Casting struct pointers in C

Question

when I compiled the following code:

AVFrameSideData* avfsd=NULL;
avfsd = av_frame_get_side_data(frame, AV_FRAME_DATA_MOTION_VECTORS);
if(avfsd != NULL) {
    int n_mvs = 0;
    if(avfsd->size > 0)
        n_mvs = avfsd->size / sizeof(struct AVMotionVector);
    struct AVMotionVector *mv = (struct AVMotionVector *) (avfsd->data);
    printf("motion vectors of this frame:\n");
    int idx;
    for(idx=0; idx<n_mvs; idx++) {
        printf("mv[%d]:\n", idx);
        if(mv[idx]->source < 0)
            printf("the current macroblock comes from the past\n");
    }
}

my compiler complained:

video_analysis.c:46:13: error: invalid type argument of ‘->’ (have ‘struct AVMotionVector’)
   if(mv[idx]->source < 0)

Frankly, there's something wrong here:

struct AVMotionVector *mv = (struct AVMotionVector *) (avfsd->data);

mv is a struct pointer, thereof it's not allowed to access through a two dimensional array manner like *mv[]. However, when I ran it in GDB, I was able to print out things like the following:

(gdb) print mv[0]
$2 = {source = -1, w = 16 '\020', h = 16 '\020', src_x = 6, src_y = 10, dst_x = 8, dst_y = 8, flags = 0, motion_x = -5, motion_y = 4, 
  motion_scale = 2}

which is exactly what I want to print out...As you can see, mv[0], which can't be accessed from my original code spinet above can now be accessed in GDB mode. How does GDB make this magic happen? If I want to access mv[0] in my program just like in GDB, how should I cast those struct pointers? Thanks in advance :-)

Indentation doesn't match with the block scopes. Please edit and fix it. — user694733
– user694733, Commented May 4, 2016 at 13:09
avfsd->data seems to be pointer to array of struct AVMotionVectors. If so, then mv[idx] is valid way to access those structures. However, in that case mv[idx]->source should probably be replaced with mv[idx].source, because you want only one redirection. — user694733
– user694733, Commented May 4, 2016 at 13:24
I noticed that there is not a single comment in your code - for your own sanity, fix that bad habit. — KevinDTimm
– KevinDTimm, Commented May 4, 2016 at 13:25

Jabberwocky · Accepted Answer · 2016-05-04 13:24:13Z

2

Change

if(mv[idx]->source < 0)

to

if(mv[idx].source < 0)

Explanation

mv is a pointer to struct AVMotionVector
mv[idx] is a struct AVMotionVector and therefore you need . instead of ->.

answered May 4, 2016 at 13:24

Jabberwocky

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