C array size given by variable -

i found code today confused me. did this:

#include <stdio.h>  int main(int argc, char **argv) {     int x = 5;     int foo[x];      foo[0] = 33;     printf("%d\n", foo[0]);     return 0; } 

my question why work?

the array foo on stack how expanded x?

i have expected thing this:

#include <stdio.h>  int main(int argc, char **argv) {     int x = 5;     int foo[] = malloc(sizeof(int)*x);      foo[0] = 33;     printf("%d\n", foo[0]);     free(foo);     return 0; } 

not prettier or but, wonder.

the snippet

int foo[x]; 

is talking advantage of called vla (variable length array) feature. introduced in c99 standard, made optional feature in c11.

this way, can create array data structure, length given (supplied) @ run-time.

point note, though created @ runtime, gcc allocates vlas on stack memory (unlike dynamic memory allocation heap memory).