3 ms·

Another neat note IIRC is that array parameter sizes don't actually do anything, they just are there for semantic purposes and get treated as raw pointers. So

Another neat note IIRC is that array parameter sizes don't actually do anything, they just are there for semantic purposes and get treated as raw pointers. So if you do void func(int x[10]); You're free to call it like int k[5]; func(k); And you won't get any warnings. Unsettling!

shift_reset 7y ago
That's what the static keyword means in those array declarators. void func(int x[static 10]); must be called with an argument that is a pointer to the start of a big enough array of int. I can't get recent GCC or Clang to warn on violations of this, though.
- spc476 7y ago
  There are cases where the compiler can't enforce it: void foo(int *p) { func(p); } How can the compiler know if `p` points to space for 10 integers?
ninkendo 7y ago
The wording in the C FAQ is that arrays “decay” into pointers when you pass them to functions. Which they explain as the reason why you can’t know the size of a passed array (at least in standard C.) The C FAQ is pretty old though, I’ve always wondered how much of that advice changed in C99/C11... from cursory googling things don’t seem to have changed much.
- cryptonector 7y ago
  It's funny that K&R chose to have arrays "decay" to pointers, but to allow structs to be passed by value. Thus you can actually pass arrays by value if and only if you wrap them in a struct: struct foo { char a[5]; }; void f(struct foo x) { x.a[4] = '\0'; printf("%s", x.a); } int main(void) { struct foo x; memcpy(x.a, "too big", sizeof(x.a)); f(x) printf("%s", x.a); /* read past end of x, crash */ return 0; } I'm thankful they didn't make structs decay into pointers!
  - jandrese 7y ago
    I figured that having arrays decay into pointers is one of those features that got grandfathered in because changing it would have broken the dozen or C programs that existed at the time. It's a real shame too because a working sizeof() for strings could have avoided a LOT of C exploits over the years.

shift_reset 7y ago

That's what the static keyword means in those array declarators. void func(int x[static 10]); must be called with an argument that is a pointer to the start of a big enough array of int. I can't get recent GCC or Clang to warn on violations of this, though.

spc476 7y ago
There are cases where the compiler can't enforce it: void foo(int *p) { func(p); } How can the compiler know if `p` points to space for 10 integers?

ninkendo 7y ago

The wording in the C FAQ is that arrays “decay” into pointers when you pass them to functions. Which they explain as the reason why you can’t know the size of a passed array (at least in standard C.) The C FAQ is pretty old though, I’ve always wondered how much of that advice changed in C99/C11... from cursory googling things don’t seem to have changed much.

cryptonector 7y ago
It's funny that K&R chose to have arrays "decay" to pointers, but to allow structs to be passed by value. Thus you can actually pass arrays by value if and only if you wrap them in a struct: struct foo { char a[5]; }; void f(struct foo x) { x.a[4] = '\0'; printf("%s", x.a); } int main(void) { struct foo x; memcpy(x.a, "too big", sizeof(x.a)); f(x) printf("%s", x.a); /* read past end of x, crash */ return 0; } I'm thankful they didn't make structs decay into pointers!
- jandrese 7y ago
  I figured that having arrays decay into pointers is one of those features that got grandfathered in because changing it would have broken the dozen or C programs that existed at the time. It's a real shame too because a working sizeof() for strings could have avoided a LOT of C exploits over the years.