tools: Allocate the priv structs with proper alignment

Previously, they could be allocated with any random alignment
matching the end of the MuxerContext/DemuxerContext. The
priv structs themselves can have members that require specific
alignment, or at least the default alignment of malloc()/calloc()
(which is sufficient for native types such as uint64_t and
doubles).

This fixes crashes in some arm builds, where GCC (correctly) wants
to use 64 bit aligned stores to write to MD5Context.

2 files changed, 6 insertions, 4 deletions

diff --git a/tools/input/input.c b/tools/input/input.c
index 4756648..2dfaa97 100644
--- a/tools/input/input.c
+++ b/tools/input/input.c
@@ -41,6 +41,7 @@
 struct DemuxerContext {
     DemuxerPriv *data;
     const Demuxer *impl;
+    uint64_t priv_data[];
 };
 
 extern const Demuxer ivf_demuxer;
@@ -109,12 +110,12 @@ int input_open(DemuxerContext **const c_out,
         }
     }
 
-    if (!(c = calloc(1, sizeof(DemuxerContext) + impl->priv_data_size))) {
+    if (!(c = calloc(1, offsetof(DemuxerContext, priv_data) + impl->priv_data_size))) {
         fprintf(stderr, "Failed to allocate memory\n");
         return DAV1D_ERR(ENOMEM);
     }
     c->impl = impl;
-    c->data = (DemuxerPriv *) &c[1];
+    c->data = (DemuxerPriv *) c->priv_data;
     if ((res = impl->open(c->data, filename, fps, num_frames, timebase)) < 0) {
         free(c);
         return res;
diff --git a/tools/output/output.c b/tools/output/output.c
index cf66d2e..f8910d4 100644
--- a/tools/output/output.c
+++ b/tools/output/output.c
@@ -46,6 +46,7 @@ struct MuxerContext {
     unsigned fps[2];
     const char *filename;
     int framenum;
+    uint64_t priv_data[];
 };
 
 extern const Muxer null_muxer;
@@ -123,12 +124,12 @@ int output_open(MuxerContext **const c_out,
         }
     }
 
-    if (!(c = malloc(sizeof(MuxerContext) + impl->priv_data_size))) {
+    if (!(c = malloc(offsetof(MuxerContext, priv_data) + impl->priv_data_size))) {
         fprintf(stderr, "Failed to allocate memory\n");
         return DAV1D_ERR(ENOMEM);
     }
     c->impl = impl;
-    c->data = (MuxerPriv *) &c[1];
+    c->data = (MuxerPriv *) c->priv_data;
     int have_num_pattern = 0;
     for (const char *ptr = filename ? strchr(filename, '%') : NULL;
          !have_num_pattern && ptr; ptr = strchr(ptr, '%'))