can not read png file with libpng - c

Question

I am new to libpng and the documentation is really confusing for me.
Below is my code which is not working and I do not see the reason why.
Can someone point me to right direction? or suggest different ( "easier" ) library?
how I understand libpng:
open the file with fopen in rb mode
create png_structp with png_create_read_struct
create png_infop with png_create_info_struct
allocate space
read data
#include <stdio.h>
#include <png.h>
int main( int argc, char **argv )
{
int x, y;
int height, width;
png_structp png_ptr;
png_infop info_ptr;
png_bytep *row_pointers;
FILE *fp = fopen( "test.png", "rb");
{
if (!fp)
printf("File could not be opened for reading");
png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
info_ptr = png_create_info_struct(png_ptr);
png_read_info(png_ptr, info_ptr);
width = png_get_image_width(png_ptr, info_ptr);
height = png_get_image_height(png_ptr, info_ptr);
row_pointers = (png_bytep*) malloc(sizeof(png_bytep) * height);
for (y=0; y<height; y++)
row_pointers[y] = (png_byte*)malloc(png_get_rowbytes(png_ptr,info_ptr));
png_read_image(png_ptr, row_pointers);
fclose(fp);
}
for (y=0; y<height; y++)
{
png_byte *row = row_pointers[y];
for (x=0; x<width; x++)
{
png_byte* ptr = &(row[x*4]);
printf("Pixel at position [ %d - %d ] has RGBA values: %d - %d - %d - %d\n", x, y, ptr[0], ptr[1], ptr[2], ptr[3]);
}
}
}

Answer 1

I similarly had failure in png_create_read_struct. It is difficult to debug without further info (in my case, stderr goes nowhere), but fortunately you can provide your own error and warning functions:
void user_error_fn(png_structp png_ptr, png_const_charp error_msg);
void user_warning_fn(png_structp png_ptr, png_const_charp warning_msg);
Using this, libpng printed helpful errors stating that the application was using a different version of the header than the libpng found at runtime. Mystery solved.
So while individual issues may differ, using libpng's error reporting should provide insight.

Answer 2

Obviously, there is no information passed to libpng about where/how to read image chunk. Use:
...
png_init_io(png_ptr, fp);
png_read_info..