ctime()返回一个字符串,为什么我们不需要free()这个字符串的内存？

函数ctime的原型是

char *ctime(const time_t *timep);

我们可以看到,它返回一个字符串.但是,哪里有刺痛？

为什么我们不应该释放字符串的内存

这是示例代码将获得大量错误消息

char *p;
p = ctime(...);
...
free(p);

*** glibc检测到*** ./a.out:free()：指针无效：0x00007f0b365b4e60 ***

解决方法

它返回一个指向静态缓冲区的指针,并且不能是free()d.从 man ctime开始：

The four functions asctime(),ctime(),gmtime() and localtime() return a pointer to static data and hence are not thread-safe.

C99标准,第7.23.3.2节ctime函数声明调用ctime(timer)函数等同于asctime(localtime(timer)),而asctime()实现(如同一文档中所示)等效于：

char *asctime(const struct tm *timeptr)
{
    static const char wday_name[7][3] = {
        "Sun","Mon","Tue","Wed","Thu","Fri","Sat"
    };

    static const char mon_name[12][3] = {
        "Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"
    };

    static char result[26];
    sprintf(result,"%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",wday_name[timeptr->tm_wday],mon_name[timeptr->tm_mon],timeptr->tm_mday,timeptr->tm_hour,timeptr->tm_min,timeptr->tm_sec,1900 + timeptr->tm_year);

    return result;
}

传递给free()的参数必须是仅通过调用malloc(),calloc()或realloc()返回的指针,否则行为是未定义的.

原文地址：https://www.jb51.cc/c/110762.html