about struct in C

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Join Date: May 2006

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yuzhang

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about struct in C

May 25th, 2006

Hi all:

I am stuck in the middle of my programme, wishing you may help.

I am writing a "quene" structure in C, and it will copy the content of a string received from a server and display it on the client monitor.

Codes:

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typedef struct Message_from_server message_from_server

void sqInitialise()
{
    struct Message_from_server
    {
        char *message;
        message_from_server *next_message;
    };
}

void sqEnqueue(const char *s)
{
    Message_from_server new_message_from_server;
    new_message_from_server.message=s;
    new_message_from_server.next_message=

Here is where I got stuck, I do not really know what I should do once I create a new instance of Message_from_server. Ideally, the pointer *next_message should point to the next incoming message, but I do not know how to realize this concept.

Thanks

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Ancient Dragon Ancient Dragon is offline

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Re: about struct in C

May 25th, 2006

This looks like it should be a linked list of messages. You need to allocate an object that represents the head of the linked list, then add nodes to it. One way might be like this

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void sqEnqueue(struct Message_from_server** head, const char *s)
{
    struct Message_from_server* new_node = malloc(sizeof(struct Message_from_server));
    new_node->s = malloc(strlen(s)+1);
    strcpy(new_node->s,s);
    new_node->next_message = NULL;
    if( *head == NULL)
    {
        *head = new_node;
    }
    else
    { // add new_node to the end of the linked list
              // locate end of list
            struct Message_from_server* tail = *head;
            while(tail->next_message != NULL)
                tail = tail->next_message;
            // found end of list, now add the new node to it
            tail->next_message = new_node;
     }
}

// this is how to call the above function
int main()
{
   struct Message_from_server* head = NULL; // top of linked list
   sqEnqueue( &head, "Hello");
   sqEnqueue( &head, "World");

// don't forget to delete the list when done with it.
}

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Join Date: May 2006

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yuzhang

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Re: about struct in C

May 25th, 2006

Thanks dragon. Would not have figured out without you.
:-)

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Join Date: May 2006

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yuzhang

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malloc()

May 25th, 2006

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Originally Posted by Ancient Dragon

This looks like it should be a linked list of messages. You need to allocate an object that represents the head of the linked list, then add nodes to it. One way might be like this
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void sqEnqueue(struct Message_from_server** head, const char *s)
{
    struct Message_from_server* new_node = malloc(sizeof(struct Message_from_server));
    new_node->s = malloc(strlen(s)+1);
    strcpy(new_node->s,s);
    new_node->next_message = NULL;
    if( *head == NULL)
    {
        *head = new_node;
    }
    else
    { // add new_node to the end of the linked list
              // locate end of list
            struct Message_from_server* tail = *head;
            while(tail->next_message != NULL)
                tail = tail->next_message;
            // found end of list, now add the new node to it
            tail->next_message = new_node;
     }
}

// this is how to call the above function
int main()
{
   struct Message_from_server* head = NULL; // top of linked list
   sqEnqueue( &head, "Hello");
   sqEnqueue( &head, "World");

// don't forget to delete the list when done with it.
} 
void sqEnqueue(struct Message_from_server** head, const char *s) { struct Message_from_server* new_node = malloc(sizeof(struct Message_from_server)); new_node->s = malloc(strlen(s)+1); strcpy(new_node->s,s); new_node->next_message = NULL; if( *head == NULL) { *head = new_node; } else { // add new_node to the end of the linked list // locate end of list struct Message_from_server* tail = *head; while(tail->next_message != NULL) tail = tail->next_message; // found end of list, now add the new node to it tail->next_message = new_node; } } // this is how to call the above function int main() { struct Message_from_server* head = NULL; // top of linked list sqEnqueue( &head, "Hello"); sqEnqueue( &head, "World"); // don't forget to delete the list when done with it. }

Hi dragon:

You used two malloc() functions above, one is to allocate memory space for the struct, and another one is to allocate memory space for the string which is a member of the struct.
Just thinking: how does the first malloc() function know how much memory it needs to allocate for the struct before knowing the length of the string?

Thanks