CODEWITHSUNDEEP
cpointersstructsegmentation-faultmemcpy
RockAndaHardPlace
RockAndaHardPlace

Reputation: 417

c - Segmentation fault when trying to write a modified structure

This is in a program meant to work with ppm image files.

I'm getting a compilation error when trying to modify a struct and then writing it to a new file.

This is the global struct (declared in ppmIO.c and ppmIO.h):

ppmIO.c:

struct Image *instance;

ppmIO.h:

struct Image
{
  int width;
  int height;
  unsigned char *data;
};

extern struct Image *instance;

This is my imageManip.h file:

#include <ppmIO.h>

void ImageInvert(struct Image **toInvert);

void ImageSwap(struct Image **toSwap);

These are the relevant parts of my imageManip.c file:

#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <ppmIO.h>
#include <imageManip.h>

void ImageInvert(struct Image **toInvert) {


  int i;
  int pix = (*toInvert->width) * (*toInvert->height);

  *toInvert = realloc(*toInvert, 2* sizeof *instance);

  for (i = 0; i < pix; i++)
    {
      *(toInvert)->data = ((unsigned char)255 - *(toInvert)->data));
      *(toInvert)->data = ((unsigned char)255 - *(toInvert)->data++));
      *(toInvert)->data = ((unsigned char)255 - *(toInvert)->data++));
    }

}

void ImageSwap(struct Image **toSwap) {

  int i;
  int pix = (*toSwap)->width * (*toSwap)->height;

  *toSwap = realloc(*toSwap, 2* sizeof *instance);

  unsigned char what = (*toSwap)->data;

  for (i = 0; i < pix-1; i++)
    {
      (*toSwap)->data = (*toSwap)->data++;
      (*toSwap)->data = (*toSwap)->data++;
      (*toSwap)->data = what;
      what = (*toSwap)->data++;
    }

}

This is the ImageWrite method, used to write the modified image structure to a file:

void ImageWrite(char *filename)
{
  int num;
  printf("%d", num);
  int size = (instance->width) * (instance->height) * 3;


  FILE *fp = fopen(filename, "w");

  if (!fp) die("cannot open file for writing\n");

  fprintf(fp, "P6\n%d %d\n%d\n", instance->width, instance->height, 255);

  num = fwrite((void *) instance->data, 1, (size_t) size, fp);

  if (num != size) die("cannot write image data to file\n");

  fclose(fp);
}

This is how I call my modifying functions from main:

  ImageInvert(&instance);
  ImageSwap(&instance);
  ImageWrite(first); //where first is a filename

This is the segmentation fault that gdb reports:

Program received signal SIGSEGV, Segmentation fault.
__mempcpy_sse2 () at ../sysdeps/x86_64/memcpy.S:166
166 ../sysdeps/x86_64/memcpy.S: No such file or directory.

Someone suggested that in my modifying funtions (ImageInvert and ImageSwap), I keep changing the pointer, not the data pointed at. If that is the case, how can I change the data pointed at instead of just the pointer?

Upvotes: 0

Views: 198

Answers (1)

Jonathan Leffler
Jonathan Leffler

Reputation: 754400

As noted in comments to a previous question, the code for ImageInvert() can be made much simpler.

Using subscript notation:

void ImageInvert(struct Image **toInvert)
{
    struct Image *image = *toInvert;
    int n_colour_vals = (image->width * image->height) * 3;
    unsigned char *data = image->data;

    for (int i = 0; i < n_colour_vals; i++)
        data[i] = 255 - data[i];
}

Using pointers:

void ImageInvert(struct Image **toInvert)
{
    struct Image *image = *toInvert;
    unsigned char *data = image->data;
    unsigned char *end = data + (image->width * image->height) * 3;

    while (data < end)
    {
        *data = 255 - *data;
        data++;
    }
}

Equivalent changes can probably be made to imageSwap(), though I am not entirely sure what it is supposed to be doing. There were problems in the previous question that might have suggested that the reallocation was a good idea, but (at least for ImageInvert()), that really isn't needed.

Upvotes: 1

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