rwpng.c

/*
** PNG read/write functions
**
** © 1998-2000 by Greg Roelofs.
** © 2009-2017 by Kornel Lesiński.
**
** See COPYRIGHT file for license.
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>

#include "png.h"  /* if this include fails, you need to install libpng (e.g. libpng-devel package) */
#include "rwpng.h"
#if USE_LCMS
#include "lcms2.h"
#endif

#ifndef Z_BEST_COMPRESSION
#define Z_BEST_COMPRESSION 9
#endif
#ifndef Z_BEST_SPEED
#define Z_BEST_SPEED 1
#endif

#ifdef _OPENMP
#include <omp.h>
#else
#define omp_get_max_threads() 1
#endif

#if PNG_LIBPNG_VER < 10400
#error libpng version 1.4 or later is required. 1.6 is recommended. You have an obsolete version of libpng or compiling on an outdated/unsupported operating system. Please upgrade.
#endif

#if PNG_LIBPNG_VER < 10500
typedef png_const_charp png_const_bytep;
#endif

static void rwpng_error_handler(png_structp png_ptr, png_const_charp msg);
pngquant_error rwpng_read_image32_cocoa(FILE *infile, uint32_t *width, uint32_t *height, size_t *file_size, rwpng_rgba **image_data);


void rwpng_version_info(FILE *fp)
{
    const char *pngver = png_get_header_ver(NULL);

#if USE_COCOA
    fprintf(fp, "   Color profiles are supported via Cocoa. Using libpng %s.\n", pngver);
#elif USE_LCMS
    fprintf(fp, "   Color profiles are supported via Little CMS. Using libpng %s.\n", pngver);
#else
    fprintf(fp, "   Compiled with no support for color profiles. Using libpng %s.\n", pngver);
#endif

#if PNG_LIBPNG_VER < 10600
    if (strcmp(pngver, "1.3.") < 0) {
        fputs("\nWARNING: Your version of libpng is outdated and may produce corrupted files.\n"
              "Please recompile pngquant with the current version of libpng (1.6 or later).\n", fp);
    } else if (strcmp(pngver, "1.6.") < 0) {
        #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
        fputs("\nWARNING: Your version of libpng is old and has buggy support for custom chunks.\n"
              "Please recompile pngquant with the current version of libpng (1.6 or later).\n", fp);
        #endif
    }
#endif
}


struct rwpng_read_data {
    FILE *const fp;
    png_size_t bytes_read;
};

#if !USE_COCOA
static void user_read_data(png_structp png_ptr, png_bytep data, png_size_t length)
{
    struct rwpng_read_data *read_data = (struct rwpng_read_data *)png_get_io_ptr(png_ptr);

    png_size_t read = fread(data, 1, length, read_data->fp);
    if (!read) {
        png_error(png_ptr, "Read error");
    }
    read_data->bytes_read += read;
}
#endif

struct rwpng_write_state {
    FILE *outfile;
    png_size_t maximum_file_size;
    png_size_t bytes_written;
    pngquant_error retval;
};

static void user_write_data(png_structp png_ptr, png_bytep data, png_size_t length)
{
    struct rwpng_write_state *write_state = (struct rwpng_write_state *)png_get_io_ptr(png_ptr);

    if (SUCCESS != write_state->retval) {
        return;
    }

    if (!fwrite(data, length, 1, write_state->outfile)) {
        write_state->retval = CANT_WRITE_ERROR;
    }

    write_state->bytes_written += length;
}

static void user_flush_data(png_structp png_ptr)
{
    // libpng never calls this :(
}


static png_bytepp rwpng_create_row_pointers(png_infop info_ptr, png_structp png_ptr, unsigned char *base, size_t height, png_size_t rowbytes)
{
    if (!rowbytes) {
        rowbytes = png_get_rowbytes(png_ptr, info_ptr);
    }

    png_bytepp row_pointers = malloc(height * sizeof(row_pointers[0]));
    if (!row_pointers) return NULL;
    for(size_t row = 0; row < height; row++) {
        row_pointers[row] = base + row * rowbytes;
    }
    return row_pointers;
}

#if !USE_COCOA
static int read_chunk_callback(png_structp png_ptr, png_unknown_chunkp in_chunk)
{
    if (0 == memcmp("iCCP", in_chunk->name, 5) ||
        0 == memcmp("cHRM", in_chunk->name, 5) ||
        0 == memcmp("gAMA", in_chunk->name, 5)) {
        return 0; // not handled
    }

    if (in_chunk->location == 0 ) {
        return 1; // ignore chunks with invalid location
    }

    struct rwpng_chunk **head = (struct rwpng_chunk **)png_get_user_chunk_ptr(png_ptr);

    struct rwpng_chunk *chunk = malloc(sizeof(struct rwpng_chunk));
    memcpy(chunk->name, in_chunk->name, 5);
    chunk->size = in_chunk->size;
    chunk->location = in_chunk->location;
    chunk->data = in_chunk->size ? malloc(in_chunk->size) : NULL;
    if (in_chunk->size) {
        memcpy(chunk->data, in_chunk->data, in_chunk->size);
    }

    chunk->next = *head;
    *head = chunk;

    return 1; // marks as "handled", libpng won't store it
}
#endif

/*
   retval:
     0 = success
    21 = bad sig
    22 = bad IHDR
    24 = insufficient memory
    25 = libpng error (via longjmp())
    26 = wrong PNG color type (no alpha channel)
 */

#if !USE_COCOA
static void rwpng_warning_stderr_handler(png_structp png_ptr, png_const_charp msg) {
    fprintf(stderr, "  libpng warning: %s\n", msg);
}

static void rwpng_warning_silent_handler(png_structp png_ptr, png_const_charp msg) {
}

static pngquant_error rwpng_read_image24_libpng(FILE *infile, png24_image *mainprog_ptr, int strip, int verbose)
{
    png_structp  png_ptr = NULL;
    png_infop    info_ptr = NULL;
    png_size_t   rowbytes;
    int          color_type, bit_depth;

    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, mainprog_ptr,
      rwpng_error_handler, verbose ? rwpng_warning_stderr_handler : rwpng_warning_silent_handler);
    if (!png_ptr) {
        return PNG_OUT_OF_MEMORY_ERROR;   /* out of memory */
    }

    info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr) {
        png_destroy_read_struct(&png_ptr, NULL, NULL);
        return PNG_OUT_OF_MEMORY_ERROR;   /* out of memory */
    }

    /* setjmp() must be called in every function that calls a non-trivial
     * libpng function */

    if (setjmp(mainprog_ptr->jmpbuf)) {
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        return LIBPNG_FATAL_ERROR;   /* fatal libpng error (via longjmp()) */
    }

#if defined(PNG_SKIP_sRGB_CHECK_PROFILE) && defined(PNG_SET_OPTION_SUPPORTED)
    png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE, PNG_OPTION_ON);
#endif

#if PNG_LIBPNG_VER >= 10500 && defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
    if (!strip) {
        /* copy standard chunks too */
        png_set_keep_unknown_chunks(png_ptr, PNG_HANDLE_CHUNK_IF_SAFE, (png_const_bytep)"pHYs\0iTXt\0tEXt\0zTXt", 4);
    }
#endif
    if (!strip) {
        png_set_read_user_chunk_fn(png_ptr, &mainprog_ptr->chunks, read_chunk_callback);
    }

    struct rwpng_read_data read_data = {infile, 0};
    png_set_read_fn(png_ptr, &read_data, user_read_data);

    png_read_info(png_ptr, info_ptr);  /* read all PNG info up to image data */

    /* alternatively, could make separate calls to png_get_image_width(),
     * etc., but want bit_depth and color_type for later [don't care about
     * compression_type and filter_type => NULLs] */

    png_get_IHDR(png_ptr, info_ptr, &mainprog_ptr->width, &mainprog_ptr->height,
                 &bit_depth, &color_type, NULL, NULL, NULL);

    /* expand palette images to RGB, low-bit-depth grayscale images to 8 bits,
     * transparency chunks to full alpha channel; strip 16-bit-per-sample
     * images to 8 bits per sample; and convert grayscale to RGB[A] */

    /* GRR TO DO:  preserve all safe-to-copy ancillary PNG chunks */

    if (!(color_type & PNG_COLOR_MASK_ALPHA)) {
#ifdef PNG_READ_FILLER_SUPPORTED
        png_set_expand(png_ptr);
        png_set_filler(png_ptr, 65535L, PNG_FILLER_AFTER);
#else
        fprintf(stderr, "pngquant readpng:  image is neither RGBA nor GA\n");
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        mainprog_ptr->retval = WRONG_INPUT_COLOR_TYPE;
        return mainprog_ptr->retval;
#endif
    }

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

    if (!(color_type & PNG_COLOR_MASK_COLOR)) {
        png_set_gray_to_rgb(png_ptr);
    }

    /* get source gamma for gamma correction, or use sRGB default */
    double gamma = 0.45455;
    if (png_get_valid(png_ptr, info_ptr, PNG_INFO_sRGB)) {
        mainprog_ptr->input_color = RWPNG_SRGB;
        mainprog_ptr->output_color = RWPNG_SRGB;
    } else {
        png_get_gAMA(png_ptr, info_ptr, &gamma);
        if (gamma > 0 && gamma <= 1.0) {
            mainprog_ptr->input_color = RWPNG_GAMA_ONLY;
            mainprog_ptr->output_color = RWPNG_GAMA_ONLY;
        } else {
            fprintf(stderr, "pngquant readpng:  ignored out-of-range gamma %f\n", gamma);
            mainprog_ptr->input_color = RWPNG_NONE;
            mainprog_ptr->output_color = RWPNG_NONE;
            gamma = 0.45455;
        }
    }
    mainprog_ptr->gamma = gamma;

    png_set_interlace_handling(png_ptr);

    /* all transformations have been registered; now update info_ptr data,
     * get rowbytes and channels, and allocate image memory */

    png_read_update_info(png_ptr, info_ptr);

    rowbytes = png_get_rowbytes(png_ptr, info_ptr);

    // For overflow safety reject images that won't fit in 32-bit
    if (rowbytes > INT_MAX/mainprog_ptr->height) {
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        return PNG_OUT_OF_MEMORY_ERROR;
    }

    if ((mainprog_ptr->rgba_data = malloc(rowbytes * mainprog_ptr->height)) == NULL) {
        fprintf(stderr, "pngquant readpng:  unable to allocate image data\n");
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        return PNG_OUT_OF_MEMORY_ERROR;
    }

    png_bytepp row_pointers = rwpng_create_row_pointers(info_ptr, png_ptr, mainprog_ptr->rgba_data, mainprog_ptr->height, 0);

    /* now we can go ahead and just read the whole image */

    png_read_image(png_ptr, row_pointers);

    /* and we're done!  (png_read_end() can be omitted if no processing of
     * post-IDAT text/time/etc. is desired) */

    png_read_end(png_ptr, NULL);

#if USE_LCMS
#if PNG_LIBPNG_VER < 10500
    png_charp ProfileData;
#else
    png_bytep ProfileData;
#endif
    png_uint_32 ProfileLen;

    cmsHPROFILE hInProfile = NULL;

    /* color_type is read from the image before conversion to RGBA */
    int COLOR_PNG = color_type & PNG_COLOR_MASK_COLOR;

    /* embedded ICC profile */
    if (png_get_iCCP(png_ptr, info_ptr, &(png_charp){0}, &(int){0}, &ProfileData, &ProfileLen)) {

        hInProfile = cmsOpenProfileFromMem(ProfileData, ProfileLen);
        cmsColorSpaceSignature colorspace = cmsGetColorSpace(hInProfile);

        /* only RGB (and GRAY) valid for PNGs */
        if (colorspace == cmsSigRgbData && COLOR_PNG) {
            mainprog_ptr->input_color = RWPNG_ICCP;
            mainprog_ptr->output_color = RWPNG_SRGB;
        } else {
            if (colorspace == cmsSigGrayData && !COLOR_PNG) {
                mainprog_ptr->input_color = RWPNG_ICCP_WARN_GRAY;
                mainprog_ptr->output_color = RWPNG_SRGB;
            }
            cmsCloseProfile(hInProfile);
            hInProfile = NULL;
        }
    }

    /* build RGB profile from cHRM and gAMA */
    if (hInProfile == NULL && COLOR_PNG &&
        !png_get_valid(png_ptr, info_ptr, PNG_INFO_sRGB) &&
        png_get_valid(png_ptr, info_ptr, PNG_INFO_gAMA) &&
        png_get_valid(png_ptr, info_ptr, PNG_INFO_cHRM)) {

        cmsCIExyY WhitePoint;
        cmsCIExyYTRIPLE Primaries;

        png_get_cHRM(png_ptr, info_ptr, &WhitePoint.x, &WhitePoint.y,
                     &Primaries.Red.x, &Primaries.Red.y,
                     &Primaries.Green.x, &Primaries.Green.y,
                     &Primaries.Blue.x, &Primaries.Blue.y);

        WhitePoint.Y = Primaries.Red.Y = Primaries.Green.Y = Primaries.Blue.Y = 1.0;

        cmsToneCurve *GammaTable[3];
        GammaTable[0] = GammaTable[1] = GammaTable[2] = cmsBuildGamma(NULL, 1/gamma);

        hInProfile = cmsCreateRGBProfile(&WhitePoint, &Primaries, GammaTable);

        cmsFreeToneCurve(GammaTable[0]);

        mainprog_ptr->input_color = RWPNG_GAMA_CHRM;
        mainprog_ptr->output_color = RWPNG_SRGB;
    }

    /* transform image to sRGB colorspace */
    if (hInProfile != NULL) {

        cmsHPROFILE hOutProfile = cmsCreate_sRGBProfile();
        cmsHTRANSFORM hTransform = cmsCreateTransform(hInProfile, TYPE_RGBA_8,
                                                      hOutProfile, TYPE_RGBA_8,
                                                      INTENT_PERCEPTUAL,
                                                      omp_get_max_threads() > 1 ? cmsFLAGS_NOCACHE : 0);
        if(!hTransform) {
            png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
            cmsCloseProfile(hOutProfile);
            cmsCloseProfile(hInProfile);
            return LCMS_FATAL_ERROR;
        }

        #pragma omp parallel for \
            if (mainprog_ptr->height*mainprog_ptr->width > 8000) \
            schedule(static)
        for (unsigned int i = 0; i < mainprog_ptr->height; i++) {
            /* It is safe to use the same block for input and output,
               when both are of the same TYPE. */
            cmsDoTransform(hTransform, row_pointers[i],
                                       row_pointers[i],
                                       mainprog_ptr->width);
        }

        cmsDeleteTransform(hTransform);
        cmsCloseProfile(hOutProfile);
        cmsCloseProfile(hInProfile);

        mainprog_ptr->gamma = 0.45455;
    }
#endif

    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);

    mainprog_ptr->file_size = read_data.bytes_read;
    mainprog_ptr->row_pointers = (unsigned char **)row_pointers;

    return SUCCESS;
}
#endif

static void rwpng_free_chunks(struct rwpng_chunk *chunk) {
    if (!chunk) return;
    rwpng_free_chunks(chunk->next);
    free(chunk->data);
    free(chunk);
}

void rwpng_free_image24(png24_image *image)
{
    free(image->row_pointers);
    image->row_pointers = NULL;

    free(image->rgba_data);
    image->rgba_data = NULL;

    rwpng_free_chunks(image->chunks);
    image->chunks = NULL;
}

void rwpng_free_image8(png8_image *image)
{
    free(image->indexed_data);
    image->indexed_data = NULL;

    free(image->row_pointers);
    image->row_pointers = NULL;

    rwpng_free_chunks(image->chunks);
    image->chunks = NULL;
}

pngquant_error rwpng_read_image24(FILE *infile, png24_image *out, int strip, int verbose)
{
#if USE_COCOA
    rwpng_rgba *pixel_data;
    pngquant_error res = rwpng_read_image32_cocoa(infile, &out->width, &out->height, &out->file_size, &pixel_data);
    if (res != SUCCESS) {
        return res;
    }
    out->gamma = 0.45455;
    out->input_color = RWPNG_COCOA;
    out->output_color = RWPNG_SRGB;
    out->rgba_data = (unsigned char *)pixel_data;
    out->row_pointers = malloc(sizeof(out->row_pointers[0])*out->height);
    for(int i=0; i < out->height; i++) {
        out->row_pointers[i] = (unsigned char *)&pixel_data[out->width*i];
    }
    return SUCCESS;
#else
    return rwpng_read_image24_libpng(infile, out, strip, verbose);
#endif
}


static pngquant_error rwpng_write_image_init(rwpng_png_image *mainprog_ptr, png_structpp png_ptr_p, png_infopp info_ptr_p, int fast_compression)
{
    /* could also replace libpng warning-handler (final NULL), but no need: */

    *png_ptr_p = png_create_write_struct(PNG_LIBPNG_VER_STRING, mainprog_ptr, rwpng_error_handler, NULL);

    if (!(*png_ptr_p)) {
        return LIBPNG_INIT_ERROR;   /* out of memory */
    }

    *info_ptr_p = png_create_info_struct(*png_ptr_p);
    if (!(*info_ptr_p)) {
        png_destroy_write_struct(png_ptr_p, NULL);
        return LIBPNG_INIT_ERROR;   /* out of memory */
    }

    /* setjmp() must be called in every function that calls a PNG-writing
     * libpng function, unless an alternate error handler was installed--
     * but compatible error handlers must either use longjmp() themselves
     * (as in this program) or exit immediately, so here we go: */

    if (setjmp(mainprog_ptr->jmpbuf)) {
        png_destroy_write_struct(png_ptr_p, info_ptr_p);
        return LIBPNG_INIT_ERROR;   /* libpng error (via longjmp()) */
    }

    png_set_compression_level(*png_ptr_p, fast_compression ? Z_BEST_SPEED : Z_BEST_COMPRESSION);
    png_set_compression_mem_level(*png_ptr_p, fast_compression ? 9 : 5); // judging by optipng results, smaller mem makes libpng compress slightly better

    return SUCCESS;
}


static void rwpng_write_end(png_infopp info_ptr_p, png_structpp png_ptr_p, png_bytepp row_pointers)
{
    png_write_info(*png_ptr_p, *info_ptr_p);

    png_set_packing(*png_ptr_p);

    png_write_image(*png_ptr_p, row_pointers);

    png_write_end(*png_ptr_p, NULL);

    png_destroy_write_struct(png_ptr_p, info_ptr_p);
}

static void rwpng_set_gamma(png_infop info_ptr, png_structp png_ptr, double gamma, rwpng_color_transform color)
{
    if (color != RWPNG_GAMA_ONLY && color != RWPNG_NONE) {
        png_set_gAMA(png_ptr, info_ptr, gamma);
    }
    if (color == RWPNG_SRGB) {
        png_set_sRGB(png_ptr, info_ptr, 0); // 0 = Perceptual
    }
}

pngquant_error rwpng_write_image8(FILE *outfile, png8_image *mainprog_ptr)
{
    png_structp png_ptr;
    png_infop info_ptr;

    if (mainprog_ptr->num_palette > 256) return INVALID_ARGUMENT;

    pngquant_error retval = rwpng_write_image_init((rwpng_png_image*)mainprog_ptr, &png_ptr, &info_ptr, mainprog_ptr->fast_compression);
    if (retval) return retval;

    struct rwpng_write_state write_state;
    write_state = (struct rwpng_write_state){
        .outfile = outfile,
        .maximum_file_size = mainprog_ptr->maximum_file_size,
        .retval = SUCCESS,
    };
    png_set_write_fn(png_ptr, &write_state, user_write_data, user_flush_data);

    // Palette images generally don't gain anything from filtering
    png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, PNG_FILTER_VALUE_NONE);

    rwpng_set_gamma(info_ptr, png_ptr, mainprog_ptr->gamma, mainprog_ptr->output_color);

    /* set the image parameters appropriately */
    int sample_depth;
#if PNG_LIBPNG_VER > 10400 /* old libpng corrupts files with low depth */
    if (mainprog_ptr->num_palette <= 2)
        sample_depth = 1;
    else if (mainprog_ptr->num_palette <= 4)
        sample_depth = 2;
    else if (mainprog_ptr->num_palette <= 16)
        sample_depth = 4;
    else
#endif
        sample_depth = 8;

    struct rwpng_chunk *chunk = mainprog_ptr->chunks;
    mainprog_ptr->metadata_size = 0;
    int chunk_num=0;
    while(chunk) {
        png_unknown_chunk pngchunk = {
            .size = chunk->size,
            .data = chunk->data,
            .location = chunk->location,
        };
        memcpy(pngchunk.name, chunk->name, 5);
        png_set_unknown_chunks(png_ptr, info_ptr, &pngchunk, 1);

        #if defined(PNG_HAVE_IHDR) && PNG_LIBPNG_VER < 10600
        png_set_unknown_chunk_location(png_ptr, info_ptr, chunk_num, pngchunk.location ? pngchunk.location : PNG_HAVE_IHDR);
        #endif

        mainprog_ptr->metadata_size += chunk->size + 12;
        chunk = chunk->next;
        chunk_num++;
    }

    png_set_IHDR(png_ptr, info_ptr, mainprog_ptr->width, mainprog_ptr->height,
      sample_depth, PNG_COLOR_TYPE_PALETTE,
      0, PNG_COMPRESSION_TYPE_DEFAULT,
      PNG_FILTER_TYPE_BASE);

    png_color palette[256];
    png_byte trans[256];
    unsigned int num_trans = 0;
    for(unsigned int i = 0; i < mainprog_ptr->num_palette; i++) {
        palette[i] = (png_color){
            .red   = mainprog_ptr->palette[i].r,
            .green = mainprog_ptr->palette[i].g,
            .blue  = mainprog_ptr->palette[i].b,
        };
        trans[i] = mainprog_ptr->palette[i].a;
        if (mainprog_ptr->palette[i].a < 255) {
            num_trans = i+1;
        }
    }

    png_set_PLTE(png_ptr, info_ptr, palette, mainprog_ptr->num_palette);

    if (num_trans > 0) {
        png_set_tRNS(png_ptr, info_ptr, trans, num_trans, NULL);
    }

    rwpng_write_end(&info_ptr, &png_ptr, mainprog_ptr->row_pointers);

    if (SUCCESS == write_state.retval && write_state.maximum_file_size && write_state.bytes_written > write_state.maximum_file_size) {
        return TOO_LARGE_FILE;
    }

    return write_state.retval;
}

pngquant_error rwpng_write_image24(FILE *outfile, const png24_image *mainprog_ptr)
{
    png_structp png_ptr;
    png_infop info_ptr;

    pngquant_error retval = rwpng_write_image_init((rwpng_png_image*)mainprog_ptr, &png_ptr, &info_ptr, 0);
    if (retval) return retval;

    png_init_io(png_ptr, outfile);

    rwpng_set_gamma(info_ptr, png_ptr, mainprog_ptr->gamma, mainprog_ptr->output_color);

    png_set_IHDR(png_ptr, info_ptr, mainprog_ptr->width, mainprog_ptr->height,
                 8, PNG_COLOR_TYPE_RGB_ALPHA,
                 0, PNG_COMPRESSION_TYPE_DEFAULT,
                 PNG_FILTER_TYPE_BASE);


    png_bytepp row_pointers = rwpng_create_row_pointers(info_ptr, png_ptr, mainprog_ptr->rgba_data, mainprog_ptr->height, 0);

    rwpng_write_end(&info_ptr, &png_ptr, row_pointers);

    free(row_pointers);

    return SUCCESS;
}

static void rwpng_error_handler(png_structp png_ptr, png_const_charp msg)
{
    rwpng_png_image  *mainprog_ptr;

    /* This function, aside from the extra step of retrieving the "error
     * pointer" (below) and the fact that it exists within the application
     * rather than within libpng, is essentially identical to libpng's
     * default error handler.  The second point is critical:  since both
     * setjmp() and longjmp() are called from the same code, they are
     * guaranteed to have compatible notions of how big a jmp_buf is,
     * regardless of whether _BSD_SOURCE or anything else has (or has not)
     * been defined. */

    fprintf(stderr, "  error: %s (libpng failed)\n", msg);
    fflush(stderr);

    mainprog_ptr = png_get_error_ptr(png_ptr);
    if (mainprog_ptr == NULL) abort();

    longjmp(mainprog_ptr->jmpbuf, 1);
}