# 向上或向下切換函數堆棧幀 ## 例子 #include <stdio.h> int func1(int a) { return 2 * a; } int func2(int a) { int c = 0; c = 2 * func1(a); return c; } int func3(int a) { int c = 0; c = 2 * func2(a); return c; } int main(void) { printf("%d\n", func3(10)); return 0; } ## 技巧 用gdb調試程序時，當程序暫停后，可以用“`up n`”或“`down n`”命令向上或向下選擇函數堆棧幀，其中`n`是層數。以上面程序為例： (gdb) b test.c:5 Breakpoint 1 at 0x40053d: file test.c, line 5. (gdb) r Starting program: /home/nanxiao/test Breakpoint 1, func1 (a=10) at test.c:5 5 return 2 * a; (gdb) bt #0 func1 (a=10) at test.c:5 #1 0x0000000000400560 in func2 (a=10) at test.c:11 #2 0x0000000000400586 in func3 (a=10) at test.c:18 #3 0x000000000040059e in main () at test.c:24 (gdb) frame 2 #2 0x0000000000400586 in func3 (a=10) at test.c:18 18 c = 2 * func2(a); (gdb) up 1 #3 0x000000000040059e in main () at test.c:24 24 printf("%d\n", func3(10)); (gdb) down 2 #1 0x0000000000400560 in func2 (a=10) at test.c:11 11 c = 2 * func1(a); 可以看到程序斷住后，先執行“`frame 2`”命令，切換到`fun3`函數。接著執行“`up 1`”命令，此時會切換到`main`函數，也就是會往外層的堆棧幀移動一層。反之，當執行“`down 2`”命令后，又會向內層堆棧幀移動二層。如果不指定`n`，則`n`默認為`1`. 還有“`up-silently n`”和“`down-silently n`”這兩個命令，與“`up n`”和“`down n`”命令區別在于，切換堆棧幀后，不會打印信息，仍以上面程序為例： (gdb) up #2 0x0000000000400586 in func3 (a=10) at test.c:18 18 c = 2 * func2(a); (gdb) bt #0 func1 (a=10) at test.c:5 #1 0x0000000000400560 in func2 (a=10) at test.c:11 #2 0x0000000000400586 in func3 (a=10) at test.c:18 #3 0x000000000040059e in main () at test.c:24 (gdb) up-silently (gdb) i frame Stack level 3, frame at 0x7fffffffe5a0: rip = 0x40059e in main (test.c:24); saved rip = 0x7ffff7a35ec5 caller of frame at 0x7fffffffe590 source language c. Arglist at 0x7fffffffe590, args: Locals at 0x7fffffffe590, Previous frame's sp is 0x7fffffffe5a0 Saved registers: rbp at 0x7fffffffe590, rip at 0x7fffffffe598 可以看到從`func3`切換到`main`函數堆棧幀時，并沒有打印出相關信息。 參見[gdb手冊](https://sourceware.org/gdb/onlinedocs/gdb/Selection.html#Selection). ## 貢獻者 nanxiao