gdb core dump
產生memory leak的錯誤會存在core檔案。
linux預設不會產生core檔案，需修改系統參數。
查看core檔案大小語法
ulimit是限制使用者資源使用量
ulimit -a
cici@cici-vm:~/test/app$ ulimit -a
real-time non-blocking time (microseconds, -R) unlimited
core file size (blocks, -c) 0
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 15194
max locked memory (kbytes, -l) 495132
max memory size (kbytes, -m) unlimited
open files (-n) 1024
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 8192
cpu time (seconds, -t) unlimited
max user processes (-u) 15194
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
由以上列表，可以發現core file size = 0
core file size (blocks, -c) 0
從以上列表，也可以看到stack大小
stack size (kbytes, -s) 8192
修改core檔案大小
-c是上述列表中有列出來的，-c代表的是core file size
將core file size設成無限unlimited
$ ulimit -c unlimited
$ ulimit -a
real-time non-blocking time (microseconds, -R) unlimited
core file size (blocks, -c) unlimited
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 15194
max locked memory (kbytes, -l) 495132
max memory size (kbytes, -m) unlimited
open files (-n) 1024
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 8192
cpu time (seconds, -t) unlimited
max user processes (-u) 15194
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
core dump
建立一個cpp檔案core_test1.cpp
以下func2()中，試圖給ptr指標設值，但ptr指標是nullptr。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
#include <cstring>
#include <iostream>
using namespace std;
void func2(const string msg) {
// 宣告nullptr的ptr指標
char * ptr = nullptr;
// 試圖給nullptr設值
*ptr = 97; // 97代表a
}
void func1(const string msg) {
func2(msg);
}
int main() {
func1("test");
return 0;
}
執行以下指令
$ g++ -o demo04 core_test1.cpp -g
$ ./demo04
程式記憶體區段錯誤 (核心已傾印
$ ls
若是沒有產生"core"開頭的檔案，把core存放路徑改為目前執行程式的目錄下
echo "core" | sudo tee /proc/sys/kernel/core_pattern
再次執行程式
$ ./demo04
$ ls
檢查是否已經產生"core"開頭的檔案
debug
輸入以下指令，core是先前步驟的檔名，每個人的檔名不同，都是以core開頭
gdb ./demo04 core
黃色是要輸入的字母
Enable debuginfod for this session? (y or [n]) y Debuginfod has been enabled. To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit. Downloading separate debug info for system-supplied DSO at 0x7fffff0d2000 --Type < RET > for more, q to quit, c to continue without paging--c [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Core was generated by `./demo04'. Program terminated with signal SIGSEGV, Segmentation fault. #0 0x00005a0b1c65d321 in func2 (msg=...) at core_test1.cpp:8 8 *ptr = 97; // 97代表a (gdb)
8 *ptr = 97; // 97代表a
會告知你錯誤的地方在第8行
bt
輸入bt，會列出呼叫函式的堆疊(call stack)
由下往上呼叫，最下面是main()呼叫func1()，func1()再呼叫func2()，最上層的是func2()
(gdb) bt
#0 0x00005a0b1c65d321 in func2 (msg="test") at core_test1.cpp:8
#1 0x00005a0b1c65d365 in func1 (msg="test") at core_test1.cpp:11
#2 0x00005a0b1c65d3d4 in main () at core_test1.cpp:14
q
輸入q就會離開gdb
顯示錯誤的行號在c++函式庫
修改程式碼
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
#include <cstring>
#include <iostream>
using namespace std;
void func2(const string msg) {
// 宣告nullptr的ptr指標
char * ptr = nullptr;
// 試圖給nullptr設值
strcpy(ptr, msg.c_str());
}
void func1(const string msg) {
func2(msg);
}
int main() {
func1("test");
return 0;
}
執行以下指令
$ g++ -o demo04 core_test1.cpp -g
$ ./demo04
程式記憶體區段錯誤 (核心已傾印)
$ gdb ./demo04 core
以下的錯誤無法看出來是什麼問題
#0 __strcpy_avx2 () at ../sysdeps/x86_64/multiarch/strcpy-avx2.S:127
使用bt可以看出錯誤
(gdb) bt
#0 __strcpy_avx2 () at ../sysdeps/x86_64/multiarch/strcpy-avx2.S:127
#1 0x00005ad51c60a37f in func2 (msg="test") at core_test1.cpp:8
#2 0x00005ad51c60a3c0 in func1 (msg="test") at core_test1.cpp:11
#3 0x00005ad51c60a42f in main () at core_test1.cpp:14
其它方式查看core dump
安裝coredump
sudo apt install systemd-coredump
執行並core dump
進入coredumpctl，使用q可離開
$ g++ -o demo03 core_test.cpp -g
$ ./demo03
程式記憶體區段錯誤 (核心已傾印
$ coredumpctl
TIME PID UID GID SIG COREFILE EXE >
Thu 2024-12-05 15:17:05 CST 25260 1000 1000 SIGSEGV present /home/cici/test/app/demo03>
Thu 2024-12-05 15:26:49 CST 25335 1000 1000 SIGSEGV present /home/cici/test/app/demo03>
$ coredumpctl debug demo03
bt
以下用黃色表示會要使用者輸入的部分與發生錯誤的行數。
bt可以列出呼叫函式的堆疊(call stack)，由下往上呼叫，最下面是main()呼叫func1()，func1()再呼叫func2()，最上層的是func2()
以下，使用q可離開
Enable debuginfod for this session? (y or [n]) y Debuginfod has been enabled. To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit. [Thread debugging using libthread_db enabled] --Type < RET > for more, q to quit, c to continue without paging--c Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Core was generated by `./demo03'. Program terminated with signal SIGSEGV, Segmentation fault. #0 0x00005893c17b7321 in func2 (msg=...) at core_test.cpp:8 8 *ptr = 97; // 97代表a (gdb) bt #0 0x00005893c17b7321 in func2 (msg="test") at core_test.cpp:8 #1 0x00005893c17b7365 in func1 (msg="test") at core_test.cpp:11 #2 0x00005893c17b73d4 in main () at core_test.cpp:14
若發生錯誤的是在c++的函式庫中，怎麼查找錯誤？
將程式碼
*ptr = 97;
修改成c++函式strcpy
strcpy(ptr, msg.c_str());
修改程式碼
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
#include <cstring>
#include <iostream>
using namespace std;
void func2(const string msg) {
// 宣告nullptr的ptr指標
char * ptr = nullptr;
// 試圖給nullptr設值
strcpy(ptr, msg.c_str());
}
void func1(const string msg) {
func2(msg);
}
int main() {
func1("test");
return 0;
}
重新編譯
$ g++ -o demo03 core_test.cpp -g
$ ./demo03
程式記憶體區段錯誤 (核心已傾印
$ coredumpctl
$ coredumpctl debug demo03
使用bt，就可以查找出來發生錯誤的函式，main()呼叫func1，func1呼叫func2，func2呼叫strcpy
#0 __strcpy_avx2 () at ../sysdeps/x86_64/multiarch/strcpy-avx2.S:127 warning: 127 ../sysdeps/x86_64/multiarch/strcpy-avx2.S: 沒有此一檔案或目錄 (gdb) bt #0 __strcpy_avx2 () at ../sysdeps/x86_64/multiarch/strcpy-avx2.S:127 #1 0x000055a25facd37f in func2 (msg="test") at core_test.cpp:8 #2 0x000055a25facd3c0 in func1 (msg="test") at core_test.cpp:11 #3 0x000055a25facd42f in main () at core_test.cpp:14
行數請比對如下
1 #include2 #include 3 using namespace std; 4 void func2(const string msg) { 5 // 宣告nullptr的ptr指標 6 char * ptr = nullptr; 7 // 試圖給nullptr設值 8 strcpy(ptr, msg.c_str()); 9 } 10 void func1(const string msg) { 11 func2(msg); 12 } 13 int main() { 14 func1("test"); 15 return 0; 16 } </pre>