Free不会释放作为va_arg列表发送的指针

Question

I want to free a variable list of pointer with the freeS macro below: freeS(s1,s2,...);

My code doesn't free the first pointer despite getting a print with the first pointer address from the freeSF function. In main, free(s1) works but it should. free(s2) in main crashes as expected.

How can I free the s1 pointer in the freeSF function?

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

#define freeS(...) freeSF("", __VA_ARGS__, NULL)
void freeSF(char *paramType, ...) {
    va_list pl;

    va_start(pl, paramType);
    paramType = va_arg(pl, char *);
    while (paramType) {
        printf("arg %p\n", paramType);
        free(paramType);
        paramType = va_arg(pl, char *);
    }
    va_end(pl);
}

int main(int ARGC, char** ARGV) {
    char *s1 = strdup("1");
    char *s2 = strdup("2");
    printf("s1 %p, s2 %p\n", s1, s2);
    freeS(s1, s2);
    free(s1);
}

Answer 1

Your program exhibits undefined behavior because you are double freeing.

With undefined behavior, anything can happen. Your program may crash, it may output strange results, or (in this case) it may appear to work properly. Adding a seemingly unrelated change, such as a call to printf or adding an unused local variable, can change the way undefined behavior manifests itself.

In this case, calling free(s1) in main isn't causing a crash. It's still undefined behavior. For example, when I run this code it doesn't crash. But if I add a call to malloc just before calling free(s1) , it does crash.

Just because the code can crash doesn't mean it will .