array of variable size in c

Question

I have this code below which creates an array of variable size that compiles and runs fine on my mac.

#include <stdio.h>
#include <stdlib.h>

int main(){
    int w = 100;
    int ar[w];
    ar[2] = 42;
    printf("%d\n",ar[2]);
}

I thought variable sized arrays were not permitted in C. what is happening here exactly? How is memory being managed? Does the memory get dynamically allocated at run time? Thanks

Answer 1

What's happening here is that variable-length arrays are a relatively new feature, only first appearing in the C standard in 1999. C standards are not adopted as quickly in the C world as JavaScript and Python; I can remember the excitement in 2007 or so when my workflow was finally able to include VLAs.

To add insult to injury, while C++ is a mostly-superset of C, VLAs are not supported, meaning that the common use-case of folks compiling C code with C++ compilers will not work.

There was enough pushback from the compiler vendors that the standard eventually (C11) made VLAs optional, mandating the feature-test macro __STDC_NO_VLA__ instead.

(see ISO stadndard 9899:2011 Programming Languages - C, section 6.7.6.2 4)