【发布时间】：2023-12-13 11:24:01
【问题描述】：
我遇到了 seg.faults 的问题。该程序运行良好，但对于少数未知字符串会导致分段错误。我用 Valgrind 运行程序，它报告“大小为 1 的无效读/写”，主要是问题与 strcpy 和 strlen 有关。
==5623== ERROR SUMMARY: 12 errors from 4 contexts (suppressed: 2 from 2)
==5623==
==5623== 1 errors in context 1 of 4:
==5623== Invalid write of size 1
==5623== at 0x4C2D812: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x400955: sameWords (main.c:62)
==5623== by 0x400A6F: main (main.c:85)
==5623== Address 0x51fd093 is 0 bytes after a block of size 3 alloc'd
==5623== at 0x4C2CD7B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x40092B: sameWords (main.c:59)
==5623== by 0x400A6F: main (main.c:85)
==5623==
==5623==
==5623== 1 errors in context 2 of 4:
==5623== Invalid write of size 1
==5623== at 0x4C2D812: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x400942: sameWords (main.c:61)
==5623== by 0x400A6F: main (main.c:85)
==5623== Address 0x51fd045 is 0 bytes after a block of size 5 alloc'd
==5623== at 0x4C2CD7B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x400913: sameWords (main.c:58)
==5623== by 0x400A6F: main (main.c:85)
==5623==
==5623==
==5623== 4 errors in context 3 of 4:
==5623== Invalid read of size 1
==5623== at 0x4C2D7B4: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x4009EC: sameWords (main.c:70)
==5623== by 0x400A6F: main (main.c:85)
==5623== Address 0x51fd093 is 0 bytes after a block of size 3 alloc'd
==5623== at 0x4C2CD7B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x40092B: sameWords (main.c:59)
==5623== by 0x400A6F: main (main.c:85)
==5623==
==5623==
==5623== 6 errors in context 4 of 4:
==5623== Invalid read of size 1
==5623== at 0x4C2D7B4: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x40099E: sameWords (main.c:65)
==5623== by 0x400A6F: main (main.c:85)
==5623== Address 0x51fd045 is 0 bytes after a block of size 5 alloc'd
==5623== at 0x4C2CD7B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5623== by 0x400913: sameWords (main.c:58)
==5623== by 0x400A6F: main (main.c:85)
==5623==
--5623--
--5623-- used_suppression: 2 dl-hack3-cond-1
==5623==
==5623== ERROR SUMMARY: 12 errors from 4 contexts (suppressed: 2 from 2)
程序应该找出两个字符串是否由相同的单词组成（并忽略不同大小的字母）。对于我的代码的出现，我感到非常抱歉，我对编程很陌生，并且仍在努力学习如何编写可以理解的代码，所以我将简要解释正在做什么（至少我是这么认为的）在哪里。 WordInString 函数从一个字符串中获取一个又一个单词，然后在另一个字符串中找到它。这个词被复制到动态分配的数组中，因为我不知道这些词可能有多长。然后在函数 sameWords 中，我将字符串复制到新数组中，这样我就可以将所有单词转换为小写字母，然后调用函数 WordInString 来搜索字符串中的单词。我的主要功能是我只用 2 个字符串调用 sameWords。
代码看起来像这样。再次为糟糕的安排感到抱歉。
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
int WordInString(const char *a, const char *b)
{
/* Selecting words one after another and finding the word in another strin.
Dynamic allocation of word string, because words lengths are unknown.*/
char * word=NULL;
int previous = 0;
int length=0;
int wordlength=0;
int i;
if((strlen(a)==0) && (strlen(b)==0))
{
return 1;
}
i=0;
while(1)
{
if (i>=wordlength) {wordlength+=250; word=(char*) malloc(wordlength*sizeof(char));}
if(isalpha(a[i]))
{
if(!isalpha(previous))
{
length=0;
}
if(length<80) word[length++] = tolower(a[i]);
}
else
{
if(length>0)
{
word[length] = '\0';
if(strstr(b, word)==NULL)
{
return 0;
}
length=0;
}
}
if(a[i] == '\0') {break;}
previous=tolower(a[i++]);
}
free(word);
return 1;
}
int sameWords( const char * a, const char * b)
{
int i=0, j=0;
char * array1=(char*) malloc(strlen(a)*sizeof(char));
char * array2=(char*) malloc(strlen(b)*sizeof(char));
/* copy a and b strings to new string, that are not cons, so they can be changed tolower */
strcpy(array1, a);
strcpy(array2, b);
/* convert strings to lower letters */
for(i=0; i<strlen(array1); i++)
{
array1[i]=tolower(array1[i]);
}
for(j=0; j<strlen(array2); j++)
{
array2[j]=tolower(array2[j]);
}
/* calling WordInString to compare */
if (WordInString(a, array2)==0) {return 0;}
if (WordInString(b, array1)==0) {return 0;}
free(array1);
free(array2);
return 1;
}
int main ( int argc, char * argv [] )
{
int res;
res=(sameWords("This is a string", "This string is a string"));
return 0;
}
我会非常感谢你的帮助。我试图查找它，但无法弄清楚。
【问题讨论】：
-
malloc(strlen(a)*sizeof(char));Size +1 需要字符串的结尾'\0' -
Please don't cast the return value of
malloc()in C。此外，按sizeof (char)缩放从来都不好，它只是 1，所以它只会增加混乱。 -
所以我应该做 malloc((strlen(a)+1)*sizeof(char)); ?而不是 sizeof，你会建议什么？我唯一能想到的就是用一些计数器遍历两个字符串，然后将计数器放在那里而不是 sizeof。
-
如果：
if (WordInString(a, array2)==0) {return 0;}不会释放 array1 和 array2。所以你会有泄漏 -
但是当我通过 Valgrind 运行它时，它说一切都被释放了。我释放了这两个函数中的所有数组。