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POgetting a substruct out of a big struct in C
 

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  1. POgetting a substruct out of a big struct in C
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    <p>I'm having a very big <code>struct</code> in an existing program. This struct includes a great number of bitfields.</p> <p>I wish to save a part of it (say, 10 fields out of 150).</p> <p>An example code I would use to save the subclass is:</p> <pre><code>typedef struct {int a;int b;char c} bigstruct; typedef struct {int a;char c;} smallstruct; void substruct(smallstruct *s,bigstruct *b) { s-&gt;a = b-&gt;a; s-&gt;c = b-&gt;c; } int save_struct(bigstruct *bs) { smallstruct s; substruct(&amp;s,bs); save_struct(s); } </code></pre> <p>I also wish that selecting which part of it wouldn't be too much hassle, since I wish to change it every now and then. The naive approach I presented before is very fragile and unmaintainable. When scaling up to 20 different fields, you have to change fields both in the <code>smallstruct</code>, and in the <code>substruct</code> function.</p> <p>I thought of two better approaches. Unfortunately both requires me to use some external <a href="http://hal.cs.berkeley.edu/cil/" rel="nofollow noreferrer">CIL</a> like tool to parse my structs.</p> <p>The first approach is automatically generating the <code>substruct</code> function. I'll just set the struct of <code>smallstruct</code>, and have a program that would parse it and generate the <code>substruct</code> function according to the fields in <code>smallstruct</code>.</p> <p>The second approach is building (with C parser) a meta-information about <code>bigstruct</code>, and then write a library that would allow me to access a specific field in the struct. It would be like ad-hoc implementation of Java's class reflection.</p> <p>For example, assuming no struct-alignment, for struct</p> <pre><code>struct st { int a; char c1:5; char c2:3; long d; } </code></pre> <p>I'll generate the following meta information:</p> <pre><code>int field2distance[] = {0,sizeof(int),sizeof(int),sizeof(int)+sizeof(char)} int field2size[] = {sizeof(int),1,1,sizeof(long)} int field2bitmask[] = {0,0x1F,0xE0,0}; char *fieldNames[] = {"a","c1","c2","d"}; </code></pre> <p>I'll get the <code>i</code><sup>th</sup> field with this function:</p> <pre><code>long getFieldData(void *strct,int i) { int distance = field2distance[i]; int size = field2size[i]; int bitmask = field2bitmask[i]; void *ptr = ((char *)strct + distance); long result; switch (size) { case 1: //char result = *(char*)ptr; break; case 2: //short result = *(short*)ptr; ... } if (bitmask == 0) return result; return (result &amp; bitmask) &gt;&gt; num_of_trailing_zeros(bitmask); } </code></pre> <p>Both methods requires extra work, but once the parser is in your makefile - changing the substruct is a breeze.</p> <p>However I'd rather do that without any external dependencies.</p> <p>Does anyone have any better idea? Where my ideas any good, is there some availible implementation of my ideas on the internet?</p>
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    1. COYou're in a good direction, but macro can help describing the structures members, using #name (to stringify) and f1##name (to concatenate)
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