C++ 使用sd设备列举设备导致seg错误

#include <stdio.h>
#include <stdlib.h>
#include <systemd/sd-device.h>
int main() {
    sd_device *device;
    sd_device_enumerator *enumerator =NULL;
    const char **subsystem =NULL;
    // *subsystem = malloc(sizeof(char *));
    *subsystem = malloc(500);
    if (subsystem == NULL)
        printf("ohh");
    // Create a device enumerator
    int r = sd_device_enumerator_new(&enumerator);
    if (r< 0)
        printf("unable to create instance");
        printf("%d\n",r);
    //sd_device_enumerator_scan(enumerator);
    for (device = sd_device_enumerator_get_device_first(enumerator); device != NULL;
         device = sd_device_enumerator_get_device_next(enumerator)) {
        
            r = sd_device_get_subsystem(device, subsystem);
            if (r < 0) 
            printf("error in subsystem");
                
                printf(" %s\n", subsystem);
                if (subsystem != NULL) 
                free((char *)subsystem); // free the previous subsystem name
                            
            
}
}

编译上述代码会导致seg错误

gcc -o dev_enum test 2. c-lsystem用于编译 请帮