C語言 --段錯誤（核心已轉儲）
阿新 • • 發佈：2019-01-03
文章原地址：https://blog.csdn.net/qq_29350001/article/details/53780697
（侵刪欠）一、什麼是段錯誤？
一旦一個程式發生了越界訪問，cpu 就會產生相應的保護，於是 segmentation fault 就出現了，通過上面的解釋，段錯誤應該就是訪問了不可訪問的記憶體，這個記憶體區要麼是不存在的，要麼是受到系統保護的，還有可能是缺少檔案或者檔案損壞。
二、段錯誤產生的原因
下面是一些典型的段錯誤的原因:
非關聯化空指標——這是特殊情況由記憶體管理硬體
試圖訪問一個不存在的記憶體地址(在程序的地址空間)
試圖訪問記憶體的程式沒有權利(如核心結構流程上下文)
試圖寫入只讀儲存器(如程式碼段)
1、訪問不存在的記憶體地址
在C程式碼,分割錯誤通常發生由於指標的錯誤使用,特別是在C動態記憶體分配。非關聯化一個空指標總是導致段錯誤,但野指標和懸空指標指向的記憶體,可能會或可能不會存在,而且可能或不可能是可讀的還是可寫的,因此會導致瞬態錯誤。
#include <stdio.h>
int main (void)
{
int *ptr = NULL;
*ptr = 0;
return 0;
}
輸出結果：
段錯誤（核心已轉儲） 現在,非關聯化這些變數可能導致段錯誤:非關聯化空指標通常會導致段錯誤,閱讀時從野指標可能導致隨機資料但沒有段錯誤,和閱讀從懸空指標可能導致有效資料,然後隨機資料覆蓋。
2、訪問系統保護的記憶體地址
#include <stdio.h>
int main (void)
{
int *ptr = (int *)0;
*ptr = 100;
return 0;
}
輸出結果:
段錯誤（核心已轉儲）3、訪問只讀的記憶體地址
寫入只讀儲存器提出了一個 segmentation fault,這個發生在程式寫入自己的一部分程式碼段或者是隻讀的資料段,這些都是由作業系統載入到只讀儲存器。
#include <stdio.h> #include <string.h> int main (void) { char *ptr = "test"; strcpy (ptr, "TEST"); return 0; } 輸出結果： 段錯誤（核心已轉儲）
#include <stdio.h>
int main (void)
{
char *ptr = "hello";
*ptr = 'H';
return 0;
}
輸出結果：
段錯誤（核心已轉儲） 包含這個程式碼被編譯程式時,字串“hello”位於rodata部分程式的可執行檔案的只讀部分資料段。當載入時,作業系統與其他字串和地方常數只讀段的記憶體中的資料。當執行時,一個變數 ptr 設定為指向字串的位置,並試圖編寫一個H字元通過變數進入記憶體,導致段錯誤。編譯程式的編譯器不檢查作業的只讀的位置在編譯時,和執行類unix作業系統產生以下執行時發生 segmentation fault。
可以糾正這個程式碼使用一個數組而不是一個字元指標,這個棧上分配記憶體並初始化字串的值:
#include <stdio.h>
int main (void)
{
char ptr[] = "hello";
ptr[0] = 'H';
return 0;
} 即使不能修改字串(相反,這在C標準未定義行為),在C char *型別,所以沒有隱式轉換原始程式碼,在c++的 const char *型別,因此有一個隱式轉換,所以編譯器通常會抓住這個特定的錯誤。
4、空指標廢棄
因為是一個很常見的程式錯誤空指標廢棄（讀或寫在一個空指標，用於C的意思是“沒有物件指標”作為一個錯誤指示器），大多數作業系統記憶體訪問空指標的地址，這樣它會導致段錯誤。
#include <stdio.h>
int main (void)
{
int *ptr = NULL;
printf ("%d\n", *ptr);
return 0;
}
輸出結果：
段錯誤（核心已轉儲） 非關聯化一個空指標，然後分配（寫一個值到一個不存在的目標）也通常會導致段錯誤。
#include <stdio.h>
int main (void)
{
int *ptr = NULL;
*ptr = 1;
return 0;
}
輸出結果：
段錯誤（核心已轉儲） 下面的程式碼包含一個空指標，但當編譯通常不會導致段錯誤，值是未使用的。因此，廢棄通常會被優化掉，死程式碼消除。
#include <stdio.h>
int main (void)
{
int *ptr = NULL;
*ptr;
return 0;
} 還有，比如malloc 動態分配記憶體，釋放、置空完成後，不可再使用該指標。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char* str=(char* )malloc(100);
if(*str)
{
return;
}
strcpy(str,"hello");
printf("%s\n",str);
free(str);
str=NULL;
strcpy(str,"abcdef");
return 0;
}
輸出結果：
hello
段錯誤 (核心已轉儲) 5、堆疊溢位
#include <stdio.h>
#include <string.h>
int main (void)
{
main ();
return 0;
}
輸出結果：
段錯誤（核心已轉儲） 上述例子的無限遞迴，導致的堆疊溢位會導致段錯誤，但無線遞迴未必導致堆疊溢位，優化執行的編譯器和程式碼的確切結構。在這種情況下，遙不可及的程式碼（返回語句）行為是未定義的。因此，編譯器可以消除它，使用尾部呼叫優化，可能導致沒有堆疊使用。其他優化可能包括將遞迴轉換成迭代，給出例子的結構功能永遠會導致程式執行，雖然可能不是其他堆疊溢位。
6、記憶體越界（陣列越界，變數型別不一致等）
#include <stdio.h>
int main (void)
{
char test[10];
printf ("%c\n", test[100000]);
return 0;
}
輸出結果：
段錯誤（核心已轉儲） 三、段錯誤資訊的獲取
程式發生段錯誤時，提示資訊很少，下面有幾種檢視段錯誤的發生資訊的途徑。
1、dmesg
dmesg 可以在應用程式崩潰時，顯示記憶體中儲存的相關資訊。如下所示，通過 dmesg 命令可以檢視發生段錯誤的程式名稱、引起段錯誤發生的記憶體地址、指令指標地址、堆疊指標地址、錯誤程式碼、錯誤原因等。
[email protected]#dmesg
[ 6357.422282] a.out[3044]: segfault at 806851c ip b75cd668 sp bf8b2100 error 4 in libc-2.15.so[b7559000+19f000] 2、-g
使用gcc編譯程式的原始碼時，加上 -g 引數，這樣可以使得生成的二進位制檔案中加入可以用於 gdb 除錯的有用資訊。
可產生供gdb除錯用的可執行檔案，大小明顯比只用-o選項編譯彙編連線後的檔案大。
[email protected]:/home/tarena/project/c_test# gcc hello.c
[email protected]:/home/tarena/project/c_test# ls -la a.out
-rwxr-xr-x 1 root root 7159 Nov 26 23:32 a.out
[email protected]:/home/tarena/project/c_test# gcc -g hello.c
[email protected]:/home/tarena/project/c_test# ls -la a.out
-rwxr-xr-x 1 root root 8051 Nov 26 23:32 a.out gdb的簡單使用：
(gdb)l 列表（list）
(gdb)r 執行（run）
(gdb)n 下一個（next）
(gdb)q 退出（quit）
(gdb)p 輸出（print）
(gdb)c 繼續（continue）
(gdb)b 4 設定斷點（break）
(gdb)d 刪除斷點（delete）
[email protected]:/home/tarena/project/c_test# gdb a.out
GNU gdb (Ubuntu/Linaro 7.4-2012.02-0ubuntu2) 7.4-2012.02
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /home/tarena/project/c_test/a.out...done.
(gdb) l
1 #include <stdio.h>
2 int main (void)
3 {
4 printf ("hello world!\n");
5 return 0;
6 }
(gdb) r
Starting program: /home/tarena/project/c_test/a.out
hello world!
[Inferior 1 (process 6906) exited normally]
(gdb) q
[email protected]:/home/tarena/project/c_test# 3、nm
使用 nm 命令列出二進位制檔案中符號表，包括符號地址、符號型別、符號名等。這樣可以幫助定位在哪裡發生了段錯誤。
[email protected]# nm a.out
08049f28 d _DYNAMIC
08049ff4 d _GLOBAL_OFFSET_TABLE_
080484ac R _IO_stdin_used
w _Jv_RegisterClasses
08049f18 d __CTOR_END__
08049f14 d __CTOR_LIST__
08049f20 D __DTOR_END__
08049f1c d __DTOR_LIST__
080485a4 r __FRAME_END__
08049f24 d __JCR_END__
08049f24 d __JCR_LIST__
0804a010 A __bss_start
0804a008 D __data_start
08048460 t __do_global_ctors_aux
08048330 t __do_global_dtors_aux
0804a00c D __dso_handle
w __gmon_start__
08048452 T __i686.get_pc_thunk.bx
08049f14 d __init_array_end
08049f14 d __init_array_start
08048450 T __libc_csu_fini
080483e0 T __libc_csu_init
U [email protected]@GLIBC_2.0
0804a010 A _edata
0804a018 A _end
0804848c T _fini
080484a8 R _fp_hw
08048294 T _init
08048300 T _start
0804a010 b completed.6159
0804a008 W data_start
0804a014 b dtor_idx.6161
08048390 t frame_dummy
080483b4 T main 4、ldd
使用 ldd 命令檢視二進位制程式的共享連結庫依賴，包括庫的名稱、起始地址，這樣可以確定段錯誤到底是發生在了自己的程式中還是依賴的共享庫中。
[email protected]# ldd a.out
linux-gate.so.1 => (0xb7765000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb75ac000)
/lib/ld-linux.so.2 (0xb7766000) 四、段錯誤的除錯方法
接下來的講解是圍繞下面的程式碼進行的：
#include <stdio.h>
int main (void)
{
int *ptr = NULL;
*ptr = 10;
return 0;
}
輸出結果：
段錯誤（核心已轉儲） 1、使用 printf 輸出資訊
這個是看似最簡單，但往往很多情況下十分有效的除錯方式，也許可以說是程式設計師用的最多的除錯方式。簡單來說，就是在程式的重要程式碼附近加上像 printf 這類輸出資訊，這樣可以跟蹤並打印出段錯誤在程式碼中可能出現的位置。
為了方便使用這種方法，可以使用條件編譯指令 #define DEBUG 和 #endif 把 printf 函式包起來。這樣在程式編譯時，如果加上 -DDEBUG 引數就可以檢視除錯資訊；否則不加上引數就不會顯示除錯資訊。
2、使用 gcc 和 gdb
1）除錯步驟
A、為了能夠使用 gdb 除錯程式，在編譯階段加上 -g 引數。
[email protected]# gcc -g test.c [email protected]# gdb a.out
GNU gdb (Ubuntu/Linaro 7.4-2012.02-0ubuntu2) 7.4-2012.02
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /home/tarena/project/c_test/a.out...done.
(gdb) C、進入 gdb 後，執行程式
(gdb) r
Starting program: /home/tarena/project/c_test/a.out
Program received signal SIGSEGV, Segmentation fault.
0x080483c4 in main () at test.c:6
6 *ptr = 10;
(gdb) 從輸出看出，程式收到 SIGSEGV 訊號，觸發段錯誤，並提示地址 0x080483c4、建立了一個空指標，然後試圖訪問它的值（讀值）。
可以通過man 7 signal檢視SIGSEGV的資訊
Signal Value Action Comment
──────────────────────────────────────────────────────────────────────
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit from keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 6 Core Abort signal from abort(3)
SIGFPE 8 Core Floating point exception
SIGKILL 9 Term Kill signal
SIGSEGV 11 Core Invalid memory reference
SIGPIPE 13 Term Broken pipe: write to pipe with no
readers
SIGALRM 14 Term Timer signal from alarm(2)
SIGTERM 15 Term Termination signal
SIGUSR1 30,10,16 Term User-defined signal 1
SIGUSR2 31,12,17 Term User-defined signal 2
SIGCHLD 20,17,18 Ign Child stopped or terminated
SIGCONT 19,18,25 Cont Continue if stopped
SIGSTOP 17,19,23 Stop Stop process
SIGTSTP 18,20,24 Stop Stop typed at tty
SIGTTIN 21,21,26 Stop tty input for background process
SIGTTOU 22,22,27 Stop tty output for background process
The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored. D、完成除錯後，輸入 q 命令退出 gdb
(gdb) q
A debugging session is active.
Inferior 1 [process 3483] will be killed.
Quit anyway? (y or n) y 2）適用場景
A、僅當能確定程式一定會發生段錯誤的情況下適用。
B、當程式的原始碼可以獲得的情況下，使用 -g 引數編譯程式
C、一般用於測試階段，生產環境下 gdb 會有副作用：使程式執行減慢，執行不夠穩定，等等。
D、即使在測試階段，如果程式過於複雜，gdb 也不能處理。
3、使用 core 檔案和 gdb
上面有提到段錯誤觸發SIGSEGV訊號，通過man 7 signal，可以看到SIGSEGV預設的處理程式（handler）會列印段錯誤資訊，併產生 core 檔案，由此我們可以藉助於程式異常退出生成的 core 檔案中的除錯資訊，使用 gdb 工具來除錯程式中的段錯誤。
1）除錯步驟
A、在一些Linux版本下，預設是不產生 core 檔案的，首先可以檢視一下系統 core 檔案的大小限制：
[email protected]# ulimit -c
0 B、可以看到預設設定情況下，本機Linux環境下發生段錯誤不會自動生成 core 檔案，下面設定下 core 檔案的大小限制（單位為KB）
[email protected]# ulimit -c 1024
[email protected]# ulimit -c
1024[email protected]# ./a.out
段錯誤 (核心已轉儲) D、載入 core 檔案，使用 gdb 工具進行除錯
[email protected]:/home/tarena/project/c_test# gdb a.out core
GNU gdb (Ubuntu/Linaro 7.4-2012.02-0ubuntu2) 7.4-2012.02
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /home/tarena/project/c_test/a.out...done.
[New LWP 3491]
warning: Can't read pathname for load map: 輸入/輸出錯誤.
Core was generated by `./a.out'.
Program terminated with signal 11, Segmentation fault.
#0 0x080483c4 in main () at test.c:6
6 *ptr = 10;
(gdb) E、完成除錯後，輸入 q 命令退出 gdb
(gdb) q 2）適用場景
A、適合於在實際生成環境下除錯程度的段錯誤（即在不用重新發生段錯誤的情況下重現段錯誤）
B、當程式很複雜，core 檔案相當大時，該方法不可用
4、使用 objdump
1）除錯步驟
A、使用 dmesg 命令，找到最近發生的段錯誤輸入資訊
[email protected]# dmesg
[ 372.350652] a.out[2712]: segfault at 0 ip 080483c4 sp bfd1f7b8 error 6 in a.out[8048000+1000] 其中，對我們接下來的除錯過程有用的是發生段錯誤的地址 0 和指令指標地址 080483c4。
有時候,“地址引起的錯”可以告訴你問題的根源。看到上面的例子,我們可以說，int *ptr = NULL; *ptr = 10;，建立了一個空指標，然後試圖訪問它的值（讀值）。
B、使用 objdump 生成二進位制的相關資訊，重定向到檔案中
[email protected]# objdump -d a.out > a.outDump
[email protected]# ls
a.out a.outDump core test.c 其中，生成的 a.outDump 檔案中包含了二進位制檔案的 a.out 的彙編程式碼
C、在 a.outDump 檔案中查詢發生段錯誤的地址
[email protected]:/home/tarena/project/c_test# grep -n -A 10 -B 10 "0" a.outDump
1-
2-a.out: file format elf32-i386
118:080483b4 <main>:
119: 80483b4: 55 push %ebp
120: 80483b5: 89 e5 mov %esp,%ebp
121: 80483b7: 83 ec 10 sub $0x10,%esp
122: 80483ba: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)
123: 80483c1: 8b 45 fc mov -0x4(%ebp),%eax
124: 80483c4: c7 00 0a 00 00 00 movl $0xa,(%eax)
125: 80483ca: b8 00 00 00 00 mov $0x0,%eax
126: 80483cf: c9 leave
127: 80483d0: c3 ret
128: 80483d1: 90 nop
129: 80483d2: 90 nop
130: 80483d3: 90 nop
131: 80483d4: 90 nop
132: 80483d5: 90 nop
133: 80483d6: 90 nop
134: 80483d7: 90 nop
135: 80483d8: 90 nop
136: 80483d9: 90 nop
137: 80483da: 90 nop
138: 80483db: 90 nop
139: 80483dc: 90 nop
140: 80483dd: 90 nop
141: 80483de: 90 nop
142: 80483df: 90 nop 2）適用場景
A、不需要 -g 引數編譯，不需要藉助於core檔案，但需要有一定的組合語言基礎。
B、如果使用 gcc 編譯優化引數（-O1，-O2，-O3）的話，生成的彙編指令將會被優化，使得除錯過程有些難度。
5、使用catchsegv
catchsegv 命令專門用來補貨段錯誤，它通過動態載入器（ld-linux.so）的預載入機制（PRELOAD）把一個事先寫好的 庫（/lib/libSegFault.so）載入上，用於捕捉段錯誤的出錯資訊。
[email protected]# catchsegv ./a.out
Segmentation fault (core dumped)
*** Segmentation fault
Register dump:
EAX: 00000000 EBX: b77a1ff4 ECX: bfd8a0e4 EDX: bfd8a074
ESI: 00000000 EDI: 00000000 EBP: bfd8a048 ESP: bfd8a038
EIP: 080483c4 EFLAGS: 00010282
CS: 0073 DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b
Trap: 0000000e Error: 00000006 OldMask: 00000000
ESP/signal: bfd8a038 CR2: 00000000
Backtrace:
??:0(main)[0x80483c4]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0xb761a4d3]
??:0(_start)[0x8048321]
Memory map:
08048000-08049000 r-xp 00000000 08:01 2102158 /home/tarena/project/c_test/a.out
08049000-0804a000 r--p 00000000 08:01 2102158 /home/tarena/project/c_test/a.out
0804a000-0804b000 rw-p 00001000 08:01 2102158 /home/tarena/project/c_test/a.out
09467000-0948c000 rw-p 00000000 00:00 0 [heap]
b75e1000-b75fd000 r-xp 00000000 08:01 1704884 /lib/i386-linux-gnu/libgcc_s.so.1
b75fd000-b75fe000 r--p 0001b000 08:01 1704884 /lib/i386-linux-gnu/libgcc_s.so.1
b75fe000-b75ff000 rw-p 0001c000 08:01 1704884 /lib/i386-linux-gnu/libgcc_s.so.1
b75ff000-b7601000 rw-p 00000000 00:00 0
b7601000-b77a0000 r-xp 00000000 08:01 1704863 /lib/i386-linux-gnu/libc-2.15.so
b77a0000-b77a2000 r--p 0019f000 08:01 1704863 /lib/i386-linux-gnu/libc-2.15.so
b77a2000-b77a3000 rw-p 001a1000 08:01 1704863 /lib/i386-linux-gnu/libc-2.15.so
b77a3000-b77a6000 rw-p 00000000 00:00 0
b77b8000-b77bb000 r-xp 00000000 08:01 1704847 /lib/i386-linux-gnu/libSegFault.so
b77bb000-b77bc000 r--p 00002000 08:01 1704847 /lib/i386-linux-gnu/libSegFault.so
b77bc000-b77bd000 rw-p 00003000 08:01 1704847 /lib/i386-linux-gnu/libSegFault.so
b77bd000-b77bf000 rw-p 00000000 00:00 0
b77bf000-b77c0000 r-xp 00000000 00:00 0 [vdso]
b77c0000-b77e0000 r-xp 00000000 08:01 1704843 /lib/i386-linux-gnu/ld-2.15.so
b77e0000-b77e1000 r--p 0001f000 08:01 1704843 /lib/i386-linux-gnu/ld-2.15.so
b77e1000-b77e2000 rw-p 00020000 08:01 1704843 /lib/i386-linux-gnu/ld-2.15.so
bfd6b000-bfd8c000 rw-p 00000000 00:00 0 [stack]五、記憶體洩漏檢測工具Valgrind
Valgrind是一款用於記憶體除錯、記憶體洩漏檢測以及效能分析的軟體開發工具。
（1）下載安裝
#tar xvf valgrind-3.13.0.tar.bz2
#cd valgrind-3.13.0
#./configure --prefix=/usr/local/webserver/valgrind
#make
#make install
新增環境變數：
#vi /etc/profile
PATH=${PATH}:/usr/local/webserver/valgrind/bin
#source /etc/profile # valgrind ls -l
==16243== Memcheck, a memory error detector
==16243== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==16243== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==16243== Command: ls -l
==16243==
valgrind: Fatal error at startup: a function redirection
valgrind: which is mandatory for this platform-tool combination
valgrind: cannot be set up. Details of the redirection are:
valgrind:
valgrind: A must-be-redirected function
valgrind: whose name matches the pattern: strlen
valgrind: in an object with soname matching: ld-linux.so.2
valgrind: was not found whilst processing
valgrind: symbols from the object with soname: ld-linux.so.2
valgrind:
valgrind: Possible fixes: (1, short term): install glibc's debuginfo
valgrind: package on this machine. (2, longer term): ask the packagers
valgrind: for your Linux distribution to please in future ship a non-
valgrind: stripped ld.so (or whatever the dynamic linker .so is called)
valgrind: that exports the above-named function using the standard
valgrind: calling conventions for this platform. The package you need
valgrind: to install for fix (1) is called
valgrind:
valgrind: On Debian, Ubuntu: libc6-dbg
valgrind: On SuSE, openSuSE, Fedora, RHEL: glibc-debuginfo
valgrind:
valgrind: Note that if you are debugging a 32 bit process on a
valgrind: 64 bit system, you will need a corresponding 32 bit debuginfo
valgrind: package (e.g. libc6-dbg:i386).
valgrind:
valgrind: Cannot continue -- exiting now. Sorry. 執行：sudo apt-get install libc6-dbg 以安裝libc6-dbg
然後再次執行： valgrind ls -l
$ valgrind ls -l
==14297== Memcheck, a memory error detector
==14297== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14297== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14297== Command: ls -l
==14297==
總用量 4
-rw-rw-r-- 1 tarena tarena 105 1月 15 10:04 test.c
==14297==
==14297== HEAP SUMMARY:
==14297== in use at exit: 13,265 bytes in 29 blocks
==14297== total heap usage: 218 allocs, 189 frees, 73,974 bytes allocated
==14297==
==14297== LEAK SUMMARY:
==14297== definitely lost: 80 bytes in 2 blocks
==14297== indirectly lost: 240 bytes in 20 blocks
==14297== possibly lost: 0 bytes in 0 blocks
==14297== still reachable: 12,945 bytes in 7 blocks
==14297== suppressed: 0 bytes in 0 blocks
==14297== Rerun with --leak-check=full to see details of leaked memory
==14297==
==14297== For counts of detected and suppressed errors, rerun with: -v
==14297== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) （2）選項：
<span style="color:#000000;">#valgrind -h
usage: valgrind [options] prog-and-args
tool-selection option, with default in [ ]:
--tool=<name> use the Valgrind tool named <name> [memcheck]
basic user options for all Valgrind tools, with defaults in [ ]:
-h --help show this message
--help-debug show this message, plus debugging options
--version show version
-q --quiet run silently; only print error msgs
-v --verbose be more verbose -- show misc extra info
--trace-children=no|yes Valgrind-ise child processes (follow execve)? [no]
--trace-children-skip=patt1,patt2,... specifies a list of executables
that --trace-children=yes should not trace into
--trace-children-skip-by-arg=patt1,patt2,... same as --trace-children-skip=
but check the argv[] entries for children, rather
than the exe name, to make a follow/no-follow decision
--child-silent-after-fork=no|yes omit child output between fork & exec? [no]
--vgdb=no|yes|full activate gdbserver? [yes]
full is slower but provides precise watchpoint/step
--vgdb-error=<number> invoke gdbserver after <number> errors [999999999]
to get started quickly, use --vgdb-error=0
and follow the on-screen directions
--vgdb-stop-at=event1,event2,... invoke gdbserver for given events [none]
where event is one of:
startup exit valgrindabexit all none
--track-fds=no|yes track open file descriptors? [no]
--time-stamp=no|yes add timestamps to log messages? [no]
--log-fd=<number> log messages to file descriptor [2=stderr]
--log-file=<file> log messages to <file>
--log-socket=ipaddr:port log messages to socket ipaddr:port
user options for Valgrind tools that report errors:
--xml=yes emit error output in XML (some tools only)
--xml-fd=<number> XML output to file descriptor
--xml-file=<file> XML output to <file>
--xml-socket=ipaddr:port XML output to socket ipaddr:port
--xml-user-comment=STR copy STR verbatim into XML output
--demangle=no|yes automatically demangle C++ names? [yes]
--num-callers=<number> show <number> callers in stack traces [12]
--error-limit=no|yes stop showing new errors if too many? [yes]
--error-exitcode=<number> exit code to return if errors found [0=disable]
--error-markers=<begin>,<end> add lines with begin/end markers before/after
each error output in plain text mode [none]
--show-below-main=no|yes continue stack traces below main() [no]
--default-suppressions=yes|no
load default suppressions [yes]
--suppressions=<filename> suppress errors described in <filename>
--gen-suppressions=no|yes|all print suppressions for errors? [no]
--input-fd=<number> file descriptor for input [0=stdin]
--dsymutil=no|yes run dsymutil on Mac OS X when helpful? [yes]
--max-stackframe=<number> assume stack switch for SP changes larger
than <number> bytes [2000000]
--main-stacksize=<number> set size of main thread's stack (in bytes)
[min(max(current 'ulimit' value,1MB),16MB)]
user options for Valgrind tools that replace malloc:
--alignment=<number> set minimum alignment of heap allocations [8]
--redzone-size=<number> set minimum size of redzones added before/after
heap blocks (in bytes). [16]
--xtree-memory=none|allocs|full profile heap memory in an xtree [none]
and produces a report at the end of the execution
none: no profiling, allocs: current allocated
size/blocks, full: profile current and cumulative
allocated size/blocks and freed size/blocks.
--xtree-memory-file=<file> xtree memory report file [xtmemory.kcg.%p]
uncommon user options for all Valgrind tools:
--fullpath-after= (with nothing after the '=')
show full source paths in call stacks
--fullpath-after=string like --fullpath-after=, but only show the
part of the path after 'string'. Allows removal
of path prefixes. Use this flag multiple times
to specify a set of prefixes to remove.
--extra-debuginfo-path=path absolute path to search for additional
debug symbols, in addition to existing default
well known search paths.
--debuginfo-server=ipaddr:port also query this server
(valgrind-di-server) for debug symbols
--allow-mismatched-debuginfo=no|yes [no]
for the above two flags only, accept debuginfo
objects that don't "match" the main object
--smc-check=none|stack|all|all-non-file [all-non-file]
checks for self-modifying code: none, only for
code found in stacks, for all code, or for all
code except that from file-backed mappings
--read-inline-info=yes|no read debug info about inlined function calls
and use it to do better stack traces. [yes]
on Linux/Android/Solaris for Memcheck/Helgrind/DRD
only. [no] for all other tools and platforms.
--read-var-info=yes|no read debug info on stack and global variables
and use it to print better error messages in
tools that make use of it (Memcheck, Helgrind,
DRD) [no]
--vgdb-poll=<number> gdbserver poll max every <number> basic blocks [5000]
--vgdb-shadow-registers=no|yes let gdb see the shadow registers [no]
--vgdb-prefix=<prefix> prefix for vgdb FIFOs [/tmp/vgdb-pipe]
--run-libc-freeres=no|yes free up glibc memory at exit on Linux? [yes]
--run-cxx-freeres=no|yes free up libstdc++ memory at exit on Linux
and Solaris? [yes]
--sim-hints=hint1,hint2,... activate unusual sim behaviours [none]
where hint is one of:
lax-ioctls lax-doors fuse-compatible enable-outer
no-inner-prefix no-nptl-pthread-stackcache fallback-llsc none
--fair-sched=no|yes|try schedule threads fairly on multicore systems [no]
--kernel-variant=variant1,variant2,...
handle non-standard kernel variants [none]
where variant is one of:
bproc android-no-hw-tls
android-gpu-sgx5xx android-gpu-adreno3xx none
--merge-recursive-frames=<number> merge frames between identical
program counters in max <number> frames) [0]
--num-transtab-sectors=<number> size of translated code cache [32]
more sectors may increase performance, but use more memory.
--avg-transtab-entry-size=<number> avg size in bytes of a translated
basic block [0, meaning use tool provided default]
--aspace-minaddr=0xPP avoid mapping memory below 0xPP [guessed]
--valgrind-stacksize=<number> size of valgrind (host) thread's stack
(in bytes) [1048576]
--show-emwarns=no|yes show warnings about emulation limits? [no]
--require-text-symbol=:sonamepattern:symbolpattern abort run if the
stated shared object doesn't have the stated
text symbol. Patterns can contain ? and *.
--soname-synonyms=syn1=pattern1,syn2=pattern2,... synonym soname
specify patterns for function wrapping or replacement.
To use a non-libc malloc library that is
in the main exe: --soname-synonyms=somalloc=NONE
in libxyzzy.so: --soname-synonyms=somalloc=libxyzzy.so
--sigill-diagnostics=yes|no warn about illegal instructions? [yes]
--unw-stack-scan-thresh=<number> Enable stack-scan unwind if fewer
than <number> good frames found [0, meaning "disabled"]
NOTE: stack scanning is only available on arm-linux.
--unw-stack-scan-frames=<number> Max number of frames that can be
recovered by stack scanning [5]
--resync-filter=no|yes|verbose [yes on MacOS, no on other OSes]
attempt to avoid expensive address-space-resync operations
--max-threads=<number> maximum number of threads that valgrind can
handle [500]
user options for Memcheck:
--leak-check=no|summary|full search for memory leaks at exit? [summary]
--leak-resolution=low|med|high differentiation of leak stack traces [high]
--show-leak-kinds=kind1,kind2,.. which leak kinds to show?
[definite,possible]
--errors-for-leak-kinds=kind1,kind2,.. which leak kinds are errors?
[definite,possible]
where kind is one of:
definite indirect possible reachable all none
--leak-check-heuristics=heur1,heur2,... which heuristics to use for
improving leak search false positive [all]
where heur is one of:
stdstring length64 newarray multipleinheritance all none
--show-reachable=yes same as --show-leak-kinds=all
--show-reachable=no --show-possibly-lost=yes
same as --show-leak-kinds=definite,possible
--show-reachable=no --show-possibly-lost=no
same as --show-leak-kinds=definite
--xtree-leak=no|yes output leak result in xtree format? [no]
--xtree-leak-file=<file> xtree leak report file [xtleak.kcg.%p]
--undef-value-errors=no|yes check for undefined value errors [yes]
--track-origins=no|yes show origins of undefined values? [no]
--partial-loads-ok=no|yes too hard to explain here; see manual [yes]
--expensive-definedness-checks=no|yes
Use extra-precise definedness tracking [no]
--freelist-vol=<number> volume of freed blocks queue [20000000]
--freelist-big-blocks=<number> releases first blocks with size>= [1000000]
--workaround-gcc296-bugs=no|yes self explanatory [no]. Deprecated.
Use --ignore-range-below-sp instead.
--ignore-ranges=0xPP-0xQQ[,0xRR-0xSS] assume given addresses are OK
--ignore-range-below-sp=<number>-<number> do not report errors for
accesses at the given offsets below SP
--malloc-fill=<hexnumber> fill malloc'd areas with given value
--free-fill=<hexnumber> fill free'd areas with given value
--keep-stacktraces=alloc|free|alloc-and-free|alloc-then-free|none
stack trace(s) to keep for malloc'd/free'd areas [alloc-and-free]
--show-mismatched-frees=no|yes show frees that don't match the allocator? [yes]
Extra options read from ~/.valgrindrc, $VALGRIND_OPTS, ./.valgrindrc
Memcheck is Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
Valgrind is Copyright (C) 2000-2017, and GNU GPL'd, by Julian Seward et al.
LibVEX is Copyright (C) 2004-2017, and GNU GPL'd, by OpenWorks LLP et al.
Bug reports, feedback, admiration, abuse, etc, to: www.valgrind.org.</span><span style="color:#993399;">
</span>（3）使用
1、使用未初始化記憶體問題
#include<stdio.h>
int main(void)
{
int a[5];
int i,s;
a[0]=a[1]=a[2]=a[3]=a[4]=0;
s=0;
for (i =0;i<5;i++)
s+=a[i];
if(s==377)
printf("sum is %d\n",s);
return 0;
}
執行：
# gcc -Wall test.c -g -o test
# valgrind --tool=memcheck --leak-check=yes ./test
$ valgrind --tool=memcheck --leak-check=yes ./test
==14496== Memcheck, a memory error detector
==14496== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14496== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14496== Command: ./test
==14496==
==14496==
==14496== HEAP SUMMARY:
==14496== in use at exit: 0 bytes in 0 blocks
==14496== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==14496==
==14496== All heap blocks were freed -- no leaks are possible
==14496==
==14496== For counts of detected and suppressed errors, rerun with: -v
==14496== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) 2、記憶體讀寫越界
#include<stdlib.h>
#include<stdio.h>
int main(int argc,char *argv[])
{
int len =4;
int* pt =(int*)malloc(len*sizeof(int));
int* p=pt;
int i;
for(i=0;i<len;i++)
{
p++;
}
*p=5;
printf("the value of p equal:%d\n",*p);
return 0;
}
執行：
# gcc -Wall test.c -g -o test
# valgrind --tool=memcheck --leak-check=yes ./test
$ valgrind --tool=memcheck --leak-check=yes ./test
==14514== Memcheck, a memory error detector
==14514== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14514== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14514== Command: ./test
==14514==
==14514== Invalid write of size 4
==14514== at 0x8048462: main (test.c:14)
==14514== Address 0x41f4038 is 0 bytes after a block of size 16 alloc'd
==14514== at 0x402BD74: malloc (vg_replace_malloc.c:270)
==14514== by 0x8048433: main (test.c:7)
==14514==
==14514== Invalid read of size 4
==14514== at 0x804846C: main (test.c:15)
==14514== Address 0x41f4038 is 0 bytes after a block of size 16 alloc'd
==14514== at 0x402BD74: malloc (vg_replace_malloc.c:270)
==14514== by 0x8048433: main (test.c:7)
==14514==
the value of p equal:5
==14514==
==14514== HEAP SUMMARY:
==14514== in use at exit: 16 bytes in 1 blocks
==14514== total heap usage: 1 allocs, 0 frees, 16 bytes allocated
==14514==
==14514== 16 bytes in 1 blocks are definitely lost in loss record 1 of 1
==14514== at 0x402BD74: malloc (vg_replace_malloc.c:270)
==14514== by 0x8048433: main (test.c:7)
==14514==
==14514== LEAK SUMMARY:
==14514== definitely lost: 16 bytes in 1 blocks
==14514== indirectly lost: 0 bytes in 0 blocks
==14514== possibly lost: 0 bytes in 0 blocks
==14514== still reachable: 0 bytes in 0 blocks
==14514== suppressed: 0 bytes in 0 blocks
==14514==
==14514== For counts of detected and suppressed errors, rerun with: -v
==14514== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0) 3、記憶體覆蓋
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main(int argc, char* argv[])
{
char x[50];
int i;
for(i=0;i<50;i++)
{
x[i]=i+1;
}
strncpy(x+20,x,20);
strncpy(x+20,x,21);
strncpy(x,x+20,20);
strncpy(x,x+20,21);
x[39]='\0';
strcpy(x,x+20);
x[39]=39;
x[50]='\0';
strcpy(x,x+20);
return 0;
}
執行：
# gcc -Wall test.c -g -o test
# valgrind --tool=memcheck --leak-check=yes ./test
$ valgrind --tool=memcheck --leak-check=yes ./test
==14551== Memcheck, a memory error detector
==14551== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14551== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14551== Command: ./test
==14551==
==14551== Source and destination overlap in strncpy(0xbec6a0c3, 0xbec6a0af, 21)
==14551== at 0x402C860: strncpy (mc_replace_strmem.c:472)
==14551== by 0x80484E6: main (test.c:14)
==14551==
==14551== Source and destination overlap in strncpy(0xbec6a0af, 0xbec6a0c3, 21)
==14551== at 0x402C860: strncpy (mc_replace_strmem.c:472)
==14551== by 0x8048524: main (test.c:16)
==14551==
==14551== Source and destination overlap in strcpy(0xbec6a09a, 0xbec6a0ae)
==14551== at 0x402C6AB: strcpy (mc_replace_strmem.c:438)
==14551== by 0x8048561: main (test.c:23)
==14551==
==14551==
==14551== HEAP SUMMARY:
==14551== in use at exit: 0 bytes in 0 blocks
==14551== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==14551==
==14551== All heap blocks were freed -- no leaks are possible
==14551==
==14551== For counts of detected and suppressed errors, rerun with: -v
==14551== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0) 4、動態記憶體管理錯誤
include<stdlib.h>
#include<stdio.h>
int main(int argc,char *argv[])
{
int i;
char* p=(char*)malloc(10);
char* pt=p;
for(i =0;i<10;i++)
{
p[i]='z';
}
delete p;
pt[1]='x';
free(pt);
return 0;
}
執行：
# g++ -Wall test.cpp -g -o test
# valgrind --tool=memcheck --leak-check=yes ./test
$ valgrind --tool=memcheck --leak-check=yes ./test
==14607== Memcheck, a memory error detector
==14607== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14607== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14607== Command: ./test
==14607==
==14607== Mismatched free() / delete / delete []
==14607== at 0x402A92A: operator delete(void*) (vg_replace_malloc.c:480)
==14607== by 0x80484F6: main (test.cpp:13)
==14607== Address 0x4324028 is 0 bytes inside a block of size 10 alloc'd
==14607== at 0x402BD74: malloc (vg_replace_malloc.c:270)
==14607== by 0x80484B8: main (test.cpp:7)
==14607==
==14607== Invalid write of size 1
==14607== at 0x80484FE: main (test.cpp:15)
==14607== Address 0x4324029 is 1 bytes inside a block of size 10 free'd
==14607== at 0x402A92A: operator delete(void*) (vg_replace_malloc.c:480)
==14607== by 0x80484F6: main (test.cpp:13)
==14607==
==14607== Invalid free() / delete / delete[] / realloc()
==14607== at 0x402AD3D: free (vg_replace_malloc.c:446)
==14607== by 0x804850C: main (test.cpp:16)
==14607== Address 0x4324028 is 0 bytes inside a block of size 10 free'd
==14607== at 0x402A92A: operator delete(void*) (vg_replace_malloc.c:480)
==14607== by 0x80484F6: main (test.cpp:13)
==14607==
==14607==
==14607== HEAP SUMMARY:
==14607== in use at exit: 0 bytes in 0 blocks
==14607== total heap usage: 1 allocs, 2 frees, 10 bytes allocated
==14607==
==14607== All heap blocks were freed -- no leaks are possible
==14607==
==14607== For counts of detected and suppressed errors, rerun with: -v
==14607== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0) 5、記憶體洩漏
//tree.h
#ifndef _BADLEAK_
#define _BAD_LEAK_
typedef struct _node{
struct _node *l;
struct _node * r;
char v;
}node;
node *mk( node *l, node *r, char val);
void nodefr(node *n);
#endif
//tree.cpp
#include <stdio.h>
#include "tree.h"
#include <stdlib.h>
node *mk(node *l, node * r, char val)
{
node *f=(node* )malloc(sizeof(node));
f->l=l;
f->r=r;
f->v=val;
return f;
}
void nodefr(node* n)
{
if(n)
{
nodefr(n->l);
nodefr(n->r);
free(n);
}
}
//badleak.cpp
#include <stdio.h>
#include <stdlib.h>
#include "tree.h"
int main()
{
node* tree1, *tree2, * tree3;
tree1=mk(mk(mk(0,0,'3'),0,'2'),0,'1');
tree2=mk(0,mk(0,mk(0,0,'6'),'5'),'4');
tree3=mk(mk(tree1,tree2,'8'),0,'7');
//nodefr(tree3);
return 0;
}
執行：
# g++ -Wall tree.cpp badleak.cpp -g -o badleak
#valgrind --tool=memcheck --leak-check=yes ./badleak
$ valgrind --tool=memcheck --leak-check=yes ./badleak
==14676== Memcheck, a memory error detector
==14676== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14676== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14676== Command: ./badleak
==14676==
==14676==
==14676== HEAP SUMMARY:
==14676== in use at exit: 96 bytes in 8 blocks
==14676== total heap usage: 8 allocs, 0 frees, 96 bytes allocated
==14676==
==14676== 96 (12 direct, 84 indirect) bytes in 1 blocks are definitely lost in loss record 8 of 8
==14676== at 0x402BD74: malloc (vg_replace_malloc.c:270)
==14676== by 0x804842B: mk(_node*, _node*, char) (tree.cpp:7)
==14676== by 0x8048560: main (badleak.cpp:10)
==14676==
==14676== LEAK SUMMARY:
==14676== definitely lost: 12 bytes in 1 blocks
==14676== indirectly lost: 84 bytes in 7 blocks
==14676== possibly lost: 0 bytes in 0 blocks
==14676== still reachable: 0 bytes in 0 blocks
==14676== suppressed: 0 bytes in 0 blocks
==14676==
==14676== For counts of detected and suppressed errors, rerun with: -v
==14676== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0) 六、一些注意事項
1）出現段錯誤時，首先應該想到段錯誤的定義，從它出發考慮引發錯誤的原因。
2）在使用指標時，定義了指標後記得初始化指標，在使用的時候記得判斷是否為 NULL
3）在使用陣列時，注意陣列是否被初始化，陣列下標是否越界，陣列元素是否存在等
4）在訪問變數，注意變數所佔地址空間是否已經被程式釋放掉
5）在處理變數時，注意變數的格式控制是否合理等