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POtrying to copy struct members to byte array in c
 

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  1. POtrying to copy struct members to byte array in c
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    <p>I am attempting to copy the members of a struct containing a mixture of ints, char's and arrays of chars into a byte array to send to a serial line. So far I have</p> <pre><code>struct msg_on_send { char descriptor_msg[5]; int address; char space; char cmdmsg[5]; char CR; char LF; }; void switch_output_on() { int member; struct msg_on_send SendMsg_on[sizeof member] = { }; unsigned char buffer [ sizeof SendMsg_on[0] ]; showbytes(buffer, serialize(buffer, SendMsg_on)); } /*************************************************************************** * Function: ArrayBuild * * Purpose: Uses memcopy to transfer the struct members sequentially * * into an array of char * * Arguments: * * Returns: size_t i = a count of the number of bytes in the array * ***************************************************************************/ size_t ArrayBuild(unsigned char *dst, const struct msg_on_send *object) { size_t i = 0; memcpy(&amp;dst[i], &amp;object-&gt;descriptor_msg, sizeof object-&gt;descriptor_msg); i += sizeof object-&gt;descriptor_msg; memcpy(&amp;dst[i], &amp;object-&gt;address, sizeof object-&gt;address); i += sizeof object-&gt;address; memcpy(&amp;dst[i], &amp;object-&gt;space, sizeof object-&gt;space); i += sizeof object-&gt;space; memcpy(&amp;dst[i], &amp;object-&gt;cmdmsg, sizeof object-&gt;cmdmsg); i += sizeof object-&gt;cmdmsg; memcpy(&amp;dst[i], &amp;object-&gt;CR, sizeof object-&gt;CR); i += sizeof object-&gt;CR; memcpy(&amp;dst[i], &amp;object-&gt;LF, sizeof object-&gt;LF); i += sizeof object-&gt;LF; return i; } /*********************************************************************** * Function: USARTWrite * * Purpose: Writes the array data to the USART data register * * Arguments: void *object = struct member * * size_t size = size of array remaining * * Returns: None * ***********************************************************************/ void USARTWrite(const void *object, size_t size) { const unsigned char *byte; for ( byte = object; size--; ++byte ) { printf("%02X", *byte); } putchar('\n'); } </code></pre> <p>As I obtained this code, I don't fully understand how it works. I can see that the memcpy takes each element of the struct and makes it into a serial stream indexed by the 'i' variable, but I don't know how the USARTWrite function packetises this into a string, or how to load the array with my struct initialisation.</p> <p>Sorry this post is a bit long, but I'm just starting this programming lark, and trying to get my head around this concept.</p> <p>Thanks Dave</p> <p>EDIT:</p> <p>wow, many good answers quickly - thanks guys.</p> <p>slaz: That seems logical to me, I hadn't really thought about that approach as I haven't really got my head around pointers yet, but I am beginning to see that they are an essential part of C, so I duly will have a look.</p> <ul> <li><p>This line of code sends the data to my UART, what would I replace the array containing the message contents with? It seems like I am missing a logical step here where I have a variable telling me where my structure starts and how big it is, but no array to send</p> <pre><code>USART_SendData(USART1, message_on_contents[array_count]); </code></pre></li> </ul> <p>Harper Shelby: Thank you for that description, it makes it much clearer in my mind.</p> <p>rgds</p> <p>Dave</p>
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    1. COThe work you're looking for is "serialization". Re-tagged.
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    2. COnot being picky, but this side of the atlantic we use an 's' instead of a 'z. Just semantics of english really, fair enough. Dave
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