从C中的列表解引用结构中的变量

Question

I am trying to dereference variables in a struct that is in a list, so here is my list and struct

struct packet {
 unsigned short block_num;
 unsigned short block_size;
 unsigned short crc;
 unsigned char  *payload;
}packet;

/* Function and type declaration for list of packets */

typedef struct List{
    struct packet *p;
    struct List *next;
}List;

And here I am trying to do a toString():

char *tostring(List *h) {
 char *str= malloc(STRSIZE);
 char num[sizeof(List)*100];
 str[0] = '\0';
 while (h != NULL) {
     sprintf(num, "package Number: %d \n", h->p->block_num);
     sprintf(num, "block size: %d \n", h->p->block_size);
     sprintf(num, "CRC: %d", h->p->crc);
     strncat(str, num, STRSIZE - strlen(str) - 1);
     h = h->next;
 }
 return str;
}

And I hit a segmentation fault, please help, thanks !

So, in C#, I will use list.ElementAt(i).block_num , how is it done in C ?

edit: new ones, the problem lies on my dereferencing

char *tostring(List *h) {
 char *str= malloc(STRSIZE);
 printf("line1");
 char num[sizeof(List)*100];
 char size[sizeof(List)*100];
 char crcs[sizeof(List)*100];
 char messages[sizeof(List)*100];
while (h != NULL) {
    sprintf(num, "package Number: %d \n", h->p->block_num);      
    sprintf(size, "block size: %d \n", h->p->block_size);       
    sprintf(crcs, "CRC: %d", h->p->crc);
    sprintf(messages, "CRC: %s", h->p->payload);
    strncat(str,num,sizeof(num));
    strncat(str,size,sizeof(size));
    strncat(str,crcs,sizeof(crcs));
    strncat(str,messages,strlen(num));
    h = h->next;

}
return str;
}

Answer 1

Are you using sizeof(*h) to determine the number of List elements in an array?

sizeof(*h) returns the size of List . Since tostring() only get's a pointer to a List , it has no way of knowing how many elements are pointed to.

In any case, it appears you are then writing a string to a character array of this size. What does the size of List have to do with the number of elements in the array? What are you trying to do?

Answer 2

There are few things that may go wrong here

1. Your SRTSIZE may be low to hold all the text that you start appending to str. For that you can start with a big buffer for now to eliminiate that.

2. As posted from previous answers you are always overwriting num. You should write to 'str' after you create num every time

3. strncat implementation seems incorrect. You should use strlen(num).

   strncat(str, num, strlen(num));

char * strncat ( char * destination, const char * source, size_t num ); The final parameter expects Maximum number of characters to be appended.

refer : http://www.cplusplus.com/reference/cstring/strncat/