Example of gcc forward parameter declaration

main.c

// Compile with: gcc -Wall -std=gnu99 main.c

#include <stddef.h>
#include <stdlib.h>

// see https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html
void foo(size_t length; char buf[length], size_t length) {
  for (size_t i=0; i<length; i++) {
    buf[i] = 0;
  }
}

int main(void) {
  char* buf = malloc(100);
  foo(buf, 128);

  return 0;
}