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POHow to create an Array of Protocol Buffer Messages in C
 

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  1. POHow to create an Array of Protocol Buffer Messages in C
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    <p>BEFORE YOU READ: The calloc you'll see that I use to initialize the buffer is what's causing the problem, but I still don't know why. Statically defining the buffer array fixes the problem as you'll see if you keep reading...</p> <p>I'm writing a UDP server that is made up of 2 threads: a receive and parse thread. The receive thread listens to the socket using recvfrom and pushes the messages it receives to the received_msgs_buf array. The parse thread pops from the received_msgs_buf array and decides what to do with it.</p> <p>The received_msgs_buf array is protected by a mutex, and a semaphore signals the parse message thread to try and pop a message off the array. The problem is, every time I try to push a message I received into the received_msgs_buf I get a segfault.</p> <p>Here is how I allocated memory for the buffer:</p> <pre><code>// this is in the header file extern UXIMessage::Wrapper* received_msgs_buf; // this is in the main.cpp file that calls pthread_create() UXIMessage::Wrapper* received_msgs_buf; // This is in the init function for the receive thread, defined in the udp.cpp file received_msgs_buf = (UXIMessage::Wrapper*)calloc(MAX_NUM_MSGS_IN_QUEUE, sizeof(UXIMessage::Wrapper)); </code></pre> <p>Here is my push function called in the receive thread:</p> <pre><code>void push_to_receive_buf(UXIMessage::Wrapper uxi_msg) { pthread_mutex_lock(&amp;received_msgs_mutex); if( num_received_msgs &lt; MAX_NUM_MSGS_IN_QUEUE ) { printf("Message to put in buffer = %s\n", uxi_msg.DebugString().c_str()); printf("Num received messages = %d\n", num_received_msgs); printf("Buf = %d\n", received_msgs_buf); // THE FOLLOWING LINE SEGFAULTS received_msgs_buf[num_received_msgs++] = uxi_msg; } pthread_mutex_unlock(&amp;received_msgs_mutex); sem_post(&amp;received_msgs_sem); } </code></pre> <p>From the print statements I can see that the number of received messages is properly initialized to 0, the received message is perfectly valid and the buffer pointer is not NULL. Here's the print out:</p> <p>Message to put in buffer = message_id: OCU_HEARTBEAT ocu_heartbeat { ocu_id: 4747 }</p> <p>Num received messages = 0</p> <p>Buf = 778112</p> <p>The segfault occurs in the CopyFrom() function, which is what's called by the = operator.</p> <p>Edit: It's late, but I'll try just using a C++ std::vector tomorrow...</p> <p>Edit2: For clarification, the mutexes and semaphores are all initialized properly in the main function as follows:</p> <pre><code>pthread_mutex_init(&amp;received_msgs_mutex); pthread_mutex_init(&amp;msgs_to_send_mutex); sem_init(&amp;received_msgs_sem, 0, 0); sem_init(&amp;msgs_to_send, 0, 0); </code></pre> <p>EDIT3: THE PROBLEM IS THE CALLOC. When I statically define the received_msgs_buf as follows:</p> <pre><code> // this is in the header file extern UXIMessage::Wrapper received_msgs_buf[MAX_NUM_MSGS_IN_BUF]; // this is in the main.cpp file that calls pthread_create() UXIMessage::Wrapper received_msgs_buf[MAX_NUM_MSGS_IN_BUF]; </code></pre> <p>the code works...does anyone know what I'm doing wrong with the calloc?</p>
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