如何使用libpng将PNG编码为缓冲区？

#include <png.h>

unsigned char *buffer;

void png_to_buffer(const char *filename, unsigned char **buffer, size_t *buffer_size) {
    FILE *fp = fopen(filename, "rb");
    if (!fp) {
        printf("Error: Unable to open file %s\n", filename);
        return;
    }

    png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (!png_ptr) {
        printf("Error: Unable to create PNG read structure\n");
        fclose(fp);
        return;
    }

    png_infop info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr) {
        printf("Error: Unable to create PNG info structure\n");
        png_destroy_read_struct(&png_ptr, NULL, NULL);
        fclose(fp);
        return;
    }

    if (setjmp(png_jmpbuf(png_ptr))) {
        printf("Error: Unable to setjmp for PNG read\n");
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        fclose(fp);
        return;
    }

    png_init_io(png_ptr, fp);
    png_read_info(png_ptr, info_ptr);

    png_uint_32 width = png_get_image_width(png_ptr, info_ptr);
    png_uint_32 height = png_get_image_height(png_ptr, info_ptr);
    int bit_depth = png_get_bit_depth(png_ptr, info_ptr);
    int color_type = png_get_color_type(png_ptr, info_ptr);

    if (bit_depth == 16) {
        png_set_strip_16(png_ptr);
    }

    if (color_type == PNG_COLOR_TYPE_PALETTE) {
        png_set_palette_to_rgb(png_ptr);
    }

    if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) {
        png_set_expand_gray_1_2_4_to_8(png_ptr);
    }

    if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
        png_set_tRNS_to_alpha(png_ptr);
    }

    if (color_type == PNG_COLOR_TYPE_RGB || color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_PALETTE) {
        png_set_filler(png_ptr, 0xFF, PNG_FILLER_AFTER);
    }

    *buffer_size = width * height * 4;
    *buffer = (unsigned char *)malloc(*buffer_size);
    if (!*buffer) {
        printf("Error: Unable to allocate memory for PNG buffer\n");
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        fclose(fp);
        return;
    }

    png_bytepp row_pointers = (png_bytepp)malloc(height * sizeof(png_bytep));
    if (!row_pointers) {
        printf("Error: Unable to allocate memory for PNG row pointers\n");
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        free(*buffer);
        fclose(fp);
        return;
    }

    for (png_uint_32 i = 0; i< height; ++i) {
        row_pointers[i] = *buffer + i * width * 4;
    }

    png_read_image(png_ptr, row_pointers);
    png_read_end(png_ptr, NULL);

    free(row_pointers);
    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
    fclose(fp);
}

int main() {
    const char *filename = "example.png";
    unsigned char *buffer;
    size_t buffer_size;

    png_to_buffer(filename, &buffer, &buffer_size);

    // 在这里使用缓冲区

    free(buffer);
    return 0;
}