Чтение и сохранение изображений PGM

#include <stdio.h>
#include <stdlib.h>
 
typedef unsigned char GSI_Byte;
 
typedef struct GrayScaleImage {
    GSI_Byte *bitmap;
    size_t width;
    size_t height;
} GrayScaleImage;
 
void
gsi_initialize(GrayScaleImage *image_ptr, size_t width, size_t height) {
    image_ptr->width = width;
    image_ptr->height = height;
    if(width != 0 && height != 0)
        image_ptr->bitmap = malloc(width * height * sizeof(GSI_Byte));
    else
        image_ptr->bitmap  = NULL;
}
 
void
gsi_fill(GrayScaleImage *image_ptr, GSI_Byte value) {
    for(int i = image_ptr->width * image_ptr->height - 1; i >= 0; i--) {
        *(image_ptr->bitmap + i) = value;
    }
}
 
GSI_Byte *
gsi_get_pixel_ptr(const GrayScaleImage *image_cptr,  size_t x, size_t y) {
    return image_cptr->bitmap + image_cptr->width * y + x;
}
 
void
gsi_free(GrayScaleImage *image_ptr) {
    if(image_ptr->width !=0 && image_ptr->height != 0) {
        image_ptr->width = 0;
        image_ptr->height = 0;
        free(image_ptr->bitmap);
        image_ptr->bitmap = NULL;
    }
}
 
void
gsi_write_PGM(const GrayScaleImage* image_cptr, const char *filename) {
    FILE *f = fopen(filename, "w");
    fprintf(f, "P5\n");
    fprintf(f,"%lu %lu\n", image_cptr->width, image_cptr->height);
    fprintf(f, "%d\n", 255);
    for(int y = image_cptr->height - 1; y >= 0; y--) {
        for(int x =  0; x < image_cptr->width; x++)
            fputc(*gsi_get_pixel_ptr(image_cptr, x, y), f);
    }
    fclose(f);
}
 
void gsi_read_PGM(GrayScaleImage* image_ptr, const char *filename) {
    FILE *f = fopen(filename, "r");
    if(!f) return;
    getc(f);
    getc(f);
    size_t width = 0, height = 0;
    fscanf(f, "%lu %lu\n", &width, &height);
    int num = 0;
    fscanf(f, "%d\n", &num);
    gsi_initialize(image_ptr, width, height);
    for(int y = height - 1; y >= 0; y--) {
        for(int x =  0; x < width; x++) {
            *gsi_get_pixel_ptr(image_ptr, x, y) = fgetc(f);
        }
    }
    fclose(f);
}
 
 
int main(void)
{
    GrayScaleImage image;
    gsi_initialize(&image, 500, 300);
    gsi_fill(&image, 120);
    gsi_write_PGM(&image, "gray.pgm");
    gsi_free(&image);
    gsi_read_PGM(&image, "gray.pgm");
    *gsi_get_pixel_ptr(&image, 100, 50) = 255; // Поставим точку белым
    gsi_write_PGM(&image, "test_with_point.pgm");
    gsi_free(&image);
    return 0;
}