Pointers c99指针和本地时间问题\r

我指定一个变量来保存当前时间：

struct tm *cur = malloc (sizeof (cur));
time_t t = time (NULL);
localtime_r (&t, cur);

然后我打印年份。这是正确的。接下来，我进入一个循环，在该循环中，我从文件中分配一个新的可变时间值：

struct stat file_stats;
struct tm *file = malloc (sizeof (file));
lstat (argv[itor], &file_stats);
//check1
localtime_r(&file_stats.st_mtime, file);
//check2

---

在“检查1”中，

cur->tm_year

打印正确合理的值。在“检查2”处，

cur->tm_year

打印“0”。这里发生了什么？我想这与我缺少指针操作有关。任何帮助都将不胜感激，尤其是对我误解的解释

我将您的代码改编为：

这很有用，因为我周围有一些旧文件。表面上看，似乎没有问题。然而，

valgrind

有一次发作：

==50495== Memcheck, a memory error detector
==50495== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==50495== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==50495== Command: ltr 3d.c const-stuff.c ltr.c madump.c pthread-1.c pthread-2.c pthread-3.c recv.c regress.c send.c strandsort.c x.c
==50495== 
==50495== WARNING: Support on MacOS 10.8 is experimental and mostly broken.
==50495== WARNING: Expect incorrect results, assertions and crashes.
==50495== WARNING: In particular, Memcheck on 32-bit programs will fail to
==50495== WARNING: detect any errors associated with heap-allocated data.
==50495== 
==50495== Invalid write of size 4
==50495==    at 0x105C48: timesub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x1058FE: _st_localsub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x10609D: localtime_r (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x100000DE8: main (ltr.c:10)
==50495==  Address 0x10001b188 is 0 bytes after a block of size 8 alloc'd
==50495==    at 0x5686: malloc (vg_replace_malloc.c:274)
==50495==    by 0x100000DC3: main (ltr.c:8)
==50495== 
==50495== Invalid write of size 4
==50495==    at 0x105C70: timesub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x1058FE: _st_localsub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x10609D: localtime_r (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x100000DE8: main (ltr.c:10)
==50495==  Address 0x10001b198 is 16 bytes after a block of size 8 alloc'd
==50495==    at 0x5686: malloc (vg_replace_malloc.c:274)
==50495==    by 0x100000DC3: main (ltr.c:8)
==50495== 

在相当长的一段时间内，产量一直保持着类似的趋势。但它突出了问题：您应该在

malloc（）

调用中使用

sizeof（*cur）

和

sizeof（*file）

。这将产生：

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <time.h>

int main(int argc, char **argv)
{
    struct tm *cur = malloc(sizeof(*cur));
    time_t t = time (NULL);
    localtime_r(&t, cur);

    printf("curr 1 year: %d\n", cur->tm_year + 1900);

    for (int itor = 1; itor < argc; itor++)
    {
        struct stat file_stats;
        struct tm *file = malloc(sizeof(*file));
        file->tm_year = 0;
        if (lstat(argv[itor], &file_stats) == 0)
        {
            printf("curr 2 year: %d\n", cur->tm_year + 1900);
            localtime_r(&file_stats.st_mtime, file);
            printf("curr 3 year: %d\n", cur->tm_year + 1900);
            printf("file 1 year: %d\n", file->tm_year + 1900);
        }
    }
    return(0);
}

#包括
#包括
#包括
#包括
int main（int argc，字符**argv）
{
struct tm*cur=malloc（sizeof（*cur））；
时间t=时间（空）；
本地时间（r&t，cur）；
printf（“当前1年：%d\n”，当前->tm\u年+1900）；
for（int-itor=1；itortm_year=0；
if（lstat（argv[itor]，&file_stats）==0）
{
printf（“当前2年：%d\n”，当前->tm\u年+1900）；
localtime\r（&file\u stats.st\u mtime，file）；
printf（“当前3年：%d\n”，当前->tm\u年+1900）；
printf（“文件1年：%d\n”，文件->tm\u年+1900）；
}
}
返回（0）；
}

---

而且，

valgrind

提供了一份干净的健康清单。

虽然这无关紧要，但为什么要动态分配呢？为什么不分配一个

struct tm

并将其地址传递给

localtime\r（）

？我不确定为什么选择使用动态分配，但我认为这确实很重要，因为在将它们更改为

struct tm

之后，事情似乎正在运行。谢谢，我猜，但是你知道为什么会这样吗？是的：你应该在

malloc（）

调用中使用

sizeof（*cur）

和

sizeof（*file）

。我正在写一个答案，意识到这就是问题所在（多亏了

valgrind

）。哦，这很有道理。所以

malloc（sizeof（*cur））

将分配cur指向的结构所需的空间量？与以前一样，使用malloc（sizeof（cur））I只是分配指针所需的空间量。我的理解正确吗？

==50495== Memcheck, a memory error detector
==50495== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==50495== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==50495== Command: ltr 3d.c const-stuff.c ltr.c madump.c pthread-1.c pthread-2.c pthread-3.c recv.c regress.c send.c strandsort.c x.c
==50495== 
==50495== WARNING: Support on MacOS 10.8 is experimental and mostly broken.
==50495== WARNING: Expect incorrect results, assertions and crashes.
==50495== WARNING: In particular, Memcheck on 32-bit programs will fail to
==50495== WARNING: detect any errors associated with heap-allocated data.
==50495== 
==50495== Invalid write of size 4
==50495==    at 0x105C48: timesub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x1058FE: _st_localsub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x10609D: localtime_r (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x100000DE8: main (ltr.c:10)
==50495==  Address 0x10001b188 is 0 bytes after a block of size 8 alloc'd
==50495==    at 0x5686: malloc (vg_replace_malloc.c:274)
==50495==    by 0x100000DC3: main (ltr.c:8)
==50495== 
==50495== Invalid write of size 4
==50495==    at 0x105C70: timesub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x1058FE: _st_localsub (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x10609D: localtime_r (in /usr/lib/system/libsystem_c.dylib)
==50495==    by 0x100000DE8: main (ltr.c:10)
==50495==  Address 0x10001b198 is 16 bytes after a block of size 8 alloc'd
==50495==    at 0x5686: malloc (vg_replace_malloc.c:274)
==50495==    by 0x100000DC3: main (ltr.c:8)
==50495== 

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <time.h>

int main(int argc, char **argv)
{
    struct tm *cur = malloc(sizeof(*cur));
    time_t t = time (NULL);
    localtime_r(&t, cur);

    printf("curr 1 year: %d\n", cur->tm_year + 1900);

    for (int itor = 1; itor < argc; itor++)
    {
        struct stat file_stats;
        struct tm *file = malloc(sizeof(*file));
        file->tm_year = 0;
        if (lstat(argv[itor], &file_stats) == 0)
        {
            printf("curr 2 year: %d\n", cur->tm_year + 1900);
            localtime_r(&file_stats.st_mtime, file);
            printf("curr 3 year: %d\n", cur->tm_year + 1900);
            printf("file 1 year: %d\n", file->tm_year + 1900);
        }
    }
    return(0);
}