[FFmpeg-devel] [PATCH 1/3] lavformat: Prepare to make avio_enum_protocols const correct

[Andreas Rheinhardt]

Using avio_enum_protocols works as follows: One initializes a pointer to
void and gives avio_enum_protocols the address of said pointer as
argument; the pointer will be updated to point to a member of the
url_protocols array. Now the address of the pointer can be reused for
another call to avio_enum_protocols.
Said array consists of constant pointers (to constant URLProtocols),
but the user now has a pointer to non-const to it; of course it was always
intended that the user is not allowed to modify what the pointer points
to and this has been enforced by hiding the real type of the underlying
object. But it is better to use a const void ** as parameter to enforce
this. This way avio_enum_protocols can be implemented without resorting
to casting a const away or ignoring constness as is done currently.

Given that this amounts to an ABI and API break, this can only be done
at the next major version bump; as usual, the break is currently hidden
behind an appropriate #if.

It was also necessary to change the type of a pointer used in
avio_enum_protocols. This makes the line that is not const correct move
as long as the old function signature is used. With the new signature,
avio_enum_protocols will be const correct.

This change will eventually force changes in their callers, e.g. to
show_protocols in fftools/cmdutils. (This function contains all currently
existing calls to avio_enum_protocols in FFmpeg's codebase. It hasn't
been touched in this commit.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
---
 libavformat/avio.h      | 4 ++++
 libavformat/protocols.c | 6 +++++-
 libavformat/version.h   | 3 +++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/libavformat/avio.h b/libavformat/avio.h
index 9141642e75..e067ea8985 100644
--- a/libavformat/avio.h
+++ b/libavformat/avio.h
@@ -805,7 +805,11 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer);
  *
  * @return A static string containing the name of current protocol or NULL
  */
+#if FF_API_NONCONST_ENUM_PROTOCOLS
 const char *avio_enum_protocols(void **opaque, int output);
+#else
+const char *avio_enum_protocols(const void **opaque, int output);
+#endif
 
 /**
  * Pause and resume playing - only meaningful if using a network streaming
diff --git a/libavformat/protocols.c b/libavformat/protocols.c
index ad95659795..c722f9a897 100644
--- a/libavformat/protocols.c
+++ b/libavformat/protocols.c
@@ -91,9 +91,13 @@ const AVClass *ff_urlcontext_child_class_next(const AVClass *prev)
 }
 
 
+#if FF_API_NONCONST_ENUM_PROTOCOLS
 const char *avio_enum_protocols(void **opaque, int output)
+#else
+const char *avio_enum_protocols(const void **opaque, int output)
+#endif
 {
-    const URLProtocol **p = *opaque;
+    const URLProtocol * const *p = *opaque;
 
     p = p ? p + 1 : url_protocols;
     *opaque = p;
diff --git a/libavformat/version.h b/libavformat/version.h
index 9814db8633..b0b9264382 100644
--- a/libavformat/version.h
+++ b/libavformat/version.h
@@ -106,6 +106,9 @@
 #ifndef FF_API_AVIOFORMAT
 #define FF_API_AVIOFORMAT               (LIBAVFORMAT_VERSION_MAJOR < 59)
 #endif
+#ifndef FF_API_NONCONST_ENUM_PROTOCOLS
+#define FF_API_NONCONST_ENUM_PROTOCOLS  (LIBAVFORMAT_VERSION_MAJOR < 59)
+#endif
 
 
 #ifndef FF_API_R_FRAME_RATE
-- 
2.21.0