MagickConstituteImage() segfaults in C - c

Question

I am trying to create an image from a float * array. However, MagickConstituteImage constantly segfaults. Here is a summary of the relevant code (the problem occurs in the function writeRawImage()):
#include <stdio.h>
#include <stdlib.h>
#include <wand/MagickWand.h>
#define COLOR_MAX 65535.0
typedef struct {
float **px;
unsigned long width;
unsigned long height;
} RawPixels;
RawPixels getRawImage (char *path)
{
MagickWand *mw;
MagickBooleanType status;
PixelIterator *iter;
MagickPixelPacket pixel;
PixelWand **pixels;
RawPixels rp;
long x;
long y;
unsigned long width;
unsigned long count;
MagickWandGenesis();
mw = NewMagickWand();
status = MagickReadImage(mw, path);
rp.px = NULL;
if (status == MagickFalse) {
return rp;
}
iter = NewPixelIterator(mw);
if (iter == (PixelIterator *) NULL) {
return rp;
}
rp.width = 0;
rp.height = (unsigned long) MagickGetImageHeight(mw);
count = 0;
for (y = 0; y < rp.height; y++) {
pixels = PixelGetNextIteratorRow(iter, &width);
rp.width = (unsigned long) width;
if (rp.px == NULL) {
rp.px = malloc(sizeof(float *) * (width * rp.height + 1));
}
if (pixels == (PixelWand **) NULL) {
break;
}
for (x = 0; x < (long) width; x++) {
count++;
rp.px[count - 1] = malloc(sizeof(float) * 3);
PixelGetMagickColor(pixels[x], &pixel);
rp.px[count - 1][0] = pixel.red / COLOR_MAX;
rp.px[count - 1][1] = pixel.green / COLOR_MAX;
rp.px[count - 1][2] = pixel.blue / COLOR_MAX;
}
}
rp.px[count] = NULL;
return rp;
}
void freeRawImage (RawPixels rp)
{
for (int i = 0; rp.px[i] != NULL; i++) {
free(rp.px[i]);
}
free(rp.px);
}
void writeRawImage (RawPixels rp, char *path)
{
// This function is the one that gives me a headache.
// Basically, I take float **rp.px, which has the following structure
// at this point:
//
// {
// {float red, float green, float blue},
// {float red, float green, float blue},
// ...
// }
//
// Now, the documentation at https://www.imagemagick.org/api/magick-image.php#MagickConstituteImage
// says the following:
//
// "The pixel data must be in scanline order top-to-bottom"
//
// So up until the call to MagickConstituteImage() I am trying
// to restructure the data and write it into float *scanline
// to satisfy that requirement. However, once I call
// MagickConstituteImage(), my program segfaults.
MagickWand *mw;
float *scanline;
unsigned long pxcount;
for (pxcount = 0; rp.px[pxcount] != NULL; pxcount++);
pxcount *= 3;
scanline = malloc(sizeof(float) * pxcount);
pxcount = 0;
for (int i = 0; rp.px[i] != NULL; i++) {
for (int j = 0; j < 3; j++) {
scanline[pxcount++] = rp.px[i][j];
printf("%f\n", scanline[pxcount - 1]);
}
}
// This function causes a segfault
MagickConstituteImage(mw, rp.width, rp.height, "RGB", FloatPixel, scanline);
free(scanline);
}
int main ()
{
RawPixels rp;
if ((rp = getRawImage("samples/tiny-white.png")).px == NULL) {
fprintf(stderr, "ERROR: Failed to process image\n");
return 1;
}
// Some image processing using raw pixels here
writeRawImage(rp, "previews/preview.jpg");
freeRawImage(rp);
return 0;
}

Answer 1

You're not initializing
MagickWand *mw;
in the function writeRawImage(...) before using it in MagickConstituteImage(...)