在函数外部访问时分配在堆上的字符串被损坏 C

问题描述 投票：0回答：1

我试图在本教程之后用C编写一些代码来替换给定字符串中的子字符串。我设法让它工作，但是该函数的问题之一是，如果我们替换的字符串比原始子字符串大，它将导致缓冲区溢出。在我的程序中，我知道我只会将动态分配的字符串传递给这个函数，所以我想我可以检查我们要替换的字符串是否大于子字符串，如果是，我可以使用 

realloc
函数调整原始字符串的大小，为我们要替换的字符串腾出空间。最初，该函数似乎可以工作，但由于某种原因，当我重复调用它来替换同一字符串上的不同子字符串时，该字符串会损坏。

在下面的代码中，我有一个名为 

GetFileContents
的函数，它将检索名为 
first_page.html
的文件的文件内容到动态分配的字符串中，然后返回所述字符串，该函数工作正常并且没有任何问题。 
StrReplaceSubstringFirstOccurance
将用新字符串替换我指定的子字符串，它还将打印出该字符串的输出，并在输出前加上 
Inside:
。在 if 语句中，我检查要替换的字符串是否大于子字符串，我有两种不同的方法来调整字符串大小。方法 2 使用 
realloc
，方法 1 使用 
malloc
分配一个新字符串，将原始字符串的内容复制到新字符串，在原始字符串上调用 
free
，然后使指向原始字符串的指针指向新字符串。两种方法都会产生同样的问题。正如您在输出中看到的，对 
StrReplaceSubstringFirstOccurance
的前两次调用成功运行，但是对函数的第三次调用（子字符串是 
[[to]]
，要替换的字符串是 
NOT CUSTOM!
），在函数内部打印出正确的字符串，但是当我尝试在函数外部打印字符串时，它显示垃圾字符。在对该函数的第四次调用中，该函数识别出原始字符串中不再出现该子字符串（因为原始字符串现在包含垃圾字符），因此它只是退出该函数，原始字符串中的垃圾字符仍然存在留在那里。

代码：

static bool StrReplaceSubstringFirstOccurance(char* source, char* substring, char* replace) {
    char* substring_occurance = strstr(source, substring);
    if (substring_occurance == NULL) {
        printf("No substring: %s found.\n", substring);
        return false;
    }

    if (strlen(replace) > strlen(substring)) {
        size_t new_size = strlen(source) + (strlen(replace)-strlen(substring))+1;
        // Approach 1
        // char* temp = malloc(new_size);
        // memcpy(temp, source, strlen(source)+1);
        // free(source);
        // source = temp;

        // Approach 2
        source = realloc(source, new_size);
    }

    substring_occurance = strstr(source, substring);
    memmove(substring_occurance + strlen(replace),
            substring_occurance + strlen(substring),
            strlen(substring_occurance) - strlen(substring)+1);
        
    memcpy(substring_occurance, replace, strlen(replace));
    printf("\nInside: %s\n", source);
    return true;
}

int main(void) {
    char* first_page = GetFileContents("first_page.html");

    StrReplaceSubstringFirstOccurance(first_page, "[[say]]", "CUSTOM!");
    StrReplaceSubstringFirstOccurance(first_page, "[[say]]", "CUSTOM!");
    printf("\nFirst Print: %s\n", first_page);
    StrReplaceSubstringFirstOccurance(first_page, "[[to]]", "NOT CUSTOM!");
    printf("\nSecond Print: %s\n", first_page);
    StrReplaceSubstringFirstOccurance(first_page, "[[to]]", "NOT CUSTOM!");
    printf("\nThird Print: %s\n", first_page);

    printf("Reached the end!\n");
    return 0;
}

输出：

Inside: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! [[to]]</title>
  <body>
    [[say]]
      <h1>This is a header!</h1>
      [[to]]
  </body>
</html>

Inside: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! [[to]]</title>
  <body>
    CUSTOM!
      <h1>This is a header!</h1>
      [[to]]
  </body>
</html>

First Print: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! [[to]]</title>
  <body>
    CUSTOM!
      <h1>This is a header!</h1>
      [[to]]
  </body>
</html>

Inside: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! NOT CUSTOM!</title>
  <body>
    CUSTOM!
      <h1>This is a header!</h1>
      [[to]]
  </body>
</html>

Second Print: α#F^Ñ☻
No substring: [[to]] found.

Third Print: α#F^Ñ☻
Reached the end!

first_page.html

<!DOCTYPE html>
<html lang="en-US">
  <title>[[say]] [[to]]</title>
  <body>
    [[say]]
      <h1>This is a header!</h1>
      [[to]]
  </body>
</html>
html c string replace substring
1个回答
0
投票

我按照 pmg 的评论设法解决了这个问题。我在 

file_path
函数中传递了一个指向 
StrReplaceSubstringFirstOccurance
变量的指针。 新代码如下所示：

static bool StrReplaceSubstringFirstOccurance(char** source, char* substring, char* replace) {
    char* substring_occurance = strstr(*source, substring);
    if (substring_occurance == NULL) {
        printf("No substring: %s found.\n", substring);
        return false;
    }

    if (strlen(replace) > strlen(substring)) {
        size_t new_size = strlen(*source) + (strlen(replace)-strlen(substring))+1;
        *source = realloc(*source, new_size);
    }

    substring_occurance = strstr(*source, substring);
    memmove(substring_occurance + strlen(replace),
            substring_occurance + strlen(substring),
            strlen(substring_occurance) - strlen(substring)+1);
        
    memcpy(substring_occurance, replace, strlen(replace));
    printf("\nInside: %s\n", *source);
    return true;
}

int main(void) {
    char* first_page = GetFileContents("first_page.html");
    StrReplaceSubstringFirstOccurance(&first_page, "[[say]]", "CUSTOM!");
    printf("\nFirst Print: %s\n", first_page);
    StrReplaceSubstringFirstOccurance(&first_page, "[[to]]", "NOT CUSTOM!");
    printf("\nThird Print: %s\n", first_page);

    printf("Reached the end!\n");
    return 0;
}

输出为：

Inside: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! [[to]]</title>
  <body>
    [[say]]
      <h1>This is a header!</h1>
      [[to]]
      [[say]][[to]]
  </body>
</html>

First Print: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! [[to]]</title>
  <body>
    [[say]]
      <h1>This is a header!</h1>
      [[to]]
      [[say]][[to]]
  </body>
</html>

Inside: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! NOT CUSTOM!</title>
  <body>
    [[say]]
      <h1>This is a header!</h1>
      [[to]]
      [[say]][[to]]
  </body>
</html>

Third Print: <!DOCTYPE html>
<html lang="en-US">
  <title>CUSTOM! NOT CUSTOM!</title>
  <body>
    [[say]]
      <h1>This is a header!</h1>
      [[to]]
      [[say]][[to]]
  </body>
</html>
Reached the end!
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