The sizeof Operator in C Language
You can use the sizeof operator to obtain the size (in bytes) of the data type of its operand. The operand may be an actual type specifier (such as int or float), as well as any valid expression. When the operand is a type name, it must be enclosed in parentheses. Here are some examples:
size_t a = sizeof(int);
size_t b = sizeof(float);
size_t c = sizeof(5);
size_t d = sizeof(5.143);
size_t e = sizeof a;
The result of the sizeof operator is of a type called size_t, which is defined in the header file <stddef.h>. size_t is an unsigned integer type, perhaps identical to unsigned int or unsigned long int; it varies from system to system.
The size_t type if often a conventient type for a look index, since it is guaranteed to be able to hold the number of elements in any array; this is not the case with int, for example.
The sizeof operator can be used to automatically compute the number of elements in an array:
#include <stddef.h>
#include <stdio.h>
static const int values[] = {1, 2, 48, 681 };
#define ARRAYSIZE(x) (sizeof x / sizeof x[0])
int main(int argc, char *argv[])
{
size_t i;
for(i = 0; i < ARRAYSIZE(values); i++) {
printf("%d\n", values[i]);
}
return 0;
}
There are two cases where this technique does not work. The first is where the array element has zero size (GCC supports zero-sized structures as a GNU extension). The second is where the array is in fact a function parameter.