缓冲区上的基名进入分段错误

Question

我现在正在调整 basename，我遇到了一个非常奇怪的案例（至少对我而言）。这是代码：

char buffer[300];
char* p;

strcpy(buffer, "../src/test/resources/constraints_0020_000");
printf("%d\n", strcmp(basename("../src/test/resources/constraints_0020_000"), "constraints_0020_000")); //works as expected
printf("assert testBasename02");
printf("%d\n", strcmp(basename(buffer), "constraints_0020_000") == 0);
printf("done 1\n"); //goes in segmentation fault
printf("%d\n", strcmp(basename(&buffer), "constraints_0020_000") == 0);
printf("done 2\n"); //goes in segmentation fault
printf("%d\n", strcmp(basename(&buffer[0]), "constraints_0020_000") == 0);
printf("done 3\n"); //goes in segmentation fault
p = malloc(strlen("../src/test/resources/constraints_0020_000") +1);
strcpy(p, "../src/test/resources/constraints_0020_000");
printf("%d\n", strcmp(basename(p), "constraints_0020_000") == 0); //works as expected
free(p);
printf("all done\n");

第一个 strcmp 完全正常；令我困惑的是第二个：为什么缓冲区会出现分段错误？我尝试以不同的方式对缓冲区进行编码，但结果是一样的。

我当然可以忍受这种行为，但是...我真的不明白如果我给他喂 const char* 或缓冲区（在end 也是一个 char*).

是否有解释此行为的文档？只有我吗？我试图寻找解释，但找不到任何解释。

这里是我电脑的规格（如果你需要的话）：

OS 系统：Ubuntu 16.4（64 位在 Windows 10 64 位上虚拟化）；
CPU（我认为没用）：Intel® Core™ i5-3230M CPU @ 2.60GHz × 2;

Answer 1

根据 man page,

Bugs

In the glibc implementation of the POSIX versions of these functions they modify their argument, and segfault when called with a static string like "/usr/". [...]

基本上，

 basename("../src/test/resources/constraints_0020_000")

invokes 调用 undefined behavior 因为这是修改字符串文字的尝试。

注意：如手册页中所述，需要更改文字。喜欢阅读，

In the glibc implementation of the POSIX versions of these functions they modify their argument, and invokes undefined behavior when called with a static string like "/usr/". [...]

分段错误是 UB 的副作用之一，但不是唯一的副作用。

FWIW，尝试修改字符串文字本身会调用 UB。引用 C11，章节 §6.4.5，字符串文字

[...] If the program attempts to modify such an array, the behavior is undefined.

编辑：

正如后续评论中所讨论的，另一个问题是缺少头文件。你需要

  #include <libgen.h>

添加以获得函数的前向声明basename()可用。

Answer 2

根据 the POSIX standard:

The basename() function may modify the string pointed to by path, and may return a pointer to internal storage. The returned pointer might be invalidated or the storage might be overwritten by a subsequent call to basename(). The returned pointer might also be invalidated if the calling thread is terminated.

根据 the Linux man page:

Both dirname() and basename() may modify the contents of path, so it may be desirable to pass a copy when calling one of these functions.

您正在使用静态字符串调用 basename()，该字符串可能是只读的，因此当 basename() 尝试修改字符串时会导致 SEGV。

缓冲区上的基名进入分段错误

basename on buffer goes into segmentation fault

c

pointers

strcmp

Bugs