No they’re not absolutely the same. While both let you store the same number and type of objects, keep in mind that: You can free() a malloced array, but you can’t free() a variable length array (although it goes out of scope and ceases to exist once the enclosing block is left). In technical jargon, … Read more
For reasons that are not entirely clear to me, almost every time the topic of C99 VLA pops up in a discussion, people start talking predominantly about the possibility of declaring run-time-sized arrays as local objects (i.e. creating them “on the stack”). This is rather surprising and misleading, since this facet of VLA functionality – … Read more
Yes, alloca is functionally equivalent to a local variable length array, i.e. this: int arr[n]; and this: int *arr = alloca(n * sizeof(int)); both allocate space for n elements of type int on the stack. The only differences between arr in each case is that 1) one is an actual array and the other is … Read more
VLA does have some overhead (compared to “ordinary” named compile-time-sized array). Firstly, it has run-time length and yet the language provides you with means to obtain the actual size of the array at run-time (using sizeof). This immediately means that the actual size of the array has to be stored somewhere. This results in some … Read more
This is not a “static memory allocation”. Your array k is a Variable Length Array (VLA), which means that memory for this array is allocated at run time. The size will be determined by the run-time value of n. The language specification does not dictate any specific allocation mechanism, but in a typical implementation your … Read more
[*] What is its purpose, why was it added? Purpose is seen when a variable length two dimentional array is used as a function parameter. The function int foo(int n, int m, int a[n][m]) {…} can be prototyped as any of the following int foo(int , int, int [][*]); int foo(int , int, int a[*][*]); … Read more
There are several different pieces at play here. The first is the difference between declaring an array as int array[n]; and int* array = malloc(n * sizeof(int)); In the first version, you are declaring an object with automatic storage duration. This means that the array lives only as long as the function that calls it … Read more
In order to answer the question, we should first clear up some concepts. What is an array and how can it be used? And what is the code in the question, if not an array? What is an array? The formal definition of an array is found in the C standard, ISO 9899:2011 6.2.5/20 Types. … Read more
Obviously a hack… but a way of squaring a number without using the * operator (this was a coding contest requirement). (&a)[n] is equivalent to a pointer to int at location (a + sizeof(a[n])*n) and thus the entire expression is (&a)[n] -a = (a + sizeof(a[n])*n -a) /sizeof(int) = sizeof(a[n])*n / sizeof(int) = sizeof(int) * … Read more
I am assuming that you are using a C99 compiler (with support for dynamically sized arrays). The problem in your code is that at the time when the compilers sees your variable declaration it cannot know how many elements there are in the array (I am also assuming here, from the compiler error that length … Read more