这里的stpncpy()有问题吗？

Question

这是一个现有代码,它找到与父路径相对应的相对路径.它已经在平台上正常工作,除了SUSE 12和GCC 4.7. 我已经提到了一个错误的输出,并在评论中预期. 我想知道为什么会这样？ 这段代码有什么问题？ 子字符串和父字符串都以NUL字符结尾.我看到的唯一另一件事是源和目标从同一个内存位置传递,换句话说我们正在尝试更新相同内存位置的值.这是真正的问题吗？

//child = /dev/shm/4/tmp/backup/datadir/performance_schema/events_stages_summary_by_account_by_event_name.frm 
//parent =  /dev/shm/4/tmp/backup

char* get_relative_path(char *child, const char *parent)
{
    char* start= child;
    static char dot[] = "." ;

    ....
    ....

    /* Check if child = parent + "/" + ....  */
    if (strstr(child, parent) == child)
    {
        int parent_len= strlen(parent);

        /* parent path may or may not have the "/" suffix. check for both. */
        if (parent[parent_len-1] == FN_LIBCHAR ||
            child[parent_len] == FN_LIBCHAR)
        {
            child+= parent_len;
            while (*child && *child == FN_LIBCHAR)
                child++;
            if (*child == 0)
                return dot;
        }
    }
   // At this point the value of 
   // start = /dev/shm/4/tmp/backup/datadir/performance_schema/events_stages_summary_by_account_by_event_name.frm 
   // child = datadir/performance_schema/events_stages_summary_by_account_by_event_name.frm

    if(start != child)
    {
        stpncpy(start, child, PATH_MAX);
    }

   // At this point  expected value of  start = datadir/performance_schema/events_stages_summary_by_account_by_event_name.frm
   // But actual value of start =  datadir/performance_schenamevents_stages_summary_by_account_by_event_name.frm 

   return start;
}

Answer 1

来自Man Page stpncpy:The strings may not overlap.

所以你应该使用memmove,否则行为是未定义的.

stpncpy(start, child, PATH_MAX); 应该
memmove(start, child, PATH_MAX > strlen(child) + 1? strlen(child) + 1: PATH_MAX);

您还应该记住,stpncpy如果在大型数组上使用小字符串,效率非常低,因为它n每次都会写字符.在你的情况下,它会'\0'在你的字符串后写入大约4000 .
Man Page说

在dest处写入正好n个字符.如果长度strlen(src)小于n,则dest指向的数组中的剩余字符将填充空字节('\ 0')