Reading ppm files and using fscanf()

Question

I'm trying to parse through a ppm file, but first need to verify if the header info is correct. A ppm file may have the following formats:

P3
100 100
255
data...

or

p3
100 100
255
data...

I'm using fscanf (file_stream, "P3 %d %d %d", &width, &height, &max_colour); to verify the header info. What I'd like to know is, how to move on to reading the data (char by char) after verifying the header info.

Answer 1

Assuming the header tells you the size of the data then allocate a block of memory that is large enough and use fread() to read it in a single call - this is MUCH faster than reading a byte at a time.

  unsigned char *data = malloc(width*height); // or whaterver size
  fread(file_stream,width*height,1,data);

Answer 2

Add a %*[\n] to the end of your fscanf string to eat the last newline in the header, then you can use fread to read raw bytes from the remainder of the file (assuming you opened it in binary mode).

Answer 3

Is there some reason not to use the netpbm library?

Answer 4

Using fscanf you can read a char with "%c".

char ch;
while (fscanf(file_stream, "%c", &ch) == 1) {
    /* process ch */
}

But instead of fscanf you can use fgetc()

int ch;
while ((ch = fgetc(file_stream)) != EOF) {
    /* process ch */
}

But, assuming a ppm file with ASCII encoding (P1, P2, or P3), fscanf is a really good option.

/* P3 format */
if (fscanf(file_stream, "%d%d%d", &red, &green, &blue) == 3) {
    /* RGB triplet read; process it */
}

Remember to open your file in binary mode if you want to deal with binary PPMs

fopen(filename, "rb");