write a jpeg with libjpeg (seg fault)

问题:

Trying to write a jpeg file from some raw data using libjpeg.

It triggers a Segmentation Fault in jpeg_start_compress()

Here is the relevant part of the code :

void write_sub_image(char *filename, int start, int end)
{
    struct jpeg_compress_struct cinfo;
    unsigned char *stride;
    JSAMPROW row_pointer[1];
    unsigned long new_width = end-start;
    int i;
    FILE *fp;

    stride = (unsigned char *)malloc( new_width * 3);

    fp = fopen(filename, "w+");

    jpeg_create_compress(&cinfo);

    jpeg_stdio_dest(&cinfo, fp);

    cinfo.image_width = new_width;
    cinfo.image_height = height;
    cinfo.input_components = 3;
    cinfo.in_color_space = JCS_RGB;

    jpeg_set_defaults(&cinfo);

    jpeg_start_compress(&cinfo, FALSE);

    for (i=0; i<height; i++) {
        memcpy (stride, image + (start + i * width) * 3, new_width * 3);
        row_pointer[0] = stride;
        jpeg_write_scanlines(&cinfo, &stride, 1);
    }

    jpeg_finish_compress(&cinfo);
    jpeg_destroy_compress(&cinfo);

    fclose(fp);
}

The problem is not with the memcpy, it does not even get to the for loop... just crash at _start_compress.

In case that is relevant, the system is Ubuntu 10.10.

回答1:

You need to set an error manager:

struct jpeg_error_mgr jerr; 
....
cinfo.err = jpeg_std_error(&jerr); 
jpeg_set_defaults(&cinfo);
....