pb_istream_from_buffer: add const to prototype

This commit changes the prototype for pb_istream_from_buffer from:
  pb_istream_t pb_istream_from_buffer(uint8_t *buf, size_t bufsize);
to
  pb_istream_t pb_istream_from_buffer(const uint8_t *buf, size_t bufsize);

This allows pb_istream_from_buffer users to point to const buffers
without having to inspect code (to ensure practical const-ness) and then be
forced to manually cast away const.

In order to not break compatibility with existing programs (by
introducing a const/non-const union in the pb_istream_t state) we simply
cast away the const in pb_istream_from_buffer and re-apply it when
possible in the callbacks.  Unfortunately we lose any compiler help in
the callbacks to ensure we are treating the buffer as const but manual
inspection is easy enough.

2 files changed, 14 insertions, 6 deletions

diff --git a/pb_decode.c b/pb_decode.c
index 50ada86a..0bf8befd 100644
--- a/pb_decode.c
+++ b/pb_decode.c
@@ -74,8 +74,8 @@ static const pb_decoder_t PB_DECODERS[PB_LTYPES_COUNT] = {
 
 static bool checkreturn buf_read(pb_istream_t *stream, uint8_t *buf, size_t count)
 {
-    uint8_t *source = (uint8_t*)stream->state;
-    stream->state = source + count;
+    const uint8_t *source = (const uint8_t*)stream->state;
+    stream->state = (uint8_t*)stream->state + count;
     
     if (buf != NULL)
     {
@@ -131,7 +131,7 @@ static bool checkreturn pb_readbyte(pb_istream_t *stream, uint8_t *buf)
     if (!stream->callback(stream, buf, 1))
         PB_RETURN_ERROR(stream, "io error");
 #else
-    *buf = *(uint8_t*)stream->state;
+    *buf = *(const uint8_t*)stream->state;
     stream->state = (uint8_t*)stream->state + 1;
 #endif
 
@@ -140,15 +140,23 @@ static bool checkreturn pb_readbyte(pb_istream_t *stream, uint8_t *buf)
     return true;    
 }
 
-pb_istream_t pb_istream_from_buffer(uint8_t *buf, size_t bufsize)
+pb_istream_t pb_istream_from_buffer(const uint8_t *buf, size_t bufsize)
 {
     pb_istream_t stream;
+    /* Cast away the const from buf without a compiler error.  We are
+     * careful to use it only in a const manner in the callbacks.
+     */
+    union {
+        void *state;
+        const void *c_state;
+    } state;
 #ifdef PB_BUFFER_ONLY
     stream.callback = NULL;
 #else
     stream.callback = &buf_read;
 #endif
-    stream.state = buf;
+    state.c_state = buf;
+    stream.state = state.state;
     stream.bytes_left = bufsize;
 #ifndef PB_NO_ERRMSG
     stream.errmsg = NULL;
diff --git a/pb_decode.h b/pb_decode.h
index 3d433155..16de3e04 100644
--- a/pb_decode.h
+++ b/pb_decode.h
@@ -103,7 +103,7 @@ void pb_release(const pb_field_t fields[], void *dest_struct);
  * Alternatively, you can use a custom stream that reads directly from e.g.
  * a file or a network socket.
  */
-pb_istream_t pb_istream_from_buffer(uint8_t *buf, size_t bufsize);
+pb_istream_t pb_istream_from_buffer(const uint8_t *buf, size_t bufsize);
 
 /* Function to read from a pb_istream_t. You can use this if you need to
  * read some custom header data, or to read data in field callbacks.