I am mindblown by this small code:
#include <stdio.h>
int main()
{
int limit = 0;
scanf("%d", &limit);
int y[limit];
for (int i = 0; i<limit; i++ ) {
y[i] = i;
}
for (int i = 0; i < limit; i++) {
printf("%d ", y[i]);
}
return 0;
}
How on earth this program is not segment-faulting as limit (size of the array) is assigned at runtime only?
Anything recently changed in C? This code shouldn't work in my understanding.
int y[limit]; is a Variable Length Array (or VLA for short) and was added in C99. If supported, it allocates the array on the stack (on systems having a stack). It's similar to using the machine- and compiler-dependent alloca function (which is called _alloca in MSVC):
Example:
#include <alloca.h>
#include <stdio.h>
int main()
{
int limit = 0;
if(scanf("%d", &limit) != 1 || limit < 1) return 1;
int* y = alloca(limit * sizeof *y); // instead of a VLA
for (int i = 0; i<limit; i++ ) {
y[i] = i;
}
for (int i = 0; i < limit; i++) {
printf("%d ", y[i]);
}
} // the memory allocated by alloca is here free'd automatically
Note that VLA:s are optional since C11, so not all C compilers support it. MSVC for example does not.
This doesnt compile in visual studio because limit "Error C2131 expression did not evaluate to a constant"
If you make limit a constexpr though then the compiler will not mind because youre telling it it wont change. You cant use 0 though as setting an array to a constant size length zero is nonsence.
What compiler does this run on for you ?