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POLosing data when sending by UDP
 

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  1. POLosing data when sending by UDP
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    <p>I've written a UDP send/receive function to send a struct and listen for another struct back. The bytes have to be sent in a particular order, but this is working OK as I'm using <code>#pragma pack(1)</code>. The only problem that I'm having now is that if any <code>Null</code> values (<code>0x00</code>) appear in the struct, the rest of the data after the <code>Null</code> disappears.</p> <p>I guess there's something fairly simple that I'm doing wrong, but here is my code:</p> <pre><code>typedef u_int8_t NN; typedef u_int8_t X; typedef int32_t S; typedef u_int32_t U; typedef char C; typedef struct{ X test; NN test2[2]; C test3[4]; S test4; } Test; int main(int argc, char** argv) { Test t; memset( &amp;t, 0, sizeof(t)); t.test = 0xde; t.test2[0]=0xad; t.test2[1]=0x00; t.test3[0]=0xbe; t.test3[1]=0xef; t.test3[2]=0xde; t.test3[3]=0xca; t.test4=0xde; LogOnResponse response; udp_send_receive(&amp;t, &amp;response); return 0; } </code></pre> <p>And here is my send/receive function:</p> <pre><code>int send_and_receive(void* message, void* reply, int do_send, int expect_reply) { struct sockaddr_in serv_addr; int sockfd, i, slen=sizeof(serv_addr); int buflen = BUFLEN; void* buf = NULL; struct timeval tv; int n_timeouts=1; int recv_retval; // printf("Message Size: %d\n", strlen(message)); if ( (strlen(message)) &gt;= BUFLEN) err("Message too big"); buf = malloc(buflen); if ((sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1) err("socket"); tv.tv_sec = timeout_seconds; tv.tv_usec = timeout_microseconds; if( setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO,&amp;tv,sizeof(tv)) &lt; 0 ){ err("Setting Timout"); } bzero(&amp;serv_addr, sizeof(serv_addr)); serv_addr.sin_family = AF_INET; serv_addr.sin_port = htons(PORT); if (inet_aton(IP_ADDRESS, &amp;serv_addr.sin_addr)==0) err("inet_aton() failed\n"); //---Timeout Send/Receive loop do{ if(do_send == TRUE){ strcpy(buf, message); if (sendto(sockfd, buf, buflen, 0, (struct sockaddr*)&amp;serv_addr, slen)==-1) err("sendto()"); } if (expect_reply == TRUE){ if( (recv_retval = recvfrom(sockfd, buf, buflen, 0, (struct sockaddr*)&amp;serv_addr, &amp;slen)) == -1){ itercount++; } } }while ((itercount &lt; itermax) &amp;&amp; (recv_retval == -1)); if ( itercount != itermax ){ memcpy(reply, buf, BUFLEN); } else{ reply=NULL; } close(sockfd); free(buf); return 0; } void udp_send_receive(void* message, void* reply) { send_and_receive(message, reply, TRUE, TRUE); } </code></pre> <p>Running the above code and capturing the packets with <code>WireShark</code> shows:</p> <p><code>Data: DEAD000000000000000000....</code></p> <p>I'd like it to show:</p> <p><code>Data: DEAD00BEEFDECADE</code></p> <p>I'd really appreciate some pointers on this.</p>
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    1. USJames Chong
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    1. COYou might also want to check what e.g. `sendto` and `recvfrom` returns, because if they send/receive correctly they return the actual length of the data sent/received. No need to copy more data than was actually received.
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      1. USSome programmer dude
 

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