Variable has incomplete type 'struct my_error_mgr'

Question

When I try to build this code, the line struct my_error_mgr jerr; gives the error "Variable has incomplete type 'struct my_error_mgr'

#import "Engine.h"
#include "jpeglib.h"

@implementation Engine
+(void)test{
    struct jpeg_decompress_struct cinfo;
    struct my_error_mgr jerr;
    FILE * infile;

    if ((infile = fopen(filename, "rb")) == NULL) {
        fprintf(stderr, "can't open %s\n", filename);
        return 0;
    }

    cinfo.err = jpeg_std_error(&jerr.pub);
    jerr.pub.error_exit = my_error_exit;

    if (setjmp(jerr.setjmp_buffer)) {
        jpeg_destroy_decompress(&cinfo);
        fclose(infile);
        return 0;
    }

    jpeg_create_decompress(&cinfo);

    jpeg_stdio_src(&cinfo, infile);

    (void) jpeg_read_header(&cinfo, TRUE);

    jvirt_barray_ptr* coeffs_array;
    coeffs_array = jpeg_read_coefficients(&cinfo);

    BOOL done = FALSE;
    for (int ci = 0; ci < 3; ci++)
    {
        JBLOCKARRAY buffer_one;
        JCOEFPTR blockptr_one;
        jpeg_component_info* compptr_one;
        compptr_one = cinfo.comp_info + ci;

        for (int by = 0; by < compptr_one->height_in_blocks; by++)
        {
            buffer_one = (cinfo.mem->access_virt_barray)((j_common_ptr)&cinfo, coeffs_array[ci], by, (JDIMENSION)1, FALSE);

            for (int bx = 0; bx < compptr_one->width_in_blocks; bx++)
            {
                blockptr_one = buffer_one[0][bx];

                for (int bi = 0; bi < 64; bi++)
                {
                    blockptr_one[bi]++;
                }                  
            }   
        } 
    }

    write_jpeg(output, &cinfo, coeffs_array); // saving modified JPEG to the output file

    jpeg_destroy_decompress(&cinfo);
    fclose(infile);
}
@end

When I try to comment out all lines related to this error, I then get a EXC_BAD_ACCESS runtime error on the line

(void) jpeg_read_header(&cinfo, infile);

I'm trying to modify the DCT coefficients of a saved JPEG image in iOS with the eventual goal of creating a JPEG steganography app. I have the following code attempting to add one to each DCT coefficient. I'm using the libjpeg library. The code is a combination of Objective-C and C.

A quick note, the variable filename is equal to "/Users/ScottBouloutian/Library/Application Support/iPhone Simulator/7.0.3/Applications/27A0450E-4685-4C3E-AAC8-A0CC6C85359E/Crypsis.app/screen.jpg", which is the path to the JPEG image I am trying to modify.

What is wrong with the code I have? Is it something dumb like a missing include or something wrong with my file path?

Answer 1

Whatever struct my_error_mgr is, the compiler can't find its declaration as a type. You need to include the header that has that declaration.

Answer 2

I was getting the following error in my code:

Variable has incomplete type 'RCTLayoutMetrics' (aka 'struct CG_BOXABLE')

I upgraded to Xcode Version 9.2 (9C40b) and it fixed the issue.