pointers - How to include a dynamic array INSIDE a struct in C? -

i have looked around have been unable find solution must asked question. here code have:

 #include <stdlib.h>  struct my_struct {     int n;     char s[] };  int main() {     struct my_struct ms;     ms.s = malloc(sizeof(char*)*50); } 

and here error gcc gives me: error: invalid use of flexible array member

i can compile if declare declaration of s inside struct

char* s 

and superior implementation (pointer arithmetic faster arrays, yes?) thought in c declaration of

char s[] 

is same as

char* s 

the way have written , used called "struct hack", until c99 blessed "flexible array member". reason you're getting error (probably anyway) needs followed semicolon:

#include <stdlib.h>  struct my_struct {     int n;     char s[]; }; 

when allocate space this, want allocate size of struct plus amount of space want array:

struct my_struct *s = malloc(sizeof(struct my_struct) + 50); 

in case, flexible array member array of char, , sizeof(char)==1, don't need multiply size, other malloc you'd need if array of other type:

struct dyn_array {      int size;     int data[]; };  struct dyn_array* my_array = malloc(sizeof(struct dyn_array) + 100 * sizeof(int)); 

edit: gives different result changing member pointer. in case, (normally) need 2 separate allocations, 1 struct itself, , 1 "extra" data pointed pointer. using flexible array member can allocate data in single block.