gdb打印技巧

打印大数组中的内容

int main()
{
  int array[201];
  int i;

  for (i = 0; i < 201; i++)
    array[i] = i;

  return 0;
}

在gdb中，如果要打印大数组的内容，缺省最多会显示200个元素：

(gdb) p array
$1 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 
  48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 
  95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 
  133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 
  170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199...}

• 可以使用如下命令，设置这个最大限制数：

(gdb) set print elements number-of-elements

• 也可以使用如下命令，设置为没有限制：

(gdb) set print elements 0

或者

(gdb) set print elements unlimited

打印数组中任意连续元素值

在gdb中，如果要打印数组中任意连续元素的值，可以使用p array[index]@num命令（p是print命令的缩写）。其中index是数组索引（从0开始计数），num是连续多少个元素。以上面代码为例：

(gdb) p array
$8 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
  32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
  63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93,
  94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119,
  120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144,
  145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169,
  170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194,
  195, 196, 197, 198, 199...}
(gdb) p array[60]@10
$9 = {60, 61, 62, 63, 64, 65, 66, 67, 68, 69}

可以看到打印了array数组第60~69个元素的值。
如果要打印从数组开头连续元素的值，也可使用这个命令：p *array@num：

(gdb) p *array@10
$2 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

打印静态变量的值

/* main.c */
extern void print_var_1(void);
extern void print_var_2(void);

int main(void)
{
  print_var_1();
  print_var_2();
  return 0;
}

/* static-1.c */
#include <stdio.h>

static int var = 1;

void print_var_1(void)
{ 
  printf("var = %d\n", var);
} 

/* static-2.c */
#include <stdio.h>

static int var = 2;

void print_var_2(void)
{ 
  printf("var = %d\n", var);
} 

在gdb中，如果直接打印静态变量，则结果并不一定是你想要的：

$ gcc -g main.c static-1.c static-2.c
$ gdb -q ./a.out
(gdb) start
(gdb) p var
$1 = 2

$ gcc -g main.c static-2.c static-1.c
$ gdb -q ./a.out
(gdb) start
(gdb) p var
$1 = 1

你可以显式地指定文件名（上下文）：

(gdb) p 'static-1.c'::var
$1 = 1
(gdb) p 'static-2.c'::var
$2 = 2

每行打印一个结构体成员

#include <stdio.h>
#include <pthread.h>

typedef struct
{
        int a;
        int b;
        int c;
        int d;
        pthread_mutex_t mutex;
}ex_st;

int main(void) {
        ex_st st = {1, 2, 3, 4, PTHREAD_MUTEX_INITIALIZER};
        printf("%d,%d,%d,%d\n", st.a, st.b, st.c, st.d);
        return 0;
}

默认情况下，gdb以一种“紧凑”的方式打印结构体。以上面代码为例：

(gdb) n
15              printf("%d,%d,%d,%d\n", st.a, st.b, st.c, st.d);
(gdb) p st
$1 = {a = 1, b = 2, c = 3, d = 4, mutex = {__data = {__lock = 0, __count = 0, __owner = 0, __nusers = 0, __kind = 0,
      __spins = 0, __list = {__prev = 0x0, __next = 0x0}}, __size = '\000' <repeats 39 times>, __align = 0}}

可以看到结构体的显示很混乱，尤其是结构体里还嵌套着其它结构体时。

可以执行“set print pretty on”命令，这样每行只会显示结构体的一名成员，而且还会根据成员的定义层次进行缩进：

(gdb) set print pretty on
(gdb) p st
$2 = {
  a = 1,
  b = 2,
  c = 3,
  d = 4,
  mutex = {
    __data = {
      __lock = 0,
      __count = 0,
      __owner = 0,
      __nusers = 0,
      __kind = 0,
      __spins = 0,
      __list = {
        __prev = 0x0,
        __next = 0x0
      }
    },
    __size = '\000' <repeats 39 times>,
    __align = 0
  }
}

打印变量的类型和所在文件

#include <stdio.h>

struct child {
  char name[10];
  enum { boy, girl } gender;
};

struct child he = { "Tom", boy };

int main (void)
{
  static struct child she = { "Jerry", girl };
  printf ("Hello %s %s.\n", he.gender == boy ? "boy" : "girl", he.name);
  printf ("Hello %s %s.\n", she.gender == boy ? "boy" : "girl", she.name);
  return 0;
}

在gdb中，可以使用如下命令查看变量的类型：

(gdb) whatis he
type = struct child

如果想查看详细的类型信息：

(gdb) ptype he
type = struct child {
    char name[10];
    enum {boy, girl} gender;
}

如果想查看定义该变量的文件：

(gdb) i variables he
All variables matching regular expression "he":

File variable.c:
struct child he;

Non-debugging symbols:
0x0000000000402030  she
0x00007ffff7dd3380  __check_rhosts_file