Embedded Files
Environment: Ubuntu 10.04
C compiler: gcc version 4.4.3
如 果 C 程式出現了下列的錯誤訊息 (glibc detected .... double free ...)，依據錯誤訊息的說明，它是 glibc 預防程式對同一塊記憶體重複 free 所進行的偵測，如果想要強制執行程式以觀察執行狀況，只要在執行程式前，將環境變數 MALLOC_CHECK_ 設定為 0 就可以關閉這項檢查，ex. MALLOC_CHECK_=0 ./test (執行檔名是 test )。
寫了下列的小程式來測試，這個程式沒做什麼事情，主要只是配置了一個動態的二維記憶體空間 (10 * 32)，再一一釋放記憶體，程式就結束了，這個程式就會引起 double free 的問題，在往下看結果之前，請研究這隻程式哪裡有問題?
1 #include <stdio.h>
2 #include <stdlib.h>
3
4 int main(int argc, char *argv[])
5 {
6
7 char **str = NULL;
8 const int x=10;
9 const int y=32;
10 int i;
11
12 str = (char**) malloc ( sizeof(char) * x );
13
14 /* NULL pointer */
15 if( ! str ) {
16 printf("NULL Pointer\n");
17 return -1;
18 }
19
20 printf("Address of str=%x, and point to %x\n", &str, str);
21
22 bzero(str, x);
23
24 printf("---------------------------------------------------\n");
25
26 for( i=0; i<x; i++) {
27
28 str[i] = (char *)malloc( sizeof(char) * y );
29
30 if( ! str[i] ) {
31 printf("NULL Pointer\n");
32 return -1;
33 }
34
35
36 printf("Address of str[%d]=%x, and point to %x\n", i, &str[i], str[i]);
37
38 bzero(str[i], y);
39 strcpy(str[i], "hello");
40
41 }
42
43
44 printf("---------------------------------------------------\n");
45
46 for( i=0; i<x; i++) {
47
48 if( str[i] ) {
49 free( str[i] );
50 }
51 }
52
53 if(str) {
54 free (str);
55 }
56
57 return 0;
58 }
59
上面的程式執行時出現了錯誤訊息
ming@linuxbox:/tmp$ gcc test.c -o test
ming@linuxbox:/tmp$ ./test
Address of str=bfe0e6ec, and point to 8884008
---------------------------------------------------
Address of str[0]=8884008, and point to 8884018
Address of str[1]=888400c, and point to 8884040
Address of str[2]=8884010, and point to 8884068
Address of str[3]=8884014, and point to 8884090
Address of str[4]=8884018, and point to 88840b8
Address of str[5]=888401c, and point to 88840e0
Address of str[6]=8884020, and point to 8884108
Address of str[7]=8884024, and point to 8884130
Address of str[8]=8884028, and point to 8884158
Address of str[9]=888402c, and point to 8884180
---------------------------------------------------
*** glibc detected *** ./6: double free or corruption (out): 0x08884018 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(+0x6b591)[0x17b591]
/lib/tls/i686/cmov/libc.so.6(+0x6cde8)[0x17cde8]
/lib/tls/i686/cmov/libc.so.6(cfree+0x6d)[0x17fecd]
./6[0x8048696]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0x126bd6]
./6[0x8048451]
======= Memory map: ========
00110000-00263000 r-xp 00000000 08:01 6685537 /lib/tls/i686/cmov/libc-2.11.1.so
00263000-00264000 ---p 00153000 08:01 6685537 /lib/tls/i686/cmov/libc-2.11.1.so
00264000-00266000 r--p 00153000 08:01 6685537 /lib/tls/i686/cmov/libc-2.11.1.so
00266000-00267000 rw-p 00155000 08:01 6685537 /lib/tls/i686/cmov/libc-2.11.1.so
00267000-0026a000 rw-p 00000000 00:00 0
003b8000-003b9000 r-xp 00000000 00:00 0 [vdso]
00ef0000-00f0d000 r-xp 00000000 08:01 6553683 /lib/libgcc_s.so.1
00f0d000-00f0e000 r--p 0001c000 08:01 6553683 /lib/libgcc_s.so.1
00f0e000-00f0f000 rw-p 0001d000 08:01 6553683 /lib/libgcc_s.so.1
00f85000-00fa0000 r-xp 00000000 08:01 6553649 /lib/ld-2.11.1.so
00fa0000-00fa1000 r--p 0001a000 08:01 6553649 /lib/ld-2.11.1.so
00fa1000-00fa2000 rw-p 0001b000 08:01 6553649 /lib/ld-2.11.1.so
08048000-08049000 r-xp 00000000 08:01 6684686 /tmp/6
08049000-0804a000 r--p 00000000 08:01 6684686 /tmp/6
0804a000-0804b000 rw-p 00001000 08:01 6684686 /tmp/6
08884000-088a5000 rw-p 00000000 00:00 0 [heap]
b7700000-b7721000 rw-p 00000000 00:00 0
b7721000-b7800000 ---p 00000000 00:00 0
b7833000-b7834000 rw-p 00000000 00:00 0
b7848000-b784b000 rw-p 00000000 00:00 0
bfdfb000-bfe10000 rw-p 00000000 00:00 0 [stack]
Aborted
上面的程式都有檢查所 要進行 free memory 的 pointer，正常的邏輯下不會對一個 pointer 進行兩次以上的 free，但是，將記憶體位址印出來就會發現問題，原因是在配置 pointer 的記憶體時，型態的大小配置錯誤 sizeof(char)，所以其實 pointer 所指向的記憶體區塊發生了問題，產生了兩個 pointers 指向同一塊 memory 的重複情況，因此，glibc 才會偵測到有 double free 發生。
ming@linuxbox:/tmp$ gcc test.c -o test
ming@linuxbox:/tmp$ MALLOC_CHECK_=0 ./test
Address of str=bfabe5ac, and point to 85af008
---------------------------------------------------
Address of str[0]=85af008, and point to 85af018
Address of str[1]=85af00c, and point to 85af040
Address of str[2]=85af010, and point to 85af068
Address of str[3]=85af014, and point to 85af090
Address of str[4]=85af018, and point to 85af0b8
Address of str[5]=85af01c, and point to 85af0e0
Address of str[6]=85af020, and point to 85af108
Address of str[7]=85af024, and point to 85af130
Address of str[8]=85af028, and point to 85af158
Address of str[9]=85af02c, and point to 85af180
---------------------------------------------------
修改程式碼第 12 行的部分，因為這塊記憶體每個 atom 的 data type 是 pointer，因此，在 malloc memory 時要注意每個 atom 的大小是配置 size of pointer，而不是 size of data type。
接著每個 str[i] 所指的記憶體每個 atom 的 data type 是 character，所以這個程式正常的配置記憶體情況將如下圖所示 (程式碼 26 ~ 41 行)
1 #include <stdio.h>
2 #include <stdlib.h>
3
4 int main(int argc, char *argv[])
5 {
6
7 char **str = NULL;
8 const int x=10;
9 const int y=32;
10 int i;
11
12 str = (char**) malloc ( sizeof(str) * x ) ;
13
14 /* NULL pointer */
15 if( ! str ) {
16 printf("NULL Pointer\n");
17 return -1;
18 }
19
20 printf("Address of str=%x, and point to %x\n", &str, str);
21
22 bzero(str, x);
23
24 printf("---------------------------------------------------\n");
25
26 for( i=0; i<x; i++) {
27
28 str[i] = (char *)malloc( sizeof(char) * y );
29
30 if( ! str[i] ) {
31 printf("NULL Pointer\n");
32 return -1;
33 }
34
35
36 printf("Address of str[%d]=%x, and point to %x\n", i, &str[i], str[i]);
37
38 bzero(str[i], y);
39 strcpy(str[i], "hello");
40
41 }
42
43
44 printf("---------------------------------------------------\n");
45
46 for( i=0; i<x; i++) {
47
48 if( str[i] ) {
49 free( str[i] );
50 }
51 }
52
53 if(str) {
54 free (str);
55 }
56
57 return 0;
58 }
59
修正後的執行結果就正確了，程式執行時也不再出現 double free 的錯誤訊息。
ming@linuxbox:/tmp$ gcc test.c -o test
ming@linuxbox:/tmp$ ./test
Address of str=bfaa32bc, and point to 8c5c008
---------------------------------------------------
Address of str[0]=8c5c008, and point to 8c5c038
Address of str[1]=8c5c00c, and point to 8c5c060
Address of str[2]=8c5c010, and point to 8c5c088
Address of str[3]=8c5c014, and point to 8c5c0b0
Address of str[4]=8c5c018, and point to 8c5c0d8
Address of str[5]=8c5c01c, and point to 8c5c100
Address of str[6]=8c5c020, and point to 8c5c128
Address of str[7]=8c5c024, and point to 8c5c150
Address of str[8]=8c5c028, and point to 8c5c178
Address of str[9]=8c5c02c, and point to 8c5c1a0
---------------------------------------------------
Reference:
Red Hat and Fedora Core compatibility tweaks, http://dag.wieers.com/howto/compatibility/.
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