c - C 中的内存分配运行时错误

Question

我已经创建了以下代码，一个用于可打印 ascii 的简单暴力破解程序。用户传入起始密码长度和结束密码长度的参数。当我的程序运行时出现以下错误:

输入:

~$ Enter START length & END length ex:(8 10): 2 3

输出:

... (more output above)
~|
~}
~~

*** glibc detected *** ./wordgen: double free or corruption (out): 0x0916e008 ***
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(+0x6cbe1)[0x874be1]
/lib/i386-linux-gnu/libc.so.6(+0x6e50b)[0x87650b]
/lib/i386-linux-gnu/libc.so.6(cfree+0x6d)[0x87969d]
./wordgen[0x80486f4]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x81ee37]
./wordgen[0x8048471]
======= Memory map: ========
00110000-0012a000 r-xp 00000000 08:06 3408733    /lib/i386-linux-gnu/libgcc_s.so.1Aborted

Valgrind 输出:

==11050== Invalid read of size 1
==11050==    at 0x804866F: main (wordgen.c:37)
==11050==  Address 0x41a2027 is 1 bytes before a block of size 3 alloc'd
==11050==    at 0x4026864: malloc (vg_replace_malloc.c:236)
==11050==    by 0x8048600: main (wordgen.c:28)
==11050== 
==11050== Invalid write of size 1
==11050==    at 0x8048675: main (wordgen.c:37)
==11050==  Address 0x41a2027 is 1 bytes before a block of size 3 alloc'd
==11050==    at 0x4026864: malloc (vg_replace_malloc.c:236)
==11050==    by 0x8048600: main (wordgen.c:28)
==11050== 
==11050== Invalid read of size 1
==11050==    at 0x8048689: main (wordgen.c:38)
==11050==  Address 0x41a2027 is 1 bytes before a block of size 3 alloc'd
==11050==    at 0x4026864: malloc (vg_replace_malloc.c:236)
==11050==    by 0x8048600: main (wordgen.c:28)
==11050==

C 代码:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
  int  sLen = 1;
  int  eLen = 0;

  printf("Enter START length & END length ex:(8 10): ");
  scanf("%d %d", &sLen, &eLen);
  int cLen = sLen;
  while (cLen <= eLen) {

    /* Allocate Memory for String  & Initialize */
    char *outStr = malloc(cLen + 1);
    memset(outStr, ' ', cLen);
    outStr[cLen] = 0;

    int outerControl = 1;
    while (outerControl == 1) {
      int cMod = 1;
      int innerControl = 1;
      while(innerControl == 1) {
        outStr[cLen-cMod] += 1;
        if((int)outStr[cLen-cMod] == 127) {
          //Exit Condition Where The Error Occurred
          if(cLen - cMod == 0) { outerControl = 0;  } 
          outStr[cLen-cMod] = 32;
          cMod += 1;
        }
        else { innerControl = 0; }
      }
      printf("%s\n",outStr);
    }
    free(outStr); // Possible source of Error?
    cLen += 1;
  }

  return 0;
}

我是 C 编程的新手，对这个错误感到非常困惑。这是什么意思？我是如何错误地创建我的程序的？我假设它与内存管理有关...

Answer 1

你的问题是:

while(innerControl == 1) {
  printf("%d %d\n", cLen, cMod);
  outStr[cLen-cMod] += 1;                       // <-- this here
  if((int)outStr[cLen-cMod] == 127) {
    //Exit Condition Where The Error Occurred
    if(cLen - cMod == 0) { outerControl = 0;  }
    outStr[cLen-cMod] = 32;
    cMod += 1;
  }
  else { innerControl = 0; }
}

在某些时候，cMod 变得比 cLen 大，所以您访问 outStr 超出了它的范围(即:outStr[-1])。此行为未定义。

这个条件:

if(cLen - cMod == 0) { outerControl = 0;  }

...似乎是为了防止这种情况发生，但只有在 (int)outStr[cLen-cMod] == 127 时才会执行。您可能应该添加如下内容:

if (cMod > cLen)
    break;

outStr[cLen-cMod] += 1;