C語言：指針參數的傳遞

指針是C語言的精華，也是C語言的難點！

今天寫程序，就犯了個很SB的指針錯誤。害我忙乎了大半天。我在這裡把問題抽象出來，給大家做個借鑒！避免以後也犯同樣的錯誤！

1. #include <stdio.h>
2. #include <stdlib.h>
3. #include <string.h>
5. void func1(char *ptr)
6. {
7. ptr[0] = 55;
8. printf("address of ptr is %p\n", (unsigned)ptr);
9. printf("value of ptr[0] is %d\n", ptr[0]);
10. }
12. void func2(char *ptr)
13. {
14. ptr = (char *)malloc(10);
15. ptr[0] = 66;
16. printf("address of ptr is %p\n", (unsigned)ptr);
17. printf("value of ptr[0] is %d\n", ptr[0]);
18. }
20. void main()
21. {
22. char *str;
23. str = (char *)malloc(10);
24. printf("*******************************\n");
25. memset(str,'\0',sizeof(str));
26. printf("address of str is %p\n", (unsigned)str);
27. printf("value of str[0] is %d\n", str[0]);
28. printf("*******************************\n");
29. memset(str,'\0',sizeof(str));
30. func1(str);
31. printf("address of str is %p\n", (unsigned)str);
32. printf("value of str[0] is %d\n", str[0]);
33. printf("*******************************\n");
34. memset(str,'\0',sizeof(str));
35. func2(str);
36. printf("address of str is %p\n", (unsigned)str);
37. printf("value of str[0] is %d\n", str[0]);
38. printf("*******************************\n");
39. }

運行結果：

[cpp]

1. [[email protected] test]# gcc parameter_deliver.c -o parameter_deliver
2. [[email protected] test]# ./parameter_deliver
3. *******************************
4. address of str is 0x995b008
5. value of str[0] is 0
6. *******************************
7. address of ptr is 0x995b008
8. value of ptr[0] is 55
9. address of str is 0x995b008
10. value of str[0] is 55
11. *******************************
12. address of ptr is 0x995b018
13. value of ptr[0] is 66
14. address of str is 0x995b008
15. value of str[0] is 0
16. *******************************
17. [[email protected] test]#

最開始我使用的是func2()的方法，一直得不到返回值，str數組的值一直不變。害我忙乎了半天，終於找到了原因。原來是我在被調函數fun2()裡面又重新malloc了，將以前的str傳遞給ptr的地址值給覆蓋了，所以我在func2()裡面對ptr的所有操作，都是局部操作，所有數據將在func2()退出的時候自動銷毀！⊙﹏⊙b汗~~~！！！