让我们考虑以下代码：
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
int main(){
char again = 'Y';
int code;
do{
printf("Please inform your option:n1 - New recordn2 - Delete recordn3 - ecovery recordn4 - Search recordsn");
scanf("%d", &code);
switch(code){
case 1:
printf("Option %dn",code);
break;
case 2:
printf("Option %dn",code);
break;
case 3:
printf("Option %dn",code);
break;
case 4:
printf("Opcao %dn",code);
break;
default:
printf("code invalido!");
}
do{
printf("Do you wnat to again? [Y - Yes / N - No]: ");
scanf("%s", &again);
again = toupper(again);
}while(again != 'Y' && again != 'N');
printf("(DEBUG)Option after reading the string %dn",code);
}while(again == 'S');
return 0;
}
我知道代码中有一个错误，因为我使用带有"%s"的scanf来读取单个字符的信息。最好使用"%c"。
然而，在这段代码中，我感兴趣的是，在执行"scanf("%s"，&again(；"之后，变量"code"的值变为零。我不知道为什么会发生这种事。
主要假设是，由于我正在读取带有"%s"的字符串，在这个过程中，scanf将两个字符的信息存储在内存中：用户提供的字符和"\0"。我认为"\0"的信息存储在分配给变量"code"的内存区域中。
这有道理吗？
致以最良好的问候。
是的，这很有道理。但直觉是一回事——让我们检查一下！
使用大量调试信息编译：
tmp$ gcc -ggdb test.c
启动gdb，运行程序：
tmp$ gdb ./a.out
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
Copyright (C) 2016 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 "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./a.out...done.
(gdb) run
Starting program: /tmp/a.out
Please inform your option:
1 - New record
2 - Delete record
3 - ecovery record
4 - Search records
^C
Program received signal SIGINT, Interrupt.
0x00007ffff7b04260 in __read_nocancel () at ../sysdeps/unix/syscall-template.S:84
84 ../sysdeps/unix/syscall-template.S: No such file or directory.
好的，所以我按下Ctrl-C来中断程序。让我们在code上添加一个观察点。
(gdb) bt
#0 0x00007ffff7b04260 in __read_nocancel () at ../sysdeps/unix/syscall-template.S:84
#1 0x00007ffff7a875e8 in _IO_new_file_underflow (fp=0x7ffff7dd18e0 <_IO_2_1_stdin_>) at fileops.c:592
#2 0x00007ffff7a8860e in __GI__IO_default_uflow (fp=0x7ffff7dd18e0 <_IO_2_1_stdin_>) at genops.c:413
#3 0x00007ffff7a69260 in _IO_vfscanf_internal (s=<optimized out>, format=<optimized out>,
argptr=argptr@entry=0x7fffffffdbe8, errp=errp@entry=0x0) at vfscanf.c:634
#4 0x00007ffff7a785df in __isoc99_scanf (format=<optimized out>) at isoc99_scanf.c:37
#5 0x00000000004006c1 in main () at test.c:12
(gdb) frame 5
#5 0x00000000004006c1 in main () at test.c:12
12 scanf("%d", &code);
(gdb) watch code
Hardware watchpoint 1: code
(gdb) cont
Continuing.
现在，当我们提供"2"作为输入时，我们可以看到值的变化：
2
Hardware watchpoint 1: code
Old value = 32767
New value = 2
0x00007ffff7a6cde7 in _IO_vfscanf_internal (s=<optimized out>, format=<optimized out>,
argptr=argptr@entry=0x7fffffffdbe8, errp=errp@entry=0x0) at vfscanf.c:1902
1902 vfscanf.c: No such file or directory.
好吧，那是第一个scanf。让我们继续第二个问题，并给出"n"作为答案。
(gdb) cont
Continuing.
Option 2
Do you wnat to again? [Y - Yes / N - No]: n
Hardware watchpoint 1: code
Old value = 2
New value = 0
_IO_vfscanf_internal (s=<optimized out>, format=<optimized out>, argptr=argptr@entry=0x7fffffffdbe8,
errp=errp@entry=0x0) at vfscanf.c:1194
1194 in vfscanf.c
(gdb) bt
#0 _IO_vfscanf_internal (s=<optimized out>, format=<optimized out>, argptr=argptr@entry=0x7fffffffdbe8,
errp=errp@entry=0x0) at vfscanf.c:1194
#1 0x00007ffff7a785df in __isoc99_scanf (format=<optimized out>) at isoc99_scanf.c:37
#2 0x000000000040076d in main () at test.c:34
是的，它确实覆盖了code！
我们写的是什么样的价值观？
(gdb) disass
Dump of assembler code for function _IO_vfscanf_internal:
[...]
0x00007ffff7a6a74e <+7886>: lea 0x1(%rax),%rbx
0x00007ffff7a6a752 <+7890>: movb $0x0,(%rax)
=> 0x00007ffff7a6a755 <+7893>: je 0x7ffff7a6a77f <_IO_vfscanf_internal+7935>
0x00007ffff7a6a757 <+7895>: mov -0x620(%rbp),%r12
0x00007ffff7a6a75e <+7902>: mov %rbx,%rsi
0x00007ffff7a6a761 <+7905>: mov (%r12),%rdi
[...]
movb表示我们正在写入一个字节。它是一个0的立即值(即常数(。它看起来、走路和嘎嘎叫都像一个字符串终结者！
如果我们真的想确定，我们可以尝试找到这个库函数的确切源文件。
(gdb) disass /s $pc-3,+10
Dump of assembler code from 0x7ffff7a6a752 to 0x7ffff7a6a75c:
vfscanf.c:
1192 in vfscanf.c
0x00007ffff7a6a752 <_IO_vfscanf_internal+7890>: movb $0x0,(%rax)
1193 in vfscanf.c
1194 in vfscanf.c
=> 0x00007ffff7a6a755 <_IO_vfscanf_internal+7893>: je 0x7ffff7a6a77f <_IO_vfscanf_internal+7935>
0x00007ffff7a6a757 <_IO_vfscanf_internal+7895>: mov -0x620(%rbp),%r12
End of assembler dump.
在我的情况下，这很容易：我可以从Ubuntu的apt存储库安装"glibc源代码"包。根据您的系统类型，您可能会遇到困难。
无论如何，请查看1192号线。这绝对是一个零终止符。
glibc-2.23$ find . -name vfscanf.c
./stdio-common/vfscanf.c
glibc-2.23$ less -N ./stdio-common/vfscanf.c
[...]
1189
1190 str = __mempcpy (str, buf, n);
1191 #endif
1192 *str++ = ' ';
1193
1194 if ((flags & MALLOC) && str - *strptr != strsize)
1195 {
1196 char *cp = (char *) realloc (*strptr, str - *strptr);