of elements (bytes in this case) actually read will be returned.