Error reading .jpeg file using libjpeg - c++

Question

I am getting NULL pointer exception in the function jpeg_read_header() in following code while the input image file is successfully read by FILE object.
#include "jpeglib.h"
#include <iostream>
using namespace std;
void decode_frame(char *filename)
{
unsigned char* raw_image;
JSAMPROW row_pointer[1];
unsigned long location = 0;
struct jpeg_error_mgr jerr;
struct jpeg_decompress_struct cinfo ;
FILE *infile = fopen(filename, "rb" );
if (infile == NULL )
{
printf("Error opening jpeg file %s\n!", filename );
return -1;
}
cinfo.err = jpeg_std_error(&jerr);
/* create decompressor */
jpeg_create_decompress(&cinfo);
/* this makes the library read from infile */
jpeg_stdio_src(&cinfo, infile );
/* read jpeg header */
jpeg_read_header(&cinfo, TRUE);
/* decompress */
jpeg_start_decompress(&cinfo);
/*allocate memory */
raw_image = (unsigned char*)malloc( cinfo.output_width*cinfo.output_height*cinfo.num_components );
/* now actually read the jpeg into the raw buffer */
row_pointer[0] = (unsigned char *)malloc( cinfo.output_width*cinfo.num_components );
/* read scanlines */
while (cinfo.output_scanline < cinfo.output_height) {
jpeg_read_scanlines( &cinfo, row_pointer, 1 );
for( int i=0; i < cinfo.image_width*cinfo.num_components;i++)
raw_image[location++] = row_pointer[0][i];
}
/* clean up */
jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);
fclose( infile );
free( row_pointer[0] );
}
int main(){
char *f = "Example.jpg";
decode_frame(f);
return 0;
}
The value of cinfo alfter jpeg_stdio_src(&cinfo, infile ) function call is :
cinfo{err=0x0020f8fc mem=0x021cb320 progress=0x00000000 ...}
progress 0x00000000 {progress_monitor=??? pass_counter=??? pass_limit=??? ...}
client_data 0xcccccccc void *
is_decompressor 1
global_state 200
src 0x021c4480 {next_input_byte=0x00000000 <Bad Ptr> bytes_in_buffer=0 init_source=0x686ccb50 ...}
image_width 0
image_height 0
num_components 0
jpeg_color_space JCS_UNKNOWN
out_color_space JCS_UNKNOWN
scale_num 0
scale_denom 0
output_gamma 0.00000000000000000
buffered_image 0
raw_data_out 0
dct_method JDCT_ISLOW
Where I am getting wrong? This is the first time I am using image library.

Answer 1

The global state 200 means (according to libjpeg source code):
#define DSTATE_START 200 /* after create_decompress */
Calling jpeg_stdio_src has only for effect to prepare the decompressor structure, i.e it allocates the internal cinfo->src buffer and initialize other decompressor state.
In other words the JPEG file parsing has not yet started: so you have no problem here.
You need at least to execute jpeg_read_header to make sure the cinfo structure is filled with all metadata info (image_width, image_height, jpeg_color_space, etc). If something goes wrong at that step, this might be related to your JPEG file that might be broken (?).
Do you see something like Not a JPEG file printed on stderr?