Calculate a file length. The file need to be in `rb` mode (reading and binary mode)

flen.c

#include <stdio.h>

int flen(FILE *f)
{
  int len;
  int origin = ftell(f);

  fseek(f, 0, SEEK_END);
  len = ftell(f);

  fseek(f, origin, SEEK_SET);

  return len;
}

you can replace fseek(f, origin, SEEK_SET) by rewind(f) do the same as getting origin and setting it back to the origin but it's in one function and in stdio.h

so no more need to use origin variable and it make the code shorter

You can't do this, rewind doesn't save the origin, it literally does fseek(f, 0, SEEK_SET), which doesn't really "rewind" the cursor