Simplified API: write-to-memory, overflow handling

This implements an API and provides a number of assist macros to allow an
application which uses the simplified API write to bypass stdio and write
directly to memory.

It also includes some warnings (png.h) and some check code to detect *possible*
overflow in the ROW_STRIDE and simplified image SIZE macros.  This disallows
image width/height/format that *might* overflow.  A quiet API change that limits
in-memory image size (uncompressed) to less that 4GByte and image row size
(stride) to less than 2GByte.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

contrib/libtests/pngstest.c

@@ -307,7 +307,7 @@ compare_16bit(int v1, int v2, int error_limit, int multiple_algorithms)
 }
 #endif /* unused */
 
-#define READ_FILE 1      /* else memory */
+#define USE_FILE 1       /* else memory */
 #define USE_STDIO 2      /* else use file name */
 #define STRICT 4         /* fail on warnings too */
 #define VERBOSE 8
@@ -320,7 +320,7 @@ compare_16bit(int v1, int v2, int error_limit, int multiple_algorithms)
 static void
 print_opts(png_uint_32 opts)
 {
-   if (opts & READ_FILE)
+   if (opts & USE_FILE)
       printf(" --file");
    if (opts & USE_STDIO)
       printf(" --stdio");
@@ -3008,14 +3008,14 @@ read_file(Image *image, png_uint_32 format, png_const_colorp background)
 static int
 read_one_file(Image *image)
 {
-   if (!(image->opts & READ_FILE) || (image->opts & USE_STDIO))
+   if (!(image->opts & USE_FILE) || (image->opts & USE_STDIO))
    {
       /* memory or stdio. */
       FILE *f = fopen(image->file_name, "rb");
 
       if (f != NULL)
       {
-         if (image->opts & READ_FILE)
+         if (image->opts & USE_FILE)
             image->input_file = f;
 
          else /* memory */
@@ -3096,6 +3096,7 @@ write_one_file(Image *output, Image *image, int convert_to_8bit)
 
    if (image->opts & USE_STDIO)
    {
+#ifdef PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
 #ifndef __COVERITY__
       FILE *f = tmpfile();
 #else
@@ -3158,10 +3159,14 @@ write_one_file(Image *output, Image *image, int convert_to_8bit)
 
       else
          return logerror(image, "tmpfile", ": open: ", strerror(errno));
+#else /* SIMPLIFIED_WRITE_STDIO */
+      return logerror(image, "tmpfile", ": open: unsupported", "");
+#endif /* SIMPLIFIED_WRITE_STDIO */
    }
 
-   else
+   else if (image->opts & USE_FILE)
    {
+#ifdef PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
       static int counter = 0;
       char name[32];
 
@@ -3181,6 +3186,48 @@ write_one_file(Image *output, Image *image, int convert_to_8bit)
 
       else
          return logerror(image, name, ": write failed", "");
+#else /* SIMPLIFIED_WRITE_STDIO */
+      return logerror(image, "stdio", ": open: unsupported", "");
+#endif /* SIMPLIFIED_WRITE_STDIO */
+   }
+
+   else /* use memory */
+   {
+      png_alloc_size_t size;
+
+      if (png_image_write_get_memory_size(image->image, size, convert_to_8bit,
+               image->buffer+16, (png_int_32)image->stride, image->colormap))
+      {
+         /* This is non-fatal: */
+         if (size > PNG_IMAGE_PNG_SIZE_MAX(image->image))
+            logerror(image, "memory", ": PNG_IMAGE_SIZE_MAX wrong", "");
+
+         initimage(output, image->opts, "memory", image->stride_extra);
+         output->input_memory = malloc(size);
+
+         if (output->input_memory != NULL)
+         {
+            output->input_memory_size = size;
+
+            if (png_image_write_to_memory(&image->image, output->input_memory,
+                  &output->input_memory_size, convert_to_8bit, image->buffer+16,
+                  (png_int_32)image->stride, image->colormap))
+            {
+               /* This is also non-fatal (maybe): */
+               if (size != output->input_memory_size)
+                  logerror(image, "memory", ": memory size wrong", "");
+            }
+
+            else
+               return logerror(image, "memory", ": write failed", "");
+         }
+
+         else
+            return logerror(image, "memory", ": out of memory", "");
+      }
+
+      else
+         return logerror(image, "memory", ": get size:", "");
    }
 
    /* 'output' has an initialized temporary image, read this back in and compare
@@ -3421,12 +3468,12 @@ main(int argc, char **argv)
       }
       else if (strcmp(arg, "--file") == 0)
 #        ifdef PNG_STDIO_SUPPORTED
-            opts |= READ_FILE;
+            opts |= USE_FILE;
 #        else
             return 77; /* skipped: no support */
 #        endif
       else if (strcmp(arg, "--memory") == 0)
-         opts &= ~READ_FILE;
+         opts &= ~USE_FILE;
       else if (strcmp(arg, "--stdio") == 0)
 #        ifdef PNG_STDIO_SUPPORTED
             opts |= USE_STDIO;

png.h

@@ -2889,12 +2889,19 @@ typedef struct
     * is the minimum 'row stride', the minimum count of components between each
     * row.  For a color-mapped image this is the minimum number of bytes in a
     * row.
+    *
+    * WARNING: this macro overflows for some images with more than one component
+    * and very large image widths.  libpng will refuse to process an image where
+    * this macro would overflow.
     */
 
 #define PNG_IMAGE_BUFFER_SIZE(image, row_stride)\
    (PNG_IMAGE_PIXEL_COMPONENT_SIZE((image).format)*(image).height*(row_stride))
    /* Return the size, in bytes, of an image buffer given a png_image and a row
     * stride - the number of components to leave space for in each row.
+    *
+    * WARNING: this macro overflows a 32-bit integer for some large PNG images,
+    * libpng will refuse to process an image where such an overflow would occur.
     */
 
 #define PNG_IMAGE_SIZE(image)\
@@ -3015,7 +3022,6 @@ PNG_EXPORT(238, void, png_image_free, (png_imagep image));
 #endif /* SIMPLIFIED_READ */
 
 #ifdef PNG_SIMPLIFIED_WRITE_SUPPORTED
-#ifdef PNG_STDIO_SUPPORTED
 /* WRITE APIS
  * ----------
  * For write you must initialize a png_image structure to describe the image to
@@ -3032,6 +3038,7 @@ PNG_EXPORT(238, void, png_image_free, (png_imagep image));
  *    values do not correspond to the colors in sRGB.
  * colormap_entries: set to the number of entries in the color-map (0 to 256)
  */
+#ifdef PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
 PNG_EXPORT(239, int, png_image_write_to_file, (png_imagep image,
    const char *file, int convert_to_8bit, const void *buffer,
    png_int_32 row_stride, const void *colormap));
@@ -3041,8 +3048,9 @@ PNG_EXPORT(240, int, png_image_write_to_stdio, (png_imagep image, FILE *file,
    int convert_to_8_bit, const void *buffer, png_int_32 row_stride,
    const void *colormap));
    /* Write the image to the given (FILE*). */
+#endif /* SIMPLIFIED_WRITE_STDIO */
 
-/* With both write APIs if image is in one of the linear formats with 16-bit
+/* With all write APIs if image is in one of the linear formats with 16-bit
  * data then setting convert_to_8_bit will cause the output to be an 8-bit PNG
  * gamma encoded according to the sRGB specification, otherwise a 16-bit linear
  * encoded PNG file is written.
@@ -3054,13 +3062,102 @@ PNG_EXPORT(240, int, png_image_write_to_stdio, (png_imagep image, FILE *file,
  *
  * With all APIs row_stride is handled as in the read APIs - it is the spacing
  * from one row to the next in component sized units (1 or 2 bytes) and if
- * negative indicates a bottom-up row layout in the buffer.  If row_stride is zero,
- * libpng will calculate it for you from the image width and number of channels.
+ * negative indicates a bottom-up row layout in the buffer.  If row_stride is
+ * zero, libpng will calculate it for you from the image width and number of
+ * channels.
  *
- * Note that the write API does not support interlacing, sub-8-bit pixels, indexed
- * PNG (color_type 3) or most ancillary chunks.
+ * Note that the write API does not support interlacing, sub-8-bit pixels or
+ * st ancillary chunks.  If you need to write text chunks (e.g. for copyright
+ * notices) you need to use one of the other APIs.
  */
-#endif /* STDIO */
+
+PNG_EXPORT(245, int, png_image_write_to_memory, (png_imagep image, void *memory,
+   png_alloc_size_t * PNG_RESTRICT memory_bytes, int convert_to_8_bit,
+   const void *buffer, png_int_32 row_stride, const void *colormap));
+   /* Write the image to the given memory buffer.  The function both writes the
+    * whole PNG data stream to *memory and updates *memory_bytes with the count
+    * of bytes written.
+    *
+    * 'memory' may be NULL.  In this case *memory_bytes is not read however on
+    * success the number of bytes which would have been written will still be
+    * stored in *memory_bytes.  On failure *memory_bytes will contain 0.
+    *
+    * If 'memory' is not NULL it must point to memory[*memory_bytes] of
+    * writeable memory.
+    *
+    * If the function returns success memory[*memory_bytes] (if 'memory' is not
+    * NULL) contains the written PNG data.  *memory_bytes will always be less
+    * than or equal to the original value.
+    *
+    * If the function returns false and *memory_bytes was not changed an error
+    * occured during write.  If *memory_bytes was changed, or is not 0 if
+    * 'memory' was NULL, the write would have succeeded but for the memory
+    * buffer being too small.  *memory_bytes contains the required number of
+    * bytes and will be bigger that the original value.
+    */
+
+#define png_image_write_get_memory_size(image, size, convert_to_8_bit, buffer,\
+   row_stride, colormap)\
+   png_image_write_to_memory(&(image), 0, &(size), convert_to_8_bit, buffer,\
+         row_stride, colormap)
+   /* Return the amount of memory in 'size' required to compress this image.
+    * The png_image structure 'image' must be filled in as in the above
+    * function and must not be changed before the actual write call, the buffer
+    * and all other parameters must also be indentical to that in the final
+    * write call.  The 'size' variable need not be initialized.
+    *
+    * NOTE: the macro returns true/false, if false is returned 'size' will be
+    * set to zero and the write failed and probably will fail if tried again.
+    */
+
+/* You can pre-allocate the buffer by making sure it is of sufficient size
+ * regardless of the amount of compression achieved.  The buffer size will
+ * always be bigger than the original image and it will never be filled.  The
+ * following macros are provided to assist in allocating the buffer.
+ */
+#define PNG_IMAGE_DATA_SIZE(image) (PNG_IMAGE_SIZE(image)+(image).height)
+   /* The number of uncompressed bytes in the PNG byte encoding of the image;
+    * uncompressing the PNG IDAT data will give this number of bytes.
+    *
+    * NOTE: while PNG_IMAGE_SIZE cannot overflow for an image in memory this
+    * macro can because of the extra bytes used in the PNG byte encoding.  You
+    * need to avoid this macro if your image size approaches 2^30 in width or
+    * height.  The same goes for the remainder of these macros; they all produce
+    * bigger numbers than the actual in-memory image size.
+    */
+#ifndef PNG_ZLIB_MAX_SIZE
+#  define PNG_ZLIB_MAX_SIZE(b) ((b)+(((b)+7U)>>3)+(((b)+63U)>>6)+11U)
+   /* An upper bound on the number of compressed bytes given 'b' uncompressed
+    * bytes.  This is based on deflateBounds() in zlib; different
+    * implementations of zlib compression may conceivable produce more data so
+    * if your zlib implementation is not zlib itself redefine this macro
+    * appropriately.
+    */
+#endif
+
+#define PNG_IMAGE_COMPRESSED_SIZE_MAX(image)\
+   PNG_ZLIB_MAX_SIZE((png_alloc_size_t)PNG_IMAGE_DATA_SIZE(image))
+   /* An upper bound on the size of the data in the PNG IDAT chunks. */
+
+#define PNG_IMAGE_PNG_SIZE_MAX_(image, image_size)\
+   ((8U/*sig*/+25U/*IHDR*/+16U/*gAMA*/+44U/*cHRM*/+12U/*IEND*/+\
+    (((image).format&PNG_FORMAT_FLAG_COLORMAP)?/*colormap: PLTE, tRNS*/\
+     12U+PNG_IMAGE_COLORMAP_SIZE(image)/*PLTE+tRNS data*/+\
+     (((image).format&PNG_FORMAT_FLAG_ALPHA)?12U/*tRNS*/:0U):0U)+\
+    12U)+(12U*((image_size)/PNG_ZBUF_SIZE))/*IDAT*/+(image_size))
+   /* A helper for the following macro; if your compiler cannot handle the
+    * following macro use this one with the result of
+    * PNG_IMAGE_COMPRESSED_SIZE_MAX(image) as the second argument (most
+    * compilers should handle this just fine.)
+    */
+
+#define PNG_IMAGE_PNG_SIZE_MAX(image)\
+   PNG_IMAGE_PNG_SIZE_MAX_(image, PNG_IMAGE_COMPRESSED_SIZE_MAX(image))
+   /* An upper bound on the total length of the PNG data stream for 'image'.
+    * The result is of type png_alloc_size_t, on 32-bit systems this may
+    * overflow even though PNG_IMAGE_DATA_SIZE does not overflow; the write will
+    * run out of buffer space but return a corrected size which should work.
+    */
 #endif /* SIMPLIFIED_WRITE */
 /*******************************************************************************
  *  END OF SIMPLIFIED API
@@ -3118,7 +3215,7 @@ PNG_EXPORT(244, int, png_set_option, (png_structrp png_ptr, int option,
  * one to use is one more than this.)
  */
 #ifdef PNG_EXPORT_LAST_ORDINAL
-  PNG_EXPORT_LAST_ORDINAL(244);
+  PNG_EXPORT_LAST_ORDINAL(245);
 #endif
 
 #ifdef __cplusplus

pngread.c

@@ -4071,58 +4071,84 @@ png_image_finish_read(png_imagep image, png_const_colorp background,
 {
    if (image != NULL && image->version == PNG_IMAGE_VERSION)
    {
-      png_uint_32 check;
+      /* Check for row_stride overflow.  This check is not performed on the
+       * original PNG format because it may not occur in the output PNG format
+       * and libpng deals with the issues of reading the original.
+       */
+      const unsigned int channels = PNG_IMAGE_PIXEL_CHANNELS(image->format);
 
-      if (row_stride == 0)
-         row_stride = PNG_IMAGE_ROW_STRIDE(*image);
+      if (image->width <= 0x7FFFFFFFU/channels) /* no overflow */
+      {
+         png_uint_32 check;
+         const png_uint_32 png_row_stride = image->width * channels;
 
-      if (row_stride < 0)
-         check = -row_stride;
+         if (row_stride == 0)
+            row_stride = (png_int_32)/*SAFE*/png_row_stride;
 
-      else
-         check = row_stride;
+         if (row_stride < 0)
+            check = -row_stride;
 
-      if (image->opaque != NULL && buffer != NULL &&
-         check >= PNG_IMAGE_ROW_STRIDE(*image))
-      {
-         if ((image->format & PNG_FORMAT_FLAG_COLORMAP) == 0 ||
-            (image->colormap_entries > 0 && colormap != NULL))
+         else
+            check = row_stride;
+
+         if (image->opaque != NULL && buffer != NULL && check >= png_row_stride)
          {
-            int result;
-            png_image_read_control display;
-
-            memset(&display, 0, (sizeof display));
-            display.image = image;
-            display.buffer = buffer;
-            display.row_stride = row_stride;
-            display.colormap = colormap;
-            display.background = background;
-            display.local_row = NULL;
-
-            /* Choose the correct 'end' routine; for the color-map case all the
-             * setup has already been done.
+            /* Now check for overflow of the image buffer calculation; this
+             * limits the whole image size to 32 bits for API compatibility with
+             * the current, 32-bit, PNG_IMAGE_BUFFER_SIZE macro.
              */
-            if ((image->format & PNG_FORMAT_FLAG_COLORMAP) != 0)
-               result =
-                  png_safe_execute(image, png_image_read_colormap, &display) &&
-                  png_safe_execute(image, png_image_read_colormapped, &display);
+            if (image->height <= 0xFFFFFFFF/png_row_stride)
+            {
+               if ((image->format & PNG_FORMAT_FLAG_COLORMAP) == 0 ||
+                  (image->colormap_entries > 0 && colormap != NULL))
+               {
+                  int result;
+                  png_image_read_control display;
+
+                  memset(&display, 0, (sizeof display));
+                  display.image = image;
+                  display.buffer = buffer;
+                  display.row_stride = row_stride;
+                  display.colormap = colormap;
+                  display.background = background;
+                  display.local_row = NULL;
+
+                  /* Choose the correct 'end' routine; for the color-map case
+                   * all the setup has already been done.
+                   */
+                  if ((image->format & PNG_FORMAT_FLAG_COLORMAP) != 0)
+                     result = png_safe_execute(image,
+                                    png_image_read_colormap, &display) &&
+                              png_safe_execute(image,
+                                    png_image_read_colormapped, &display);
 
-            else
-               result =
-                  png_safe_execute(image, png_image_read_direct, &display);
+                  else
+                     result =
+                        png_safe_execute(image,
+                              png_image_read_direct, &display);
+
+                  png_image_free(image);
+                  return result;
+               }
 
-            png_image_free(image);
-            return result;
+               else
+                  return png_image_error(image,
+                     "png_image_finish_read[color-map]: no color-map");
+            }
+
+            else
+               return png_image_error(image,
+                  "png_image_finish_read: image too large");
          }
 
          else
             return png_image_error(image,
-               "png_image_finish_read[color-map]: no color-map");
+               "png_image_finish_read: invalid argument");
       }
 
       else
          return png_image_error(image,
-            "png_image_finish_read: invalid argument");
+            "png_image_finish_read: row_stride too large");
    }
 
    else if (image != NULL)

pngwrite.c

@@ -12,9 +12,9 @@
  */
 
 #include "pngpriv.h"
-#if defined(PNG_SIMPLIFIED_WRITE_SUPPORTED) && defined(PNG_STDIO_SUPPORTED)
+#ifdef PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
 #  include <errno.h>
-#endif
+#endif /* SIMPLIFIED_WRITE_STDIO */
 
 #ifdef PNG_WRITE_SUPPORTED
 
@@ -1452,7 +1452,6 @@ png_write_png(png_structrp png_ptr, png_inforp info_ptr,
 
 
 #ifdef PNG_SIMPLIFIED_WRITE_SUPPORTED
-# ifdef PNG_STDIO_SUPPORTED /* currently required for png_image_write_* */
 /* Initialize the write structure - general purpose utility. */
 static int
 png_image_write_init(png_imagep image)
@@ -1504,6 +1503,10 @@ typedef struct
    png_const_voidp first_row;
    ptrdiff_t       row_bytes;
    png_voidp       local_row;
+   /* Byte count for memory writing */
+   png_bytep        memory;
+   png_alloc_size_t memory_bytes; /* not used for STDIO */
+   png_alloc_size_t output_bytes; /* running total */
 } png_image_write_control;
 
 /* Write png_uint_16 input to a 16-bit PNG; the png_ptr has already been set to
@@ -1931,9 +1934,43 @@ png_image_write_main(png_voidp argument)
       png_set_benign_errors(png_ptr, 0/*error*/);
 #   endif
 
-   /* Default the 'row_stride' parameter if required. */
-   if (display->row_stride == 0)
-      display->row_stride = PNG_IMAGE_ROW_STRIDE(*image);
+   /* Default the 'row_stride' parameter if required, also check the row stride
+    * and total image size to ensure that they are within the system limits.
+    */
+   {
+      const unsigned int channels = PNG_IMAGE_PIXEL_CHANNELS(image->format);
+
+      if (image->width <= 0x7FFFFFFFU/channels) /* no overflow */
+      {
+         png_uint_32 check;
+         const png_uint_32 png_row_stride = image->width * channels;
+
+         if (display->row_stride == 0)
+            display->row_stride = (png_int_32)/*SAFE*/png_row_stride;
+
+         if (display->row_stride < 0)
+            check = -display->row_stride;
+
+         else
+            check = display->row_stride;
+
+         if (check >= png_row_stride)
+         {
+            /* Now check for overflow of the image buffer calculation; this
+             * limits the whole image size to 32 bits for API compatibility with
+             * the current, 32-bit, PNG_IMAGE_BUFFER_SIZE macro.
+             */
+            if (image->height > 0xFFFFFFFF/png_row_stride)
+               png_error(image->opaque->png_ptr, "memory image too large");
+         }
+
+         else
+            png_error(image->opaque->png_ptr, "supplied row stride too small");
+      }
+
+      else
+         png_error(image->opaque->png_ptr, "image row stride too large");
+   }
 
    /* Set the required transforms then write the rows in the correct order. */
    if ((format & PNG_FORMAT_FLAG_COLORMAP) != 0)
@@ -2110,6 +2147,122 @@ png_image_write_main(png_voidp argument)
    return 1;
 }
 
+
+static void (PNGCBAPI
+image_memory_write)(png_structp png_ptr, png_bytep/*const*/ data,
+   png_size_t size)
+{
+   png_image_write_control *display = png_voidcast(png_image_write_control*,
+      png_ptr->io_ptr/*backdoor: png_get_io_ptr(png_ptr)*/);
+   const png_alloc_size_t ob = display->output_bytes;
+
+   /* Check for overflow; this should never happen: */
+   if (size <= ((png_alloc_size_t)-1) - ob)
+   {
+      /* I don't think libpng ever does this, but just in case: */
+      if (size > 0)
+      {
+         if (display->memory_bytes >= ob+size) /* writing */
+            memcpy(display->memory+ob, data, size);
+
+         /* Always update the size: */
+         display->output_bytes = ob+size;
+      }
+   }
+
+   else
+      png_error(png_ptr, "png_image_write_to_memory: PNG too big");
+}
+
+static void (PNGCBAPI
+image_memory_flush)(png_structp png_ptr)
+{
+   PNG_UNUSED(png_ptr)
+}
+
+static int
+png_image_write_memory(png_voidp argument)
+{
+   png_image_write_control *display = png_voidcast(png_image_write_control*,
+      argument);
+
+   /* The rest of the memory-specific init and write_main in an error protected
+    * environment.  This case needs to use callbacks for the write operations
+    * since libpng has no built in support for writing to memory.
+    */
+   png_set_write_fn(display->image->opaque->png_ptr, display/*io_ptr*/,
+         image_memory_write, image_memory_flush);
+
+   return png_image_write_main(display);
+}
+
+int PNGAPI
+png_image_write_to_memory(png_imagep image, void *memory,
+   png_alloc_size_t * PNG_RESTRICT memory_bytes, int convert_to_8bit,
+   const void *buffer, png_int_32 row_stride, const void *colormap)
+{
+   /* Write the image to the given buffer, or count the bytes if it is NULL */
+   if (image != NULL && image->version == PNG_IMAGE_VERSION)
+   {
+      if (memory_bytes != NULL && buffer != NULL)
+      {
+         /* This is to give the caller an easier error detection in the NULL
+          * case and guard against uninitialized variable problems:
+          */
+         if (memory == NULL)
+            *memory_bytes = 0;
+
+         if (png_image_write_init(image) != 0)
+         {
+            png_image_write_control display;
+            int result;
+
+            memset(&display, 0, (sizeof display));
+            display.image = image;
+            display.buffer = buffer;
+            display.row_stride = row_stride;
+            display.colormap = colormap;
+            display.convert_to_8bit = convert_to_8bit;
+            display.memory = png_voidcast(png_bytep, memory);
+            display.memory_bytes = *memory_bytes;
+            display.output_bytes = 0;
+
+            result = png_safe_execute(image, png_image_write_memory, &display);
+            png_image_free(image);
+
+            /* write_memory returns true even if we ran out of buffer. */
+            if (result)
+            {
+               /* On out-of-buffer this function returns '0' but still updates
+                * memory_bytes:
+                */
+               if (memory != NULL && display.output_bytes > *memory_bytes)
+                  result = 0;
+
+               *memory_bytes = display.output_bytes;
+            }
+
+            return result;
+         }
+
+         else
+            return 0;
+      }
+
+      else
+         return png_image_error(image,
+            "png_image_write_to_memory: invalid argument");
+   }
+
+   else if (image != NULL)
+      return png_image_error(image,
+         "png_image_write_to_memory: incorrect PNG_IMAGE_VERSION");
+
+   else
+      return 0;
+}
+
+#ifdef PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
 int PNGAPI
 png_image_write_to_stdio(png_imagep image, FILE *file, int convert_to_8bit,
    const void *buffer, png_int_32 row_stride, const void *colormap)
@@ -2117,7 +2270,7 @@ png_image_write_to_stdio(png_imagep image, FILE *file, int convert_to_8bit,
    /* Write the image to the given (FILE*). */
    if (image != NULL && image->version == PNG_IMAGE_VERSION)
    {
-      if (file != NULL)
+      if (file != NULL && buffer != NULL)
       {
          if (png_image_write_init(image) != 0)
          {
@@ -2167,7 +2320,7 @@ png_image_write_to_file(png_imagep image, const char *file_name,
    /* Write the image to the named file. */
    if (image != NULL && image->version == PNG_IMAGE_VERSION)
    {
-      if (file_name != NULL)
+      if (file_name != NULL && buffer != NULL)
       {
          FILE *fp = fopen(file_name, "wb");
 
@@ -2225,6 +2378,6 @@ png_image_write_to_file(png_imagep image, const char *file_name,
    else
       return 0;
 }
-# endif /* STDIO */
+#endif /* SIMPLIFIED_WRITE_STDIO */
 #endif /* SIMPLIFIED_WRITE */
 #endif /* WRITE */

scripts/pnglibconf.dfa

@@ -873,9 +873,12 @@ option SIMPLIFIED_READ_BGR enables FORMAT_BGR,
 
 # Write:
 option SIMPLIFIED_WRITE,
-   requires WRITE STDIO, SETJMP, WRITE_SWAP, WRITE_PACK,
+   requires WRITE, SETJMP, WRITE_SWAP, WRITE_PACK,
       WRITE_tRNS, WRITE_gAMA, WRITE_sRGB, WRITE_cHRM
 
+# 1.6.22: allow simplified write without stdio support:
+option SIMPLIFIED_WRITE_STDIO requires SIMPLIFIED_WRITE STDIO
+
 option SIMPLIFIED_WRITE_AFIRST enables FORMAT_AFIRST,
    requires SIMPLIFIED_WRITE WRITE_SWAP_ALPHA
 

scripts/pnglibconf.h.prebuilt

@@ -109,6 +109,7 @@
 #define PNG_SIMPLIFIED_READ_SUPPORTED
 #define PNG_SIMPLIFIED_WRITE_AFIRST_SUPPORTED
 #define PNG_SIMPLIFIED_WRITE_BGR_SUPPORTED
+#define PNG_SIMPLIFIED_WRITE_STDIO_SUPPORTED
 #define PNG_SIMPLIFIED_WRITE_SUPPORTED
 #define PNG_STDIO_SUPPORTED
 #define PNG_STORE_UNKNOWN_CHUNKS_SUPPORTED

scripts/symbols.def

@@ -249,3 +249,4 @@ EXPORTS
  png_set_check_for_invalid_index @242
  png_get_palette_max @243
  png_set_option @244
+ png_image_write_to_memory @245