用新指针构造字符串并重新分配给原始指针

Question

我有以下程序，它是通过创建新的字符指针 (newRow)、为新指针分配内存、将第一个字符设置为“a”，将字符“a”重复附加到字符串的最小示例，将先前更新的字符串复制到新字符串中'a'之后的位置，然后将原始字符串重新分配给新的字符指针。但是，有两件事没有按我的预期工作。

首先，我希望对于每次迭代，newRow 的地址应该改变，因为它是由 malloc 新创建的(并且之前的还没有被释放)，然后一旦 newRow 被分配给行，行的地址应该是以前的迭代 newRow，但事实并非如此。

其次，在第 18 次迭代之后，额外的字符被追加到字符串的末尾，我不知道为什么。

下面是在 Linux 上使用 gcc -Wall -Wextra -pedantic -std=c99 -g c_programs/playground.c 编译的源代码，以及运行程序的输出。

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

int main()
{
  char lineBuf[] = "hello";
  char *row = (char *)malloc(strlen(lineBuf) + 1);
  strcpy(row, lineBuf);

  for (size_t i = 0; i < 40; i++)
  {
    char *newRow = (char *)malloc(strlen(row) + 1);
    printf("%ld:  &row: %p, &newRow: %p, len: %ld", i, (void *)&row, (void *)&newRow, strlen(row));
    newRow[0] = 'a';
    strcpy(newRow + 1, row);
    printf(", string is: %s\n", newRow);
    row = newRow;
  }
}

0:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 5, string is: ahello
1:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 6, string is: aahello
2:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 7, string is: aaahello
3:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 8, string is: aaaahello
4:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 9, string is: aaaaahello
5:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 10, string is: aaaaaahello
6:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 11, string is: aaaaaaahello
7:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 12, string is: aaaaaaaahello
8:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 13, string is: aaaaaaaaahello
9:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 14, string is: aaaaaaaaaahello
10:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 15, string is: aaaaaaaaaaahello
11:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 16, string is: aaaaaaaaaaaahello
12:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 17, string is: aaaaaaaaaaaaahello
13:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 18, string is: aaaaaaaaaaaaaahello
14:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 19, string is: aaaaaaaaaaaaaaahello
15:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 20, string is: aaaaaaaaaaaaaaaahello
16:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 21, string is: aaaaaaaaaaaaaaaaahello
17:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 22, string is: aaaaaaaaaaaaaaaaaahello
18:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 23, string is: aaaaaaaaaaaaaaaaaaahello
19:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 25, string is: aaaaaaaaaaaaaaaaaaaahello1
20:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 26, string is: aaaaaaaaaaaaaaaaaaaaahello1
21:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 27, string is: aaaaaaaaaaaaaaaaaaaaaahello1
22:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 28, string is: aaaaaaaaaaaaaaaaaaaaaaahello1
23:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 29, string is: aaaaaaaaaaaaaaaaaaaaaaaahello1
24:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 30, string is: aaaaaaaaaaaaaaaaaaaaaaaaahello1
25:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 31, string is: aaaaaaaaaaaaaaaaaaaaaaaaaahello1
26:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 32, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaahello1
27:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 33, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaahello1
28:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 34, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1
29:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 35, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1
30:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 36, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1
31:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 37, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1
32:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 38, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1
33:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 39, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1
34:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 41, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1A
35:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 42, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1A
36:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 43, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1A
37:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 44, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1A
38:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 45, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1A
39:  &row: 0x7fff662312f8, &newRow: 0x7fff66231300, len: 46, string is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahello1A

Answer 1

strcpy(newRow + 1, row); 失败，因为 newRow 指向内存不足(1)。
这会导致未定义的行为 (UB)。

 // char *newRow = (char *)malloc(strlen(row) + 1);
 char *newRow = (char *)malloc(strlen(row) + 1 + 1);
 //                                          ^   ^
 //                              new character   \0