2

以下代码会导致分段错误。可执行文件名为“./struct”

#include <stdio.h>
#define VERSION_NUMBER_LEN 32
#define MAX_DESCRIPTION_COUNT 32
#define DESCRIPTION_LEN 128
int main(void)
{
    struct foo {
        char number[VERSION_NUMBER_LEN + 1];
        char description[MAX_DESCRIPTION_COUNT][DESCRIPTION_LEN];
    };

    struct foo asdf = {
        "1.1", { "clap", "clap", "stomp", NULL }
    };

    struct foo hjkl = {
        "1.2", { "clop", "clop", "stamp", NULL }
    };

    int i;
    printf( "%s\n", asdf.number );
    for( i = 0; (asdf.description)[i] != NULL; i++ ){
        printf( "\t%s\n", (asdf.description)[i]);
    }
    printf("\n");
    printf( "%s\n", hjkl.number );
    for( i = 0; (hjkl.description)[i] != NULL; i++ ){
        printf( "\t%s\n", (hjkl.description)[i]);
    }
}

输出如下所示：

1.1
    clap
    clap
    stomp






��
    N���~�����������ջ�����e���t�����������A���P���b���������������̽��㽊����,���V���g���y���������������̾��    ���k�����������Ͽ��迊�
    迊�





    ome/tiger
    56
    y
    vZxy/ssh
    ptop:/tmp/.ICE-unix/2710
    usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:~/bin:~/vitetris-0.3.6:/var/lib/gems/1.8/bin/
    baz
    GNOME_KEYRING_PID=2692
    t \w\n\$
    XDG_DATA_DIRS=/usr/share/gnome:/usr/local/share/:/usr/share/
    9a6bf0ef61ded7872065094fca55d1
    se
Segmentation fault

我跑了 valgrind：

$ valgrind -v --leak-check=full --track-origins=yes ./struct  

<snip>

==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x402605B: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x4026067: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228== Invalid read of size 1
==15228==    at 0x4026058: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0xbec1007c is not stack'd, malloc'd or (recently) free'd
==15228==
==15228==
==15228== Process terminating with default action of signal 11 (SIGSEGV)
==15228==  Access not within mapped region at address 0xBEC1007C
==15228==    at 0x4026058: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  If you believe this happened as a result of a stack
==15228==  overflow in your program's main thread (unlikely but
==15228==  possible), you can try to increase the size of the
==15228==  main thread stack using the --main-stacksize= flag.
==15228==  The main thread stack size used in this run was 8388608.
==15228== Syscall param write(buf) points to uninitialised byte(s)
==15228==    at 0x4107DC3: __write_nocancel (syscall-template.S:82)
==15228==    by 0x40B0A1E: new_do_write (fileops.c:530)
==15228==    by 0x40B0D35: _IO_do_write@@GLIBC_2.1 (fileops.c:503)
==15228==    by 0x40B181C: _IO_file_overflow@@GLIBC_2.1 (fileops.c:881)
==15228==    by 0x40B2DED: _IO_flush_all_lockp (genops.c:849)
==15228==    by 0x40B3A4F: _IO_cleanup (genops.c:1010)
==15228==    by 0x41670F0: ??? (in /lib/tls/i686/cmov/libc-2.11.1.so)
==15228==    by 0x401F4F3: _vgnU_freeres (vg_preloaded.c:62)
==15228==    by 0xBEC0D5F7: ???
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0x402a054 is not stack'd, malloc'd or (recently) free'd
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)

<snip>

==15228== HEAP SUMMARY:
==15228==     in use at exit: 0 bytes in 0 blocks
==15228==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==15228==
==15228== All heap blocks were freed -- no leaks are possible
==15228==
==15228== ERROR SUMMARY: 7 errors from 4 contexts (suppressed: 12 from 7)
==15228==
==15228== 1 errors in context 1 of 4:
==15228== Syscall param write(buf) points to uninitialised byte(s)
==15228==    at 0x4107DC3: __write_nocancel (syscall-template.S:82)
==15228==    by 0x40B0A1E: new_do_write (fileops.c:530)
==15228==    by 0x40B0D35: _IO_do_write@@GLIBC_2.1 (fileops.c:503)
==15228==    by 0x40B181C: _IO_file_overflow@@GLIBC_2.1 (fileops.c:881)
==15228==    by 0x40B2DED: _IO_flush_all_lockp (genops.c:849)
==15228==    by 0x40B3A4F: _IO_cleanup (genops.c:1010)
==15228==    by 0x41670F0: ??? (in /lib/tls/i686/cmov/libc-2.11.1.so)
==15228==    by 0x401F4F3: _vgnU_freeres (vg_preloaded.c:62)
==15228==    by 0xBEC0D5F7: ???
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0x402a054 is not stack'd, malloc'd or (recently) free'd
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228==

==15228== 1 errors in context 2 of 4:
==15228== Invalid read of size 1
==15228==    at 0x4026058: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0xbec1007c is not stack'd, malloc'd or (recently) free'd
==15228==
==15228==
==15228== 1 errors in context 3 of 4:
==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x402605B: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228==
==15228== 4 errors in context 4 of 4:
==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x4026067: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
--15228--
--15228-- used_suppression:     12 dl-hack3-cond-1
==15228==
==15228== ERROR SUMMARY: 7 errors from 4 contexts (suppressed: 12 from 7)

好的......所以我有几个内存位置，valgrind 显示为未初始化，但我不明白......内部的结构和字符串是如何静态定义的，并且结构的每个实例都被显式声明。

我认为当访问第二个结构（hjkl）中的元素之一时会发生分段错误。

跑gdb...

(gdb) p asdf
$1 = {number = "1.1", '\000' <repeats 29 times>, description = {"clap", '\000' <repeats 123 times>, "clap", '\000' <repeats 123 times>, 
"stomp", '\000' <repeats 122 times>, '\000' <repeats 127 times> <repeats 29 times>}}

(gdb) p hjkl
$2 = {number = "1.2", '\000' <repeats 29 times>, description = {"clop", '\000' <repeats   123 times>, "clop", '\000' <repeats 123 times>, 
"stamp", '\000' <repeats 122 times>, '\000' <repeats 127 times> <repeats 29 times>}}

我只是没有看到导致分段错误的原因......

4

3 回答 3

4

你的测试(asdf.description)[i] != NULL失败了，永远不会是真的。这是修复：

#include <stdio.h>
#define VERSION_NUMBER_LEN 32
#define MAX_DESCRIPTION_COUNT 32
#define DESCRIPTION_LEN 128
int main(void)
{
    struct foo {
        char number[VERSION_NUMBER_LEN + 1];
        char description[MAX_DESCRIPTION_COUNT][DESCRIPTION_LEN];
    };

    struct foo asdf = {
        "1.1", { "clap", "clap", "stomp", "" }
    };

    struct foo hjkl = {
        "1.2", { "clop", "clop", "stamp", "" }
    };

    int i;
    printf( "%s\n", asdf.number );
    for( i = 0; (asdf.description)[i][0] != 0; i++ ){
        printf( "\t%s\n", (asdf.description)[i]);
    }
    printf("\n");
    printf( "%s\n", hjkl.number );
    for( i = 0; (hjkl.description)[i][0] != 0; i++ ){
        printf( "\t%s\n", (hjkl.description)[i]);
    }
}
于 2012-06-04T04:47:29.823 回答
4

问题不在于您的初始化字符串本身，而在于字符数组与字符指针的混淆。

特别是，这不会像您认为的那样：

struct foo asdf = {
    "1.1", { "clap", "clap", "stomp", NULL }
};

您正在foo::description[][]使用字符串 clap、clap 和 stomp OK 进行初始化，但随后您将第二个字符串的第一个字符NULL分配给. 如果您分配给指针，这将起作用，但是您尝试转换NULL为字符数组而不是将指向字符的指针设置为NULL，如果这有任何意义的话。

您正在检查指针是否为 NULL，因为数组是预先声明的，所以它永远不会是。

所以简单地改变

for( i = 0; (hjkl.description)[i] != NULL; i++ ){

到

for( i = 0; *(hjkl.description)[i] != NULL; i++ ){

也就是说，检查数组中每个NULL 字符串的第一个索引中的字符，而不是尝试检查字符指针本身的（不可能的）条件是NULL.

最终完成的代码：

#include <stdio.h>
#define VERSION_NUMBER_LEN 32
#define MAX_DESCRIPTION_COUNT 32
#define DESCRIPTION_LEN 128
int main(void)
{
    struct foo {
        char number[VERSION_NUMBER_LEN + 1];
        char description[MAX_DESCRIPTION_COUNT][DESCRIPTION_LEN];
    };

    struct foo asdf = {
        "1.1", { "clap", "clap", "stomp", NULL }
    };

    struct foo hjkl = {
        "1.2", { "clop", "clop", "stamp", NULL }
    };

    int i;
    printf( "%s\n", asdf.number );
    for( i = 0; *(asdf.description)[i] != NULL; i++ ){
        printf( "\t%s\n", (asdf.description)[i]);
    }
    printf("\n");
    printf( "%s\n", hjkl.number );
    for( i = 0; *(hjkl.description)[i] != NULL; i++ ){
        printf( "\t%s\n", (hjkl.description)[i]);
    }
}

如果结构如下所示，您的代码将起作用：

struct foo {
    char number[VERSION_NUMBER_LEN + 1];
    char *description[MAX_DESCRIPTION_COUNT];
};

基本上，（据我所知）没有真正需要对实际描述字符数组本身进行硬编码，只需要对它们的数量进行硬编码。您的其余代码将按原样工作。

此外，如果您在编译期间查看警告（假设在大多数现代 C 编译器上NULL定义为((void *)0)），您将收到以下警告：

test.c:13:43: warning: incompatible pointer to integer conversion initializing
      'char' with an expression of type 'void *';
        "1.1", { "clap", "clap", "stomp", NULL }
                                          ^~~~
/usr/include/stdio.h:82:14: note: expanded from:
#define NULL __DARWIN_NULL
             ^
/usr/include/sys/_types.h:91:23: note: expanded from:
#define __DARWIN_NULL ((void *)0)
                      ^~~~~~~~~~~
test.c:13:43: warning: suggest braces around initialization of subobject
      [-Wmissing-braces]
        "1.1", { "clap", "clap", "stomp", NULL }
                                          ^~~~
/usr/include/stdio.h:82:14: note: expanded from:
#define NULL __DARWIN_NULL
             ^
/usr/include/sys/_types.h:91:23: note: expanded from:
#define __DARWIN_NULL ((void *)0)
                      ^~~~~~~~~~~
test.c:17:43: warning: incompatible pointer to integer conversion initializing
      'char' with an expression of type 'void *';
        "1.2", { "clop", "clop", "stamp", NULL }
                                          ^~~~
/usr/include/stdio.h:82:14: note: expanded from:
#define NULL __DARWIN_NULL
             ^
/usr/include/sys/_types.h:91:23: note: expanded from:
#define __DARWIN_NULL ((void *)0)
                      ^~~~~~~~~~~
test.c:17:43: warning: suggest braces around initialization of subobject
      [-Wmissing-braces]
        "1.2", { "clop", "clop", "stamp", NULL }
                                          ^~~~
/usr/include/stdio.h:82:14: note: expanded from:
#define NULL __DARWIN_NULL
             ^
/usr/include/sys/_types.h:91:23: note: expanded from:
#define __DARWIN_NULL ((void *)0)
                      ^~~~~~~~~~~
4 warnings generated.
于 2012-06-04T04:50:01.853 回答
3

虽然宏旨在NULL（至少是由那些首先定义它的人）专门用作指针值，但它通常是d 一个简单的：#define

#define NULL 0

这可能是您的实现的情况（在这里并不重要，除了说明；它可以定义为(void *)0不更改下面的结果——但这会导致编译时对您的初始化程序的抱怨）。for考虑到上述内容，让我们扩展第一个循环：

for (i = 0; (asdf.description)[i] != 0; i++) {
    printf( "\t%s\n", (asdf.description)[i]);
}

（旁注：这里不需要括号，因为.和下标运算符的绑定已经是括号强制的。）每个都asdf.description[i]命名一个完整的数组（大小DESCRIPTION_LEN）char。因此，您正在比较：

<some array of char> != 0

数组对象的“值”是指向数组第一个元素的指针，所以这与以下含义相同：

&asdf.description[i][0] != 0

指针值 ( &asdf.description[i][0]) 与整数常量零的比较测试指针是否为 NULL（不是“宏NULL，而是“系统的 NULL 指针的内部表示”）。有效指针的地址永远不会比较等于 0，所以循环运行（实际上）“永远”（当然直到 i >= 32）。

最终，调用printf传递一个指针值，导致您看到的分段错误。

大概你真正想要做的是用全零字节chars（或至少一个初始零字节）在最后一个有效数组之后初始化数组。在这种情况下，循环测试应为：

asdf.description[i][0] != '\0'

您还可以考虑 32 元素数组（of 的数组）被有效的字符数组完全填充的DESCRIPTION_LEN可能性char。i在这种情况下，您应该在查看之前检查 的值asdf.description[i][anything]：

i < MAX_DESCRIPTION_COUNT && asdf.description[i][0] != '\0'
于 2012-06-04T04:54:28.703 回答