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POGetting at C binary data from OCaml
 

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    <p>(Ignoring endianness for the sake of argument - this is just a test case/proof of concept - and I would never use <code>strcpy</code> in real code either!)</p> <p>Consider the following trivial C code:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; /* variables of type message_t will be stored contiguously in memory */ typedef struct { int message_id; char message_text[80]; } message_t; int main(int argc, char**argv) { message_t* m = (message_t*)malloc(sizeof(message_t)); m-&gt;message_id = 1; strcpy(m-&gt;message_text,"the rain in spain falls mainly on the plain"); /* write the memory to disk */ FILE* fp = fopen("data.dat", "wb"); fwrite((void*)m, sizeof(int) + strlen(m-&gt;message_text) + 1, 1, fp); fclose(fp); exit(EXIT_SUCCESS); } </code></pre> <p>The file it writes can easily be read back in from disk:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; typedef struct { int message_id; char message_text[80]; } message_t; int main(int argc, char**argv) { message_t* m = (message_t*)malloc(sizeof(message_t)); FILE* fp = fopen("data.dat", "rb"); fread((void*)m, sizeof(message_t), 1, fp); fclose(fp); /* block of memory has structure "overlaid" onto it */ printf("message_id=%d, message_text='%s'\n", m-&gt;message_id, m-&gt;message_text); exit(EXIT_SUCCESS); } </code></pre> <p>E.g.</p> <pre><code>$ ./write $ ./read message_id=1, message_text='the rain in spain falls mainly on the plain' </code></pre> <p>My question is, in OCaml, if all I have is:</p> <pre><code>type message_t = {message_id:int; message_text:string} </code></pre> <p>How would I get at that data? <code>Marshal</code> can't do it, nor can <code>input_binary_int</code>. I can call out to helper functions in C like "what is <code>sizeof(int)</code>" then get n bytes and call a C function to "convert these bytes into an int" for example but in this case I can't add any new C code, the "unpacking" has to be done in OCaml, based on what I know it "should" be. Is it just a matter of iterating over the string either in blocks of <code>sizeof</code>s or looking for '\0' or is there a clever way? Thanks!</p>
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    1. CODon't know about OCaml; if there are padding bytes between the members of the struct, the statement `fwrite((void*)m, sizeof(int) + strlen(m->message_text) + 1, 1, fp);` will not write what you might expect. For portability I suggest you write each struct member by itself. Also you're not reading the same number of bytes you are writing.
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    2. COYep - that however is representative of my problem!
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