Is passing a cast pointer by value valid in C? If not why is this working?
#include <stdio.h>
typedef struct
{
int size;
char* str;
} MY_STRUCT;
void print_my_struct_prt(MY_STRUCT* mstrct)
{
printf("%s%d%s%s\n", "size: ", mstrct->size, " content: ", mstrct->str);
}
int main()
{
MY_STRUCT my_ptr_strct;
my_ptr_strct.size = 3;
my_ptr_strct.str = "first test";
print_my_struct_prt(&my_ptr_strct);
print_my_struct_prt(&(MY_STRUCT) { 6, "second test"});
return 0;
}
output:
size: 3 content: first test
size: 6 content: second test
In the first test a stuct is created, the values are set, and the address is passed to print_my_struct_prt().
In the second test I can only guess the values 6 and "second test" are stored in memory and the address to the location in memory is passed to print_my_struct_prt(). Is this correct?
Aside from obvious readability issues, is this acceptable in C?
Is passing a cast pointer by value valid in C? If not why is this working?
C has only pass by value. When you pass a pointer, you pass it by value. There is nothing inherently wrong with passing a pointer value obtained via a cast.
But I don't think that's what you mean to ask. Judging from the code, I think you're concerned about passing a pointer to the result of a cast, as opposed to passing the result of casting a pointer. In that case, passing in general and pass-by-value in particular have nothing to do with it. The result of a cast expression is not an lvalue, so you cannot take its address in the first place. You cannot obtain a pointer to the result of a cast.
On the third hand, the code you've presented demonstrates something yet again different. Although this construct ...
(MY_STRUCT) { 6, "second test"}
... resembles a cast expression, it is in fact a compound literal of type MY_STRUCT. Unlike the results of casts, compound literals are lvalues, so you can obtain their addresses. Having done so, you can pass the resulting pointer to a function or otherwise do whatever other thing you want that is permitted by its type.