1 #include <stdio.h>
 2 #include <stdlib.h>
 3 
 4 int main(void)
 5 {
 6     int *p_old,*p_new,i;
 7 
 8     p_old = (int *)malloc(2 * sizeof(int));
 9     *p_old = 1;
10     *(p_old + 1) = 2;
11     p_new = (int *)realloc(p_old,3 * sizeof(int));
12     *(p_old + 2) = 3;
13     
14     for(i = 0; i < 3; ++i)
15         printf("%d ",*(p_new + i));
16     putchar('\n');
17 
18     for(i = 0; i < 3; ++i)
19         printf("%d ",*(p_old + i));//point to the same memory area
20     putchar('\n');
21 
22     for(i = 0; i < 3; ++i)
23         printf("%d ",p_old[i]);//as above
24     //free(p_old);   此处不需要释放 p_old,因为 realloc() 已经释放过了
25     free(p_new);
26 
27     return 0;
28 }

结果：

1 2 3
1 2 3
1 2 3

 

realloc() should only be used for dynamically allocated memory. If the memory is not dynamically allocated, then behavior is undefined. https://www.geeksforgeeks.org/g-fact-66/

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 
 4 int main(void)
 5 {
 6     int arr[2] = {1,2};
 7     int *p_old,*p_new;
 8 
 9     for(int i = 0;i < 2; ++i)
10         printf("%d ",arr[i]);
11     p_old = arr;
12     p_new = (int *)realloc(p_old,3 * sizeof(int));
13     arr[2] = 3;
14     for(int i = 0;i < 3; ++i)
15         printf("%d ",arr[i]);
16     free(p_old);
17     free(p_new);
18 
19     return 0;
20 }

编译运行：

1 2

 

GDB 调试：

(gdb) l
1 #include <stdio.h>
2 #include <stdlib.h>
3
4 int main(void)
5 {
6 int arr[2] = {1,2};
7 int *p_old,*p_new;
8
9 for(int i = 0;i < 2; ++i)
10 printf("%d ",arr[i]);
(gdb)
11 p_old = arr;
12 p_new = (int *)realloc(p_old,3 * sizeof(int));
13 arr[2] = 3;
14 for(int i = 0;i < 3; ++i)
15 printf("%d ",arr[i]);
16 free(p_old);
17 free(p_new);
18
19 return 0;
20 }
(gdb)
Line number 21 out of range; err_realloc.c has 20 lines.
(gdb) b 11
Breakpoint 1 at 0x4015f5: file err_realloc.c, line 11.
(gdb) r
Starting program: C:\Users\luwudang\c\err.exe
[New Thread 6188.0x1254]
[New Thread 6188.0x3e4]
1 2
Breakpoint 1, main () at err_realloc.c:11
11 p_old = arr;
(gdb) p p_old
$1 = (int *) 0x0
(gdb) p arr
$2 = {1, 2}
(gdb) p &arr[0]
$3 = (int *) 0x22fe30
(gdb) s
12 p_new = (int *)realloc(p_old,3 * sizeof(int));
(gdb) p p_old
$4 = (int *) 0x22fe30
(gdb) p p_new
$5 = (int *) 0x1c
(gdb) s
warning: Critical error detected c0000374


Program received signal SIGTRAP, Trace/breakpoint trap.
0x0000000077c1f3b0 in ntdll!RtlUnhandledExceptionFilter () from C:\windows\SYSTEM32\ntdll.dll
(gdb) p p_new
No symbol "p_new" in current context.