fread from binary returns the same regardless of flags

Question

First I have this:

int main (void)
{
   int m = 10;
   double x[3] = {1.5, -3.5, 3.25};

   int n1, n2; FILE *izTok;
   izTok = fopen ("podaci.bin", "wb");

   n1 = fwrite (&m, sizeof(m), 1, izTok);
   n2 = fwrite (x, sizeof(x[0]), 3, izTok);
   fclose(izTok);

   return 0;
}

And after that I try to read from it with

   FILE *stream;
   stream = fopen("podaci.bin", "r");

   n1 = fread(&n, sizeof(n), 1, stream);
   n2 = fread(arr, sizeof(arr[0]), 3, stream);

   printf("%d %f %f %f", n, arr[0], arr[1], arr[2]);

And regardless of whether I put

 stream = fopen("podaci.bin", "r");

or

 stream = fopen("podaci.bin", "rb");

the output is the same

 10 1.500000 -3.500000 3.250000

Whats the point of the flags if it does the same thing both times?

score 3 · Accepted Answer · answered Jan 29 '14 at 15:07

On all POSIX systems, the b flag is ignored and has no meaning. From man 3 fopen on a Linux system:

The mode string can also include the letter 'b' either as a last character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with C89 and has no effect; the 'b' is ignored on all POSIX conforming systems, including Linux. (Other systems may treat text files and binary files differently, and adding the 'b' may be a good idea if you do I/O to a binary file and expect that your program may be ported to non-UNIX environments.)

On Windows, b means that the data is read unaltered. Otherwise, Text mode is engaged, which is described here. Among others, line endings are transformed and CTRL+Z is interpreted as end-of-file.

score 1 · Answer 2 · answered Jan 29 '14 at 15:08

The difference between text and binary depends on your system. Many of the *nixes make a point that on those systems, there is no difference between them. Windows will, in text mode, convert between "\n" and "\r\n" (adding a byte when writing and subtracting when reading). If you don't happen to have those bytes in your binary data, then you won't notice a difference there either.

score 1 · Answer 3 · answered Jan 29 '14 at 15:10

The point of the flags is that some Operating Systems treat text and binary streams differently. Yours may not. But it still may: if the double values as binary values contain a byte with value '\r' you may be surprised to find that the numbers you read depend on whether you used "r" or "rb" to open the file.

fread from binary returns the same regardless of flags

3 Answers3