Q:

Read in JPEG, resize it and save it to disk

What I want:
Resize an existing jpeg into a new, smaller one.  
Example: before_3000x2000.jpeg →→→ after_1024x768.jpeg
I think I'll need pseudo-functions like these:

foo( before_file_path, after_file_path, new_width, new_height )

which makes use of

decompress_jpeg( before_file_path )
resize( decompressed_data, new_width, new_height )
compress_jpeg( resized_data )
save_jpeg( compressed_data, after_file_path )

Please correct my approach if I misunderstood something. 
What I assume:

Since the resolutions will vary, first you have to decompress the existing jpeg so you can properly work with the pixel data.  
Second, you resize the pixeldata to a smaller buffer with some function foo(). 
Third, you compress said buffer and save it to disk. 

What I tried (and failed):
For starters, I tried playing around with the 

write_JPEG_file (char * filename, int quality)

function provided in example.c of jpeglib-9b.  I just want to write out a test-picture in whatever resolution I specify with a single colour.  But I run into a problem.  I think I'm writing out random memory and I dont know why. ( here is a picture: img_100x100) Notice how at the top there is white colour up to a certain point.  This part changes accordingly to what I assign in the first loop.  But after a certain amount it changes to random noise.  Why? 
What I changed:
I defined image_width, image_height, image_buffer
I replaced all fopen(...) with fopen_s(...). 
I hardcoded the quality to 100 in jpeg_set_quality(...) 
Code:
#include <iostream>
#include "jpeg-9b\jpeglib.h"
#include "jpeg-9b\jerror.h"

int TEST_WRITE_JPEG(char* file_name)
{
    // DEFINE A PIC HERE
    const int image_width = 100;        /* Number of columns in image */
    const int image_height = 100;   /* Number of rows in image */
    JSAMPLE* image_buffer = new JSAMPLE[image_width * image_height]();  /* Points to large array of R,G,B-order data */
    for (int i = 0; i < image_width*image_height; i++)
    {
        image_buffer[i] = 255;
    }

    /* Step 1: allocate and initialize JPEG compression object */
    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr jerr;
    FILE * outfile;
    JSAMPROW row_pointer[1];
    int row_stride;

    cinfo.err = jpeg_std_error(&jerr);
    jpeg_create_compress(&cinfo);

    /* Step 2: specify data destination (eg, a file) */
    if (fopen_s(&outfile, file_name, "wb") ! = NULL)
    {
        fprintf(stderr, "can't open %s\n", file_name);
        exit(1);
    }
    jpeg_stdio_dest(&cinfo, outfile);

    /* Step 3: set parameters for compression */
    cinfo.image_width = image_width;    /* image width and height, in pixels */
    cinfo.image_height = image_height;
    cinfo.input_components = 3;         /* # of color components per pixel */
    cinfo.in_color_space = JCS_RGB;     /* colorspace of input image */

    jpeg_set_defaults(&cinfo);
    jpeg_set_quality(&cinfo, 100, TRUE);

    /* Step 4: Start compressor */
    jpeg_start_compress(&cinfo, TRUE);

    /* Step 5: while (scan lines remain to be written) */
    row_stride = image_width * 3;

    while (cinfo.next_scanline < cinfo.image_height)
    {
        row_pointer[0] = (JSAMPROW)&image_buffer[cinfo.next_scanline * row_stride];
        (void)jpeg_write_scanlines(&cinfo, row_pointer, 1);
    }

    /* Step 6: Finish compression */
    jpeg_finish_compress(&cinfo);
    /* After finish_compress, we can close the output file. */ 
    fclose(outfile);

    /* Step 7: release JPEG compression object */
    /* This is an important step since it will release a good deal of memory. */ 
    jpeg_destroy_compress(&cinfo);

    return 0;
}

Clear questions:
Why do I write out this picture and not a plain white one? 
Why does the program crash if image_buffer contains more than 128x128 unsigned chars? 

A:

3 bytes are used for each pixels, so you have to allocate 3 * image_width * image_height elements while you allocated only image_width * image_height elements.  Allocate and initialize sufficient elements.