Segmentation Fault when loading font with FreeType

I'm trying to use the FreeType library together with libpng to output a PNG image of a glyph. My code compiles just fine but I then run into a segmentation fault. Here's some debugging output from gdb (I'm not sure what to make of it):

(gdb) run
Starting program: /home/david/Desktop/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff76a58a5 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) where
#0  0x00007ffff76a58a5 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff7bcb298 in png_write_row (png_ptr=0x611320, row=<optimized out>)
    at /usr/include/x86_64-linux-gnu/bits/string3.h:52
#2  0x00007ffff7bcb56c in png_write_image (png_ptr=0x611320, image=0x60d218)
    at pngwrite.c:608
#3  0x0000000000400c35 in main ()

And here's the code:

#include <stdlib.h>
#include <stdio.h>
#include <png.h>

#include <ft2build.h>
#include FT_FREETYPE_H

main() {

    // Declare FreeType variables
    FT_Library library;
    FT_Face face;
    FT_GlyphSlot slot;
    FT_UInt glyph_index = 30;
    char* font_file = "/usr/share/fonts/truetype/freefont/FreeMono.ttf";

    // Declare PNG variables
    png_uint_32 width = 100;
    png_uint_32 height = 100;
    int bit_depth = 16;
    int color_type = PNG_COLOR_TYPE_GRAY;
    char* file_name = "/home/david/Desktop/out.png";
    png_structp png_ptr;
    png_infop info_ptr;

    // Render font
    int error;
    error = FT_Init_FreeType(&library);
    error = FT_New_Face(library, font_file, 0, &face);
    error = FT_Set_Pixel_Sizes(face, 100, 100);
    error = FT_Load_Glyph(face, glyph_index, FT_LOAD_DEFAULT);
    error = FT_Render_Glyph(face->glyph, FT_RENDER_MODE_NORMAL);

    if (error) {
        printf("ERROR CODE: %d", error);
    }

    // Create a PNG file
    FILE *fp = fopen(file_name, "wb");

    // Create the PNG in memory
    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    info_ptr = png_create_info_struct(png_ptr);
    png_init_io(png_ptr, fp);

    // Write the header
    png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth, color_type, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
    png_write_info(png_ptr, info_ptr);

    // Write image data
    slot = face->glyph;
    png_write_image(png_ptr, &slot->bitmap.buffer);

    // End write
    png_write_end(png_ptr, NULL);

    fclose(fp);
}

UPDATE

I'm trying a slightly new approach (using png_write_row instead of png_write_image).

... SAME AS BEFORE ABOVE ....

// Write image data
slot = face->glyph;

int x, y, offset;
for (y = 0; y < slot->bitmap.rows; y++) {
    for (x = 0; x < slot->bitmap.width; x++) {
        offset = y*(slot->bitmap.width) + x;
        png_write_row(png_ptr, &slot->bitmap.buffer[offset]);
    }
}

// End write
png_write_end(png_ptr, NULL);

fclose(fp);

}

Now I get some more useful libpng errors, but I'm still not sure what to make of them:

libpng warning: zstream not in use (internal error)
libpng error: stream error
Aborted (core dumped)

Two suggestions:

1. Take a look for all compiler warnings in order to see that the pointers types match properly.

2. Trace the possible errors better. For example write something like this:

   int error;
   error = FT_Init_FreeType(&library);
   if (error) {
       printf("ERROR CODE: %d", error);
   }
   error = FT_New_Face(library, font_file, 0, &face);
   if (error) {
       printf("ERROR CODE: %d", error);
   }
   error = FT_Set_Pixel_Sizes(face, 100, 100);
   if (error) {
       printf("ERROR CODE: %d", error);
   }
   error = FT_Load_Glyph(face, glyph_index, FT_LOAD_DEFAULT);
   if (error) {
       printf("ERROR CODE: %d", error);
   }
   error = FT_Render_Glyph(face->glyph, FT_RENDER_MODE_NORMAL);
   
   if (error) {
       printf("ERROR CODE: %d", error);
   }
   

Any error on one of this calls may cause a misplaced pointer that may trigger the segmentation fault.

This can be obvious:

FILE *fp = fopen(file_name, "wb");
if ( fp == null ) {
    printf ("Unable to open file for writing"):
}

But if for example you are attempting to write your file in a directory without the permissions, you will have fp pointing to NULL and the further segmentation fault.