typedef void *Pgm;

Pgm read_pgm(char *);   /* read a pgm file from the named file (or standard
                           input if NULL */

Pgm empty_pgm(int, int); /* Create a pgm with the given number of rows and
                            columns.  Make all of the pixels zero. */

int write_pgm(Pgm, char *); /* Write the pgm into the named file, (or standard
                            output if NULL */

int pgm_rows(Pgm);     /* return the number of rows in the PGM */
int pgm_cols(Pgm);     /* return the number of columns in the PGM */

int *pgm_pixels(Pgm); /* return a pointer to the pixels of the PGM.
                         This should *not* call malloc.  */