how to initialize struct array members inside function by reference - c

Question

im trying to make an array of struct and initialize struct array member, but I don't know how to access struct member, i used (st->ch)[t] = 'c'; and others similar syntax but i did not succeed.
best regards.
struct ST
{
char ch;
};
bool init(ST* st, int num)
{
st = (ST*)malloc(num * sizeof(ST));
if (st == NULL) return false;
for (int t = 0; t < num; t++) (st->ch)[t] = 'c';
return true;
}
int main()
{
ST* s = NULL;
init(s, 2);
putchar(s[1].ch);
}

Answer 1

You declared in main a pointer
ST* s = NULL;
that in C shall be declared like
struct ST* s = NULL;
because you declared the type specifier struct ST (that in C is not the same as just ST)
struct ST
{
char ch;
};
that you are going to change within a function. To do that you have to pass the pointer to the function by reference. That is the function declaration will look at least like
bool init( struct ST **st, int num );
and the function is called like
init( &s, 2);
if ( s ) putchar( s[1].ch );
The function itself can be defined like
bool init( struct ST **st, int num )
{
*st = malloc( num * sizeof( struct ST ) );
if ( *st )
{
for ( int i = 0; i < num; i++) ( *st )[i].ch = 'c';
}
return *st != NULL;
}
If you are using a C++ compiler then substitute this statement
*st = malloc( num * sizeof( struct ST ) );
for
*st = ( struct ST * )malloc( num * sizeof( struct ST ) );
When the array of structures will not be needed you should free the memory occupied by the array like
free( s );

Answer 2

You can access struct member using following:
st[t].ch

Answer 3

As mentioned by #kaylum st being a local variable in the init() and doesn't update the variable s in the main function hence an alternative could be you either pass the add the address of the variable s to init() or could just return the allocated memory as shown in the code snippet below. Instead of using bool as return type to check you can use the ST* as return type and if it returns NULL or mem address to get mem alloc status.
Also you would have to typedef the struct typedef struct ST ST; to be able to directly use the type as ST else you would have to stick to using struct ST
typedef struct ST
{
char ch;
}ST;
ST* init(int num)
{
ST *st;
// Create num elems of ST type
st = (ST*)malloc(num * sizeof(ST));
// return NULL is st unintialised
if (st == NULL) {
return st;
}
// Assign ch member variable of the 't'th st element wit 'c'
for (int t = 0; t < num; t++) {
st[t].ch = 'c';
}
return st;
}
int main()
{
ST* s;
// creates an array of size two of type st
s = init(2);
putchar(s[1].ch);
return 0;
}