We already saw with the mmap function how to do this using the fstat function.

    1   #include <unistd.h>
    2   #include <sys/stat.h>
    3   
    4   int main()
    5   {
    6      struct stat file_stat;
    7   
    8      stat("foo.txt", &file_stat);
    9   
   10      printf("File foo.txt has %u bytes\n", file_stat.st_size);
   11   
   12   
   13   }

The stat function returns -1 for error.

Instead of using the stat function with the file name, there is a function, fstat that can be used if you have already opened the file and have a file descriptor, fd for the file.

    8      fstat(fd, &file_stat);