Suppose I have the following code:
typedef struct
{
char **p;
} STRUCT;
int main()
{
STRUCT s;
*(s.p) = "hello";
printf("%s\n", *(s.p));
return 0;
}
which obviously doesn't work, but it should show what I want to do. How would I go about initialising, accessing, printing, etc the array of strings in the structure?
You have two * where you want just one, I think. Try:
typedef struct
{
char *p;
} STRUCT;
int main()
{
STRUCT s;
s.p = "hello";
printf("%s\n", s.p);
return 0;
}
If you do really want to have the double indirection, you need to allocate some space for the pointer you're dereferencing. *(s.p) in your original program dereferences an uninitialized pointer. In this case:
typedef struct
{
char **p;
} STRUCT;
int main()
{
STRUCT s;
s.p = malloc(sizeof(char *));
*(s.p) = "hello";
printf("%s\n", *(s.p));
free(s.p);
return 0;
}
This second program allocates space for just one string pointer; if you want an array, just allocate the appropriate amount of space.
There is no array at the moment, but I assume you want to create one. You need to first allocate as many char *s as you want strings:
int main()
{
STRUCT s;
int N = 10; // number of strings you want
s.p = (char **)malloc(N * sizeof(char *));
s.p[0] = "hello";
s.p[1] = "world";
...
printf("%s\n", s.p[0]);
free(s.p);
return 0;
}
char** p, the first pointer points to a string and the second one points to the characters in the string.
You're going to need to know how many strings are contained in the array, either by adding a count member to the struct or by using a NULL sentinel value. The following examples use the NULL sentinel:
Allocating and initializing:
STRUCT s;
s.p = malloc(sizeof *s.p * (number_of_strings + 1));
if (s.p)
{
size_t i;
for (i = 0; i < number_of_strings; i++)
{
s.p[i] = malloc(length_of_ith_string + 1);
if (s.p[i])
strcpy(s.p[i], ith_string);
}
s.p[i] = NULL;
}
for appropriate values of number_of_strings, length_of_ith_string, and ith_string.
Accessing/printing:
for (i = 0; s.p[i] != NULL; i++)
printf("String %d: %s\n", i, s.p[i]);
Deallocating:
for (i = 0; s.[i] != NULL; i++)
free(s.p[i]);
free(s.p);
