cgo调用内存泄露

package main

/*
   #include <stdlib.h>
   #include <malloc.h>
   void mal(char **p) {
    int l = 1000000;
    *p= (char *)malloc(l);
    *(*p+l-2) = 'a';
    *(*p+l-1) = 'b';
   }
*/
import "C"
import (
    "fmt"
    "sync"
    "time"
    "unsafe"
)

func main() {
    time.Sleep(time.Second * 10)
    var wg sync.WaitGroup
    for i := 0; i < 1000; i++ {
        wg.Add(1)
        go func() {
            var p *C.char

            C.mal(&p)
            defer C.free(unsafe.Pointer(p))
            l := 1000000
            ss := C.GoStringN(p, C.int(l))
            if string(ss[l-2:]) != "ab" {
                fmt.Println("not ab")
            }

            wg.Done()
        }()
    }
    wg.Wait()
    fmt.Println("complete!")
    time.Sleep(time.Second * 300)
}

C.GoStringN(p, C.int(l))会产生内存泄露，生成的string不会被GC，大家有遇到这个情况吗