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256 lines
8.1 KiB
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[#]: subject: "7 pro tips for using the GDB step command"
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[#]: via: "https://opensource.com/article/22/12/gdb-step-command"
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[#]: author: "Alexandra https://opensource.com/users/ahajkova"
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[#]: collector: "lkxed"
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[#]: translator: "chai001125"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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7 pro tips for using the GDB step command
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======
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A debugger is software that runs your code and examines any problems it finds. [GNU Debugger][1] (GBD) is one of the most popular debuggers, and in this article, I examine GDB's `step` command and related commands for several common use cases. Step is a widely used command but there are a few lesser known things about it which might be confusing. Also, there are ways to step into a function without actually using the `step` command itself such as using the less known `advance` command.
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### No debugging symbols
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Consider a simple example program:
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```
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#include <stdio.h>
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int num() {
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return 2;
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}
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void bar(int i) {
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printf("i = %d\n", i);
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}
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int main() {
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bar(num());
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return 0;
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}
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```
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If you compile without the debugging symbols first, set a breakpoint on `bar` and then try to step within it. The GDB gives an error message about no line number information:
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```
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gcc exmp.c -o exmp
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gdb ./exmp
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(gdb) b bar
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Breakpoint 1 at 0x401135
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(gdb) r
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Starting program: /home/ahajkova/exmp
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Breakpoint 1, 0x0000000000401135 in bar ()
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(gdb) step
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Single stepping until exit from function bar,
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which has no line number information.
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i = 2
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0x0000000000401168 in main ()
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```
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### Stepi
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It is still possible to step inside the function that has no line number information but the `stepi` command should be used instead. Stepi executes just one instruction at a time. When using GDB's `stepi` command, it's often useful to first do `display/i $pc`. This causes the program counter value and corresponding machine instruction to be displayed after each step:
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```
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(gdb) b bar
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Breakpoint 1 at 0x401135
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(gdb) r
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Starting program: /home/ahajkova/exmp
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Breakpoint 1, 0x0000000000401135 in bar ()
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(gdb) display/i $pc
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1: x/i $pc
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=> 0x401135 <bar+4>: sub $0x10,%rsp
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```
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In the above `display` command, the `i` stands for machine instructions and `$pc` is the program counter register.
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It can be useful to use info registers and print some register contents:
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```
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(gdb) info registers
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rax 0x2 2
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rbx 0x7fffffffdbc8 140737488346056
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rcx 0x403e18 4210200
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(gdb) print $rax
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$1 = 2
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(gdb) stepi
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0x0000000000401139 in bar ()
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1: x/i $pc
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=> 0x401139 <bar+8>: mov %edi,-0x4(%rbp)
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```
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### Complicated function call
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After recompiling the example program with debugging symbols you can set the breakpoint on the `bar` call in main using its line number and then try to step into `bar` again:
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```
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gcc -g exmp.c -o exmp
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gdb ./exmp
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(gdb) b exmp.c:14
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Breakpoint 1 at 0x401157: file exmp.c, line 14.
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(gdb) r
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Starting program: /home/ahajkova/exmp
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Breakpoint 1, main () at exmp.c:14
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14 bar(num());
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```
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Now, let's step into`bar()`:
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```
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(gdb) step
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num () at exmp.c:4
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4 return 2;
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```
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The arguments for a function call need to be processed before the actual function call, so `num()` is expected to execute before `bar()`is called. But how do you step into the `bar` as was desired? You need to use the `finish` command and `step` again:
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```
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(gdb) finish
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Run till exit from #0 num () at exmp.c:4
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0x0000000000401161 in main () at exmp.c:14
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14 bar(num());
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Value returned is $1 = 2
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(gdb) step
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bar (i=2) at exmp.c:9
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9 printf("i = %d\n", i);
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```
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### Tbreak
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The `tbreak` command sets a temporary breakpoint. It's useful for situations where you don't want to set a permanent breakpoint. For example, if you want to step into a complicated function call like `f(g(h()), i(j()), ...)` , in such a case you need a long sequence of `step/finish/step` to step into `f` . Setting a temporary breakpoint and then using continue can help to avoid using such sequences. To demonstrate this, you need to set the breakpoint to the `bar` call in `main` as before. Then set the temporary breakpoint on `bar`. As a temporary breakpoint it is automatically removed after being hit:
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```
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(gdb) r
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Starting program: /home/ahajkova/exmp
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Breakpoint 1, main () at exmp.c:14
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14 bar(num());
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(gdb) tbreak bar
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Temporary breakpoint 2 at 0x40113c: file exmp.c, line 9.
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```
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After hitting the breakpoint on the call to `bar` and setting a temporary breakpoint on `bar`, you just need to continue to end up in `bar`.
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```
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(gdb) continue
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Continuing.
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Temporary breakpoint 2, bar (i=2) at exmp.c:9
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9 printf("i = %d\n", i);
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```
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### Disable command
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Alternatively, you could set a normal breakpoint on `bar` , continue, and then disable this second breakpoint when it's no longer needed. This way you can achieve the same results as with the `tbreak` with one extra command:
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```
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(gdb) b exmp.c:14
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Breakpoint 1 at 0x401157: file exmp.c, line 14.
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(gdb) r
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Starting program: /home/ahajkova/exmp
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Breakpoint 1, main () at exmp.c:14
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14 bar(num());
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(gdb) b bar
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Breakpoint 2 at 0x40113c: file exmp.c, line 9.
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(gdb) c
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Continuing.
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Breakpoint 2, bar (i=2) at exmp.c:9
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9 printf("i = %d\n", i);
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(gdb) disable 2
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```
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As you can see, the `info breakpoints` command displays `n` under `Enb`which means it’s disabled but you can enable it later if it’s needed again.
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```
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(gdb) info breakpoints
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Num Type Disp Enb Address What
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1 breakpoint keep y 0x0000000000401157 in main at exmp.c:14
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breakpoint already hit 1 time
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2 breakpoint keep n 0x000000000040113c in bar at exmp.c:9
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breakpoint already hit 1 time
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(gdb) enable 2
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(gdb) info breakpoints
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Num Type Disp Enb Address What
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1 breakpoint keep y 0x000000000040116a in main at exmp.c:19
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breakpoint already hit 1 time
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2 breakpoint keep y 0x0000000000401158 in bar at exmp.c:14
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breakpoint already hit 1 time
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```
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### Advance location
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Another option you can use is an `advance` command. Instead of `tbreak bar ; continue` , you can simply do `advance bar` . This command continues running the program up to the given location.
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The other cool thing about `advance` is that if the location that you try to advance to is not reached, GDB will stop after the current frame's function finishes. Thus, execution of the program is constrained:
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```
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Breakpoint 1 at 0x401157: file exmp.c, line 14.
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(gdb) r
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Starting program: /home/ahajkova/exmp
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Breakpoint 1, main () at exmp.c:14
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14 bar(num());
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(gdb) advance bar
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bar (i=2) at exmp.c:9
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9 printf("i = %d\n", i);
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```
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### Skipping a function
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Yet another way to step into the `bar,` avoiding `num`, is using the `skip` command:
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```
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(gdb) b exmp.c:14
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Breakpoint 1 at 0x401157: file exmp.c, line 14.
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(gdb) skip num
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Function num will be skipped when stepping.
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(gdb) r
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Starting program: /home/ahajkova/exmp
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Breakpoint 1, main () at exmp.c:14
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14 bar(num());
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(gdb) step
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bar (i=2) at exmp.c:9
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9 printf("i = %d\n", i);
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```
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To know which functions are currently skipped, `info skip` is used. The `num` function is marked as enabled to be skipped by `y`:
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```
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(gdb) info skip
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Num Enb Glob File RE Function
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1 y n <none> n num
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```
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If `skip` is not needed any more it can be disabled (and re-enabled later) or deleted altogether. You can add another `skip` and disable the first one and then delete them all. To disable a certain `skip`, its number has to be specified, if not specified, each `skip`is disabled. It works the same for enabling or deleting a `skip`:
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```
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(gdb) skip bar
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(gdb) skip disable 1
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(gdb) info skip
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Num Enb Glob File RE Function
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1 n n <none> n num
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2 y n <none> n bar
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(gdb) skip delete
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(gdb) info skip
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Not skipping any files or functions.
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```
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### GDB step command
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Using GDB's `step` command is a useful tool for debugging your application. There are several ways to step into even complicated functions, so give these GDB techniques a try next time you're troubleshooting your code.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/22/12/gdb-step-command
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作者：[Alexandra][a]
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选题：[lkxed][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/ahajkova
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[b]: https://github.com/lkxed
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[1]: https://opensource.com/article/21/3/debug-code-gdb
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