I want to calculate the length of an (at first) uninitialized char array.
My code is:
#include <stdio.h>
int main()
{
char *string_t;
int loc = sizeof(string_t)/sizeof(*string_t);
printf("%d", loc);
}
I expect loc to be 0 but instead loc is 8. Can someone tell me why this is and how I can "fix" this to be the answer I expect?
sizeof is a compile time operator (with the exception of VLA). The value of sizeof(string_t)/sizeof(*string_t) is equivalent to sizeof(char *) / sizeof(char), which is the size of a pointer, usually 8 on 64-bit machines.
For uninitialized char * pointer, there is no correct size, you have to initialize it:
char *string_t = "";
and use strlen(string_t) to get 0. However, since it's a pointer to a string literal, it's somehow pointless though.
string_t and then check the size again, strlen causes a pointer error for me
char string_t[1024] = "", or use dynamic allocation.you don't have a char array, you have a char *. To get what you want you need to something like :-
#include <stdio.h>
int main()
{
char string[100];
string[0]=0;
int loc = strlen(string);
printf("%d", loc);
}
char *string_t is a pointer, not an uninitialized char array. Its size is fixed to whatever is the size of a pointer on your system, which appears to be eight.
sizeof(*string_t) is the same as sizeof(char), so it is fixed at 1 by the C standard. There is no way to produce the behavior that you expect without changing from pointers to arrays.
If you would like to make an array that you resize by realloc-ing, you need to make a struct that keeps track of both the pointer and the size (or has two pointers pointing to the beginning and to the end of the allocated block).
struct dynamic_char_array {
char *array;
size_t size;
};
void add_char(struct dynamic_char_array* a, char c) {
char *tmp = realloc(a->array, (a->size)+1);
if (tmp) {
a->array = tmp;
} else {
// Do something about a failed allocation
...
return;
}
a->array[size++] = c;
}
Note: You could also use variable-length arrays, but once their size is set, it cannot be changed.
To calculate the length of an uninitialized char array, you have to declare one first. In your code, you wrote char *string_t;. This statement declares a pointer to char, rather than an array of char.
An uninitialized char array should be declared using something like char s[1024];. To create an initialized one, use char s[1024] = "foo";, or char s[1024] = {'f', 'o', 'o'};.
So, actually, you are trying to calculate the length of an undeclared array, which is obviously impossible.
'char *' is just pointer to char, not char array.
so you can't count array.
use like this
char array[100];
int len = sizeof(array);
0exactly?sizeof(string_t)/sizeof(*string_t)is how you find the length of a char array[ ].