Brian Dessent wrote: > > chimi wrote: > >> I've just heard that teachers from one of the university of technology >> say >> that: >> >> int a[atoi(argv[1])]; >> >> is correct way to dynamic allocation... what is more it looks like x = >> a[999999]; works! o_O >> but I run memusage test-prog 1000000 and it doesn't call *alloc nor free >> at >> all, but works well... >> >> in the other hand >> int a[32]; >> doesn't allow to acces its 1000000 element (it's ok of course) >> >> I wanted to know what gcc does with atoi(argv[2])? Does it only OS >> allocate >> so huge heap so it works? > > This is not a question about gcc, it's a question about the C language, > so it's off topic for this list. A better place would be the comp.std.c > newsgroup. > > There are two different things going on here. > > When you declare "char foo[n]" and n is a compile-time constant then you > get a standard array of characters allocated on the stack (assuming this > is in a function not a global file-level scope.) There will be no call > to any allocate or free function, as that is how the stack works -- you > just decrease a pointer. IF there was not enough memory remaining on > the stack then you generally get a runtime fault and the program aborts. > > In traditional C the 'n' must be a compile-time constant. If you try to > compile your atoi() example on a ANSI or C90 compiler, it will fail. It > is only recently in C99 that the standard allows for a new type of > array, called variable length array (VLA) in which the 'n' can be > unknown at compile time, as in your example. VLAs work just like > alloca(), i.e. the memory still comes from the stack and is invalidated > whenever the block goes out of scope. Thus it's only valid for > allocations that are small and temporary. > > It is debatable whether use of VLAs should really be called "the correct > way for dynamic allocation." For one thing, it's a C99 feature and > there are many compilers out there that have not implemented parts or > any of C99, so it makes your code very much less portable. > > Also there is the general argument of stack vs. heap allocation, and > there are many circumstances where it would be a very bad idea to try to > use a VLA in place of malloc(). Your teachers should go over all of > this. > > Brian > > Thank you very much for the explanation. Now I understand why and how it works. chimi -- View this message in context: http://www.nabble.com/Strange-memory-allocation-int-a-atoi%28argv-1-%29---%21-tf3902478.html#a11202139 Sent from the gcc - Help mailing list archive at Nabble.com.
