C 字符串、strlen 和 Valgrind

Question

我试图理解为什么 Valgrind 会吐出：

==3409== Invalid read of size 8
==3409==    at 0x4EA3B92: __GI_strlen (strlen.S:31)

每当我在动态分配的字符串上应用 strlen 时？

这是一个简短的测试用例：

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

int main() {

  char *hello = "Hello World";
  char *hello2;

  /* Step 1 */
  printf("Step 1\n");
  printf("strlen : %lu\n",(unsigned long)strlen(hello));

  /* Step 2 */
  hello2 = calloc(12,sizeof(char));
  hello2[0] = 'H';
  hello2[1] = 'e';
  hello2[2] = 'l';
  hello2[3] = 'l';
  hello2[4] = 'o';
  hello2[5] = ' ';
  hello2[6] = 'W';
  hello2[7] = 'o';
  hello2[8] = 'r';
  hello2[9] = 'l';
  hello2[10] = 'd';
  hello2[11] = 0;  

  printf("Step 2\n");
  printf("strlen : %lu\n",(unsigned long)strlen(hello2));
  free(hello2);

  return 0;
}

这是 Valgrind 的结果输出：

lenain@perseus:~/work/leaf$ valgrind ./leaf
==3409== Memcheck, a memory error detector
==3409== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==3409== Using Valgrind-3.5.0-Debian and LibVEX; rerun with -h for copyright info
==3409== Command: ./leaf
==3409== 
Step 1
strlen : 11
Step 2
==3409== Invalid read of size 8
==3409==    at 0x4EA3B92: __GI_strlen (strlen.S:31)
==3409==    by 0x40098A: main (in /home/lenain/work/leaf/leaf)
==3409==  Address 0x5189048 is 8 bytes inside a block of size 12 alloc'd
==3409==    at 0x4C234CB: calloc (vg_replace_malloc.c:418)
==3409==    by 0x4008F0: main (in /home/lenain/work/leaf/leaf)
==3409== 
strlen : 11
==3409== 
==3409== HEAP SUMMARY:
==3409==     in use at exit: 0 bytes in 0 blocks
==3409==   total heap usage: 1 allocs, 1 frees, 12 bytes allocated
==3409== 
==3409== All heap blocks were freed -- no leaks are possible
==3409== 
==3409== For counts of detected and suppressed errors, rerun with: -v
==3409== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)

避免这些警告的正确方法是什么？它们是真正的警告吗？

Answer 1

这很可能与此错误报告有关：

https://bugzilla.redhat.com/show_bug。 cgi?id=518247

正如 Paul 已经建议的那样，intel 平台上的 strlen() 可以选择使用 SSE 优化来加速 strlen 和其他函数。这种加速涉及分配块后面的安全读取，这是旧版本的 valgrind 尚不理解的。所以升级你的 valgrind 就可以了。

Answer 2

我的猜测是，您的 strlen 实现已经过优化，可以一次读取 8 个字节，并测试 64 位字中任意位置的第一个零字节（可能使用 MMX/SSE）。这意味着对于 12 字节字符串示例，它会读取超出字符串末尾的 4 个字节。关于这是否是 strlen 实现中的错误是有争议的。我想你只能忽略它。或者确保您的字符串分配始终是 8 字节的倍数。