Reading bytes of PPM into a flexible member array in a struct

Question

I would like to read through the bytes of a ppm image and store it in my flexible member array, which is contained in a structure. I hope I didn't mess up the allocation or something. This is how it looks now:

typedef struct ppm {
    unsigned xsize;
    unsigned ysize;
    char data[];
} PPMImage;

int main(void)
{
    int c = 0;
    unsigned int rgb = 0;
    char arr[2];
    FILE *handle;
    PPMImage img;

    if((handle = fopen(filename, "rb")) == NULL)
        return 1;

    fscanf(handle, "%c%c", &arr[0], &arr[1]); // scanning width and height

    if(arr[0] != 'P' || arr[1] != '6')
        // error handling...

    c = getc(handle);
    while(c == '#') // getting rid of comments
    {
            while(getc(handle) != '\n');
            c = getc(handle);
    }
    ungetc(c, handle);
    if(fscanf(handle, "%u %u", &img.xsize, &img.ysize) != 2)
        // error handling...

    if(fscanf(handle, "%u", &rgb) != 1)
        // error handling...

    PPMImage *data = (PPMImage *)malloc(RANGE);

    if(fread(data, 3 * img.xsize, img.ysize, handle) != img.ysize)
        // error handling...

    for(int i = 0; i < RANGE; i++)
        printf("%c\n", data[i]); // ERROR POINT

    return 0;
}

I guess I can't figure out where the data will be saved or if the arguments of fread are right.. any ideas? This is the output:

warning: format ‘%c’ expects argument of type ‘int’, but argument 2 has type ‘PPMImage’ [-Wformat]

[Please don't cast the return value of `malloc()` in C](http://stackoverflow.com/a/605858/28169). — unwind, Mar 25 '14 at 10:03
I certainly think so, click the comment for the reasoning (if that wasn't obvious). — unwind, Mar 25 '14 at 10:05

Ferenc Deak · Accepted Answer · 2014-03-25T10:09:28.657

2

so, PPMImage *data = (PPMImage *)malloc(RANGE); creates a new local variable, of type PPMImage (a structure!) and is not accessing the img.data which I think you want ...

Edit to answer the question in the comment

modify struct ppm to have a pointer to a char:

typedef struct ppm {
    unsigned xsize;
    unsigned ysize;
    char* data;
} PPMImage;

and then (assuming there is a byte matrix with R,G,B):

img.data = malloc(3 * img.xsize * img.ysize);

// do error checking ...

and then

fread(img.data, 3 * img.xsize, img.ysize, handle)

// do error checking ...

edited Mar 25 '14 at 10:09

answered Mar 25 '14 at 10:02

Ferenc Deak

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So if I want to store it in img.data, what would I have to do? – imre Mar 25 '14 at 10:04
I need to keep the char data[]; in the struct, I can't change it (it has been assigned). Is that a problem? – imre Mar 25 '14 at 10:12
Also, how do I free that? `free(img.data)`? – imre Mar 25 '14 at 10:13
You need the char*, you cannot dynamically allocate an array of unknown size. And yes, just free(img.data) – Ferenc Deak Mar 25 '14 at 10:17
So the flexible array member is impossible to work with at this point? Even without allocation? I'll try it with the char. – imre Mar 25 '14 at 10:20

Reading bytes of PPM into a flexible member array in a struct

1 Answers1