How to encode PNG to buffer using libpng?

I'm currently using the following to write a PNG to a file:

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

/* Pixels in this bitmap structure are stored as BGR. */
typedef struct _RGBPixel {
    uint8_t blue;
    uint8_t green;
    uint8_t red;
} RGBPixel;

/* Structure for containing decompressed bitmaps. */
typedef struct _RGBBitmap {
    RGBPixel *pixels;
    size_t width;
    size_t height;
    size_t bytewidth;
    uint8_t bytes_per_pixel;
} RGBBitmap;

/* Returns pixel of bitmap at given point. */
#define RGBPixelAtPoint(image, x, y) \
    *(((image)->pixels) + (((image)->bytewidth * (y)) \
                        + ((x) * (image)->bytes_per_pixel)))

/* Attempts to save PNG to file; returns 0 on success, non-zero on error. */
int save_png_to_file(RGBBitmap *bitmap, const char *path)
{
    FILE *fp = fopen(path, "wb");
    png_structp png_ptr = NULL;
    png_infop info_ptr = NULL;
    size_t x, y;
    png_uint_32 bytes_per_row;
    png_byte **row_pointers = NULL;

    if (fp == NULL) return -1;

    /* Initialize the write struct. */
    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (png_ptr == NULL) {
        fclose(fp);
        return -1;
    }

    /* Initialize the info struct. */
    info_ptr = png_create_info_struct(png_ptr);
    if (info_ptr == NULL) {
        png_destroy_write_struct(&png_ptr, NULL);
        fclose(fp);
        return -1;
    }

    /* Set up error handling. */
    if (setjmp(png_jmpbuf(png_ptr))) {
        png_destroy_write_struct(&png_ptr, &info_ptr);
        fclose(fp);
        return -1;
    }

    /* Set image attributes. */
    png_set_IHDR(png_ptr,
                 info_ptr,
                 bitmap->width,
                 bitmap->height,
                 8,
                 PNG_COLOR_TYPE_RGB,
                 PNG_INTERLACE_NONE,
                 PNG_COMPRESSION_TYPE_DEFAULT,