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POFind the no. of elements in an array of structs?
 

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  1. POFind the no. of elements in an array of structs?
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    <p>It's weird because I have done it before but it just is not working. I have a structure <code>xmp_frame</code> like this.</p> <pre><code>typedef struct { short int attr_dtype; short int attr_code; short int attr_len; const char *attr; }xmp_frame; </code></pre> <p>Now I create an array of <code>xmp_frame</code> and use them like:</p> <pre><code>xmp_frame frame_array[]={ {1,2,strlen("Hello there"),"Hello there"}, {1,3,strlen("This is not working"),"This is not working"}, {0,3,strlen("But why??"),"But why??"} }; </code></pre> <p>Now I have a routine which basically writes the <code>frame_array</code> in to file:</p> <pre><code>short int write_frames(xmp_frame frame_array[],FILE *outfp){ } </code></pre> <p>Before I write the <code>frame_array</code> I need to get the no. of elements in <code>frame_array[]</code> for doing some processing. So this is how we do it (generally):</p> <pre><code>short int write_frames(xmp_frame frame_array[],FILE *outfp) { short intnum_frames=sizeof(frame_array) / sizeof(frame_array[0]); /*But i get the value of num_frames as 0. I will print the outout of some debugging.*/ fprintf(stderr,"\n Size of frame_array : %lu",sizeof(frame_array)); //prints 8 fprintf(stderr,"\n Size of frame_array[0] : %lu",sizeof(frame_array[0])); //prints 16 fprintf(stderr,"\n So num. of frames to write : %d", (sizeof(frame_array))/(sizeof(frame_array[0]))); //prints 0 } </code></pre> <p>Of course if <code>frame_array</code> is 8 bytes and <code>frame_array[0]</code> is 16 bytes then <code>num_frames</code> is going to be 0.</p> <p>But the question is how can size of an array be smaller than one of its elements? I heard about byte padding.</p> <p>I don't have much of an idea if it is causing the problem. Here is one of the links I referred to: <a href="https://stackoverflow.com/questions/1898657/sizeof-array-of-structs-in-c">Result of &#39;sizeof&#39; on array of structs in C?</a></p> <p>Although I have found a few workarounds to determine the size of an array of structs.</p> <ol> <li><p>Get the no. of elements from the caller and </p></li> <li><p>another is compulsorily get the last struct element as <code>{0,0,0,NULL}</code> and then in <code>write()</code> check for it's presence and stop scanning <code>frame_array</code> any further.</p></li> </ol> <p>But both depend on the caller, something which you can't trust. So where is the real problem. And how could I determine the value of <code>num_frames</code>?</p>
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