fread and fgetc in the same file stream not working

Question

I have a binary file with the following repeating format: 6 float values + 3 unsigned char (byte`, integer value from 0 to 255) values.

I am parsing it like this:

FILE *file = fopen("file.bin", "r");
bool valid = true;

while(!feof(file)) {

  float vals[6];
  valid = valid && (fread((void*)(&vals), sizeof(float), 6, file) == 6);
  unsigned char a,b,c;
  a = fgetc(file); b = fgetc(file); c = fgetc(file); 

  (...)

}

This works fine for the first 30 iterations or so, but after that it simply stops parsing (way way before the end of the file).

What could be wrong?

I also tried parsing the unsigned char bytes with

fread((void*)&(a), sizeof(unsigned char), 1, file);

Make it `valid = (fread((void*)(&vals), 1, 6 * sizeof(float), file) == 6 * sizeof(float));` and(!) check that `valid` — , Apr 29 '15 at 19:02
@AndyG this is using C syntax but I am compiling it as C++ code — manatttta, Apr 29 '15 at 19:03
[Why is “while ( !feof (file) )” always wrong?](http://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — Eugene Sh., Apr 29 '15 at 19:06
@manatttta: Regarding your latest edit, what is `(...)`? Please, for the love of programming create a [MCVE](http://stackoverflow.com/help/mcve) — AndyG, Apr 29 '15 at 19:21
Try opening in binary `fopen("file.bin", "rb")` so that characters equal to the ASCII EOF (aka 26 aka 0x1A aka Ctrl-Z) are not treated as the end of the file. — Ben Voigt, Apr 29 '15 at 19:23

Ben Voigt · Accepted Answer · 2015-04-29T19:57:57.670

it simply stops parsing (way way before the end of the file).

You and the C Standard library are having a difference of opinion about where the end of the file is. ASCII character EOF (for DOS/Windows: decimal 26, hex 1A, aka Ctrl+Z, for Unix/Linux: decimal 4, hex 04, aka Ctrl+D) is a control character meaning "end of file". There's also the file length stored by the filesystem metadata.

The C stdio functions can operate in several modes: text, default, binary, and these control several behaviors:

Newline translations (implementation-defined): enabled in text mode, disabled in binary mode, default: ???
End of file: Implementation-defined, but usually EOF character in text mode, by filesystem file length in binary mode, default: ???

Since your file contains binary data, you should force binary mode by using "b" in the mode string to fopen, e.g.

FILE* file = fopen("file.bin", "rb");

When you do so, characters with value 26 are treated like any other byte and lose their "EOF" meaning.

fread and fgetc in the same file stream not working

1 Answers1