c - segmentation fault assigning to 2d array - c

Question

I'm trying to take a bmp file and make a grayscale copy. I'm learning about dynamic allocation and I have to dynamically allocate a 2D array which I import the bmp file to and manipulate it from, so it will work with various picture sizes. But near the end (I labeled where) I get a Seg Fault and I have no idea why. Everything works great if I don't dynamically allocate "pixels".
#include <stdio.h>
#include <stdlib.h>
int main(void) {
const int HEADER_SIZE = 54;
FILE *infile = fopen("test1.bmp", "rb");
FILE *outfile1 = fopen("copy1.bmp", "wb");
int i, width, height, r, c, bmpsize;
char header[HEADER_SIZE], filename[32];
char **pixels; //initialize pixels here
puts("Enter the filename: ");
i = -1;
while(filename[i] != '\n')
{
i++;
scanf("%c", &filename[i]);
}
filename[i] = '.';
filename[i+1] = 'b';
filename[i+2] = 'm';
filename[i+3] = 'p';
filename[i+4] = '\0';
i = -1;
while(filename[i] != '\0')
{
i++;
printf("%c", filename[i]);
}
infile = fopen(filename, "rb");
puts("Enter the height and width (in pixels): ");
scanf("%d%d", &height, &width);
bmpsize = 3 * width * height;
pixels = malloc(height * sizeof(char*)); //DA part 1
for(i = 0; i < height; i++)
{
pixels[i] = malloc((width * 3) * sizeof(char)); //DA part 2
}
fread(header, 1 , HEADER_SIZE, infile);
fread(pixels, 1 , bmpsize, infile);
for( r = 0; r < height; r++) {
for ( c = 0; c < width*3; c += 3) {
int avg= 0;
puts("THIS PUTS PRINTS: THE NEXT LINE IS MY PROBLEM");
avg = ((int) pixels[r][c] + (int) pixels[r][c+1] + (int) pixels[r][c+2]) / 3;//This is my problem line, why?
puts("THIS PUTS DOESN'T PRINT. ERROR: SEG. FAULT(core dumped)");
pixels[r][c] = (char) avg;
pixels[r][c+1] = (char) avg;
pixels[r][c+2] = (char) avg;
}
}
puts("Done. Check the generated images.");
fwrite(header, sizeof(char) , HEADER_SIZE, outfile1);
fwrite(pixels, sizeof(char) , bmpsize, outfile1);
fclose(infile);
fclose(outfile1);
return 0;
}

Answer 1

I think the problem is with your fread call and how you are setting up the input buffer for the file contents
pixels = malloc(height * sizeof(char*)); //DA part 1
for(i = 0; i < height; i++)
{
pixels[i] = malloc((width * 3) * sizeof(char)); //DA part 2
}
fread(pixels, 1 , bmpsize, infile);
The memory you have allocated in total is bmpsize bytes but the length of pixels is just height bytes - and yet you are passing it to fread as if it were a buffer of bmpsize bytes in length. Each element in pixels is a char* - the address of each is block of dynamically allocated array but this does not mean that you can treat your pixels array as a contiguous block of memory.
These arrays allocated dynamically in your loops are thus not initialised which could be leading to the segfaults when you read from them later in your loop (reading uninitialised variables is undefined behaviour).
This probably explains why your code works when you use a non dynamically allocated 2D array - because such a 2D array is a contiguous in memory.

Answer 2

The answer of mathematician1975 is almost correct, since you are accessing data from outside of the area you've allocated, but the last index of c in the inner loop is not 3*width-1 nor 3*width-3, but it's actually width*3.
You should adapt the inner loop to:
for ( c = 0; c < width*3-3; c += 3)
This way c will go until width*3-3 and then you can do c, c+1 and c+2 and you will visit the last bytes of the lines with c+2.