4 ms·

Another great one ; http://icube-icps.unistra.fr/img_with.php/d/db/ModernC.pdf http://icube-icps.unistra.fr/img_with.php/d/db/ModernC.pdf

mirekrusin 10y ago
It's already in the list.
- splesjaz 10y ago
  Oops just woke up
aktau 10y ago
Scrolling randomly through the document my eyes almost popped when I saw [Level 3, Chapter 16: Performance]: A pointer to a collection of objects of unknown number. These functions should use the VLA notation: void func(size_t n, double a[n]); I had no idea this was possible. Do compilers actually do anything with this information or is it just informative? EDIT: I didn't get a warning out of gcc #include <stddef.h> int last(size_t n, int a[n]) { return a[n-1]; } int main() { int a[] = {0, 1, 2, 3, 4}; size_t siz = sizeof(a) / sizeof(a[0]); return last(siz+1, a); } Compile: $ cc --version cc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4 Copyright (C) 2013 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. $ cc -Wall -Wextra -std=c11 staticarr.c -o staticarr && ./staticarr ; echo $? 254 clang 3.8 warns about differing things: $ clang-3.8 -Weverything -std=c11 staticarr.c -o staticarr && ./staticarr ; echo $? staticarr.c:3:25: warning: variable length array used [-Wvla] int last(size_t n, int a[n]) { ^ staticarr.c:3:5: warning: no previous prototype for function 'last' [-Wmissing-prototypes] int last(size_t n, int a[n]) { ^ 2 warnings generated. Still, it's a neat idea from a documentation point of view, and GNU-like compilers seem to accept it. A bit before is more explanation of on the arr[static ?] notation. I was aware of this and usually prefer to use (Clang/GCC) function annotations to indicate NULL-ness of pointers and such, but I see the advantage in some situations. I don't agree with everything in the document, but it contains a lot of cool things.
- kps 10y ago
  VLAs were added in C99, but not universally adopted (particularly by embedded systems compilers, for obvious reasons), and then made optional in C11.
  - pantalaimon 10y ago
    I don't see an obvious reason - that's all information that's available on compile-time, there should be no additional run-time cost - or what are you implying?
    - gcp 10y ago
      VLA allows for run-time size determination, not compile time
  - DSMan195276 10y ago
    You're completely right, but I think it's worth clarifying that VLA notation in parameter lists, and actual VLAs are two completely different things. The example he showed doesn't actually use a VLA, it just looks like it does.
- DSMan195276 10y ago
  I think it's worth clarifying that arrays declared in parameter lists are simply just pointers. It's 100% syntactic sugar. So the `int a[n]` is simply treated as `int * a`, and the value inside of the `[]` is ignored completely. This leads to the weird situation that you declared it as `int a[n]`, but `sizeof(a)` just gives the size of a pointer, not the size of the array. The only special case is `int a[static n]`, which does tell the compiler that `a` has at-least `n` entries. But `a` is still just a pointer in this instance, so `sizeof(a)` still gives the size of a pointer. Personally, I'd recommend avoiding the syntax purely because of the `sizeof` issue, but its up to you.
linker3000 10y ago
That link did not work. From the article though: http://icube-icps.unistra.fr/img_auth.php/d/db/ModernC.pdf http://icube-icps.unistra.fr/img_auth.php/d/db/ModernC.pdf

mirekrusin 10y ago

It's already in the list.

splesjaz 10y ago
Oops just woke up

aktau 10y ago

Scrolling randomly through the document my eyes almost popped when I saw [Level 3, Chapter 16: Performance]: A pointer to a collection of objects of unknown number. These functions should use the VLA notation: void func(size_t n, double a[n]); I had no idea this was possible. Do compilers actually do anything with this information or is it just informative? EDIT: I didn't get a warning out of gcc #include <stddef.h> int last(size_t n, int a[n]) { return a[n-1]; } int main() { int a[] = {0, 1, 2, 3, 4}; size_t siz = sizeof(a) / sizeof(a[0]); return last(siz+1, a); } Compile: $ cc --version cc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4 Copyright (C) 2013 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. $ cc -Wall -Wextra -std=c11 staticarr.c -o staticarr && ./staticarr ; echo $? 254 clang 3.8 warns about differing things: $ clang-3.8 -Weverything -std=c11 staticarr.c -o staticarr && ./staticarr ; echo $? staticarr.c:3:25: warning: variable length array used [-Wvla] int last(size_t n, int a[n]) { ^ staticarr.c:3:5: warning: no previous prototype for function 'last' [-Wmissing-prototypes] int last(size_t n, int a[n]) { ^ 2 warnings generated. Still, it's a neat idea from a documentation point of view, and GNU-like compilers seem to accept it. A bit before is more explanation of on the arr[static ?] notation. I was aware of this and usually prefer to use (Clang/GCC) function annotations to indicate NULL-ness of pointers and such, but I see the advantage in some situations. I don't agree with everything in the document, but it contains a lot of cool things.

kps 10y ago
VLAs were added in C99, but not universally adopted (particularly by embedded systems compilers, for obvious reasons), and then made optional in C11.
- pantalaimon 10y ago
  I don't see an obvious reason - that's all information that's available on compile-time, there should be no additional run-time cost - or what are you implying?
  - gcp 10y ago
    VLA allows for run-time size determination, not compile time
- DSMan195276 10y ago
  You're completely right, but I think it's worth clarifying that VLA notation in parameter lists, and actual VLAs are two completely different things. The example he showed doesn't actually use a VLA, it just looks like it does.
DSMan195276 10y ago
I think it's worth clarifying that arrays declared in parameter lists are simply just pointers. It's 100% syntactic sugar. So the `int a[n]` is simply treated as `int * a`, and the value inside of the `[]` is ignored completely. This leads to the weird situation that you declared it as `int a[n]`, but `sizeof(a)` just gives the size of a pointer, not the size of the array. The only special case is `int a[static n]`, which does tell the compiler that `a` has at-least `n` entries. But `a` is still just a pointer in this instance, so `sizeof(a)` still gives the size of a pointer. Personally, I'd recommend avoiding the syntax purely because of the `sizeof` issue, but its up to you.

linker3000 10y ago

That link did not work. From the article though: http://icube-icps.unistra.fr/img_auth.php/d/db/ModernC.pdf http://icube-icps.unistra.fr/img_auth.php/d/db/ModernC.pdf