strncat看起來像它保存呼叫數據？

Question

我在string.h標準庫中的strncat函數中看到一些奇怪的行爲，並希望能夠幫助您理解正在發生的事情。

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我的問題的關鍵是在功能我創建了名爲readLine與返回的文件作爲char *字符串的行末尾沒有新行終結的目的。這個功能看起來是這樣的：

char * readLine(FILE * fp) { 
    char * chunk = NULL; 
    char * line = NULL; 
    int count = 0; 

    // iterate through chunks of a line until we reach the end (or an error) 
    while (fgets((chunk = malloc(sizeof(char) * BUFFER_SIZE)), BUFFER_SIZE, fp) != NULL) { 

     // realloc on a null pointer works like malloc 
     line = realloc(line, ++count * BUFFER_SIZE * sizeof(char));  

     printf("chunk's contents: %s\n", chunk); 

     // does chunk contain the end of a line? 
     if(strchr(chunk, '\n') == NULL) { 
      // concatenate string parts and continue loop 
      strncat(line, chunk, strlen(chunk) + 1);   
      free(chunk); 

     } else { 
      // we want to return a \0 terminated string without the \n 
      // expected position of \n in chunk is ({length of chunk}-1) 
      chunk[(strlen(chunk) - 1)] = '\0'; 

      // concatenate string parts 
      strncat(line, chunk, strlen(chunk) + 1); 
      printf("readLine line: %s\n", line); 
      free(chunk); 

      break;   
     }   
    } 
    return line; 
} 

我把它稱爲主在一個循環中，看起來像這樣：

FILE * fp = NULL; 

if ((fp = fopen(FILE_PATH, "r")) != NULL) { 
    char * line = NULL; 

    while ((line = readLine(fp)) != NULL) { 
     printf("main line:  %s\n\n", line); 
     free(line); 
    } 

    fclose(fp); 
} 

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現在的古怪行爲來到我的#define BUFFER_SIZE 1000定義。有了它設置一樣，我得到下面的輸出（這不是我想要的）：

chunk's contents: I am on line 1 
readLine line: I am on line 1 
main line:  I am on line 1 

chunk's contents: Over here I am on line 2 
readLine line: I am on line 1Over here I am on line 2 
main line:  I am on line 1Over here I am on line 2 

chunk's contents: Line 3 here 
readLine line: I am on line 1Over here I am on line 2Line 3 here 
main line:  I am on line 1Over here I am on line 2Line 3 here 

chunk's contents: Look out for 4 
readLine line: I am on line 1Over here I am on line 2Line 3 hereLook out for 4 
main line:  I am on line 1Over here I am on line 2Line 3 hereLook out for 4 

chunk's contents: Johnny 5 alive! 
readLine line: I am on line 1Over here I am on line 2Line 3 hereLook out for 4Johnny 5 alive! 
main line:  I am on line 1Over here I am on line 2Line 3 hereLook out for 4Johnny 5 alive! 

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但，如果我這一定義更改爲類似#define BUFFER_SIZE 20，我得到我想要的輸出：

chunk's contents: I am on line 1 

readLine line: I am on line 1 
main line:  I am on line 1 

chunk's contents: Over here I am on l 
chunk's contents: ine 2 

readLine line: Over here I am on line 2 
main line:  Over here I am on line 2 

chunk's contents: Line 3 here 

readLine line: Line 3 here 
main line:  Line 3 here 

chunk's contents: Look out for 4 

readLine line: Look out for 4 
main line:  Look out for 4 

chunk's contents: Johnny 5 alive! 

readLine line: Johnny 5 alive! 
main line:  Johnny 5 alive! 

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我想我已經縮小到strncat(line, chunk, strlen(chunk) + 1);線的問題。我不明白爲什麼當我的BUFFER_SIZE足夠高時，前面的line被包括在內。

Answer 1

line = realloc(line, ++count * BUFFER_SIZE * sizeof(char)); 

不初始化分配的內存。因此，如果readLine中的第一個realloc可以讓您恢復之前調用中獲得的大量內存 - 這並非不可能，您可能仍然在其中。

無論如何，對於未初始化的內存，第一個strncat可能會調用未定義的行爲，因爲分配的內存中不需要有0字節。

在進入循環之前將緩衝區分配爲line，並將0寫入第一個字節。

另外，不要使用

line = realloc(line, ++count * BUFFER_SIZE * sizeof(char)); 

如果realloc失敗，則泄漏內存。你應該檢查的realloc返回值，

char *temp = realloc(line, ++count * BUFFER_SIZE * sizeof(char)); 
if (temp == NULL) { 
    // Oops 
} else { 
    line = temp; 
} 

而且不malloc到chunk在fgets電話，

while (fgets((chunk = malloc(sizeof(char) * BUFFER_SIZE)), BUFFER_SIZE, fp) != NULL) 

如果malloc失敗，太調用未定義的行爲。malloc並呼籲fgets前檢查，

while ((chunk = malloc(sizeof(char) * BUFFER_SIZE)) && fgets(chunk, BUFFER_SIZE, fp) != NULL) 

Answer 2

您可以realloc堅持和memset您的緩衝區之前到零，雖然。

Answer 3

你的問題是在這裏：

line = realloc(line, ++count * BUFFER_SIZE * sizeof(char));  

按照手冊頁realloc：

"realloc(3) does not guarantee that the additional memory is also 
zero-filled." 

和

"If ptr is NULL, realloc() is identical to a call to malloc() for size bytes." 

所以你得到任何新的內存很可能被填補與非零字節，這意味着第一次被稱爲，它可能不會有0 f或第一個字節，這意味着strncat將會追加到分配中的任何垃圾字節。