c - Dynamic array of Strings inside a Struct realloc -

i have array of strings inside struct looks this

#define id_len 20  struct person{ char name[id_len]; int num_items; char **items; };  int main(){ int num_persons = 10; struct person *ptr[num_persons]; 

i start array of 10 persons. persons have 0 items list of items malloc(0).

for(int = 0; < num_persons; i++){         ptr[i] = (struct person *) malloc(sizeof(struct person) + 1);         ptr[i]->num_items = 0;         ptr[i]->items = malloc(0 * sizeof(char*)); } 

at point want name persons , add them items this.

strcpy(ptr[0]->name, "john"); ptr[0]->num_items = 1; ptr[0]->items = (char **)realloc(ptr[0]->items, ptr[0]->num_items * sizeof(char*)); strcpy(ptr[0]->items[0], "pencil");   printf("name: %s\n", ptr[0]->name); printf("number of items: %d\n", ptr[0]->num_items); for(int = 0; < ptr[0]->num_items; i++){     printf("item %d %s\n", i, ptr[0]->items[i]); } 

i'm getting segmentation fault: 11. i'm not sure if have done realloc correctly or not.

you have several problems here

first id_len

nowhere check if copying name surpass id_len

so instead of

strcpy(ptr[0]->name, "john");  

use

strcpy_s(ptr[0]->name,sizeof(ptr[0]->name),"john or whatever"); // c11 

you allocate

ptr[0]->items = (char **)realloc(ptr[0]->items, ptr[0]->num_items * sizeof(char*)) 

but

you should not cast return value of malloc/realloc (search on web explanation, has been described ad nauseum)

when using realloc, should first check return value, may fail.

char** tmp = realloc(ptr[0]->items, ptr[0]->num_items * sizeof(char*)) if (tmp != null) {   ptr[0]->items = tmp; } else {   abort(); } 

the memory realloc returns partly uninitialized (old pointers remain new ones uninitialized. in case did not have previous pointers 1 items[0] uninitialized.

so when

strcpy(ptr[0]->items[0], "pencil"); 

it fail since items[0] pointing arbitrary memory location.

after realloc pointers need initialize them point memory large enough accommodate string

e.g.

ptr[0]->items[0] = strdup("pencil");  // malloc copies string 

it not efficient use realloc everytime need add 1 new item, instead allocate bunch of items set ones not using null keep track of how many left, once run out allocate bunch