Question

When I run this code to load a jpeg file I get a crash in jpeg_read_scanlines
I’m using windows 7 64 bit with VC++ 2010

The image I’m loading is a 100×75 jpg image.

If you need any more details just ask

The crash message is:
Unhandled exception at 0x012db29e in LibTest.exe: 0xC0000005: Access violation writing location 0xcdcdcdcd.

void JPG_Load (const char *path, image_t *img) 
{
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
int infile;
JSAMPARRAY buffer;  
int row_stride;     
unsigned char *out;

infile = fopen(path,"rb");
if (infile == 0) {
    memset (img, 0, sizeof(image_t));
    return;
}

cinfo.err = jpeg_std_error(&jerr);

jpeg_create_decompress(&cinfo);
jpeg_stdio_src(&cinfo, (FILE *)infile);
jpeg_read_header(&cinfo, TRUE);
jpeg_start_decompress(&cinfo);
row_stride = cinfo.output_width * cinfo.output_components;
out = malloc(cinfo.output_width*cinfo.output_height*cinfo.output_components);

img->pixels = out;
img->width = cinfo.output_width;
img->height = cinfo.output_height;
img->bytesPerPixel = cinfo.out_color_components;

while (cinfo.output_scanline < cinfo.output_height) {
    buffer = (JSAMPARRAY)out+(row_stride*cinfo.output_scanline);
    jpeg_read_scanlines(&cinfo, buffer, 1);
}

jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);

fclose(infile);
}

image_t is defined as:

typedef struct {
int width;
int height;
int bytesPerPixel;
byte *pixels;
} image_t;

Answer 1

Don’t do this.

buffer = (JSAMPARRAY)out+(row_stride*cinfo.output_scanline); // WRONG

You are casting to JSAMPARRAY, which is basically void **. The result is garbage, since that’s not the kind of data you have: you have an array of bytes.

The jpeg_read_scanlines function, if you look at the documentation, does not take a pointer to your buffer. It takes a pointer to an array of scanlines, and each scanline is a pointer to row data.

while (cinfo.output_scanline < cinfo.output_height) {
    unsigned char *rowp[1];
    rowp[0] = (unsigned char *) out + row_stride * cinfo.output_scanline;
    jpeg_read_scanlines(&cinfo, rowp, 1);
}

Recommendation: Adding a cast to fix a compiler error only works if you know the cast is correct. Don’t cast to any type unless you know what the type is.