c - 非全局使用的 getopt (C) 错误？

Question

我正在编写一段代码来解析文本文件中的一些命令，我​​发现我认为这是 getopt 中的一个错误。只是想知道是否有人以前看过这个？

基本上我有一个循环，在每次迭代中我读取一个字符串，我用“strtok”将它转换为可以提供给 getopt 的东西并检查提供的选项。它在某些情况下有效:

$ ./test_short
cmd -ia
  -- option i
  -- option a


cmd -a
  -- option a


cmd -i
  -- option i

但它失败了，例如:

$ ./test_short
cmd -ia
  -- option i
  -- option a


cmd -i -a
cmd: invalid option -- '-'
  -- option unknown
  -- option a
  -- option a

代码可以从https://github.com/angel-devicente/test-getopt中克隆出来，但为了完整起见，我也将简短选项放在下面。

是否知道这是一个真正的错误，还是有一些我忘记重置的其他全局变量？

谢谢，

#include <stdio.h>     /* for printf */
#include <stdlib.h>    /* for exit */
#include <string.h>
#include <unistd.h>

#define MAX_ARGS 10
#define MAX_C 100


int main(int argc, char **argv) {
  int c;
  char       cmd[MAX_C+1];

  int argc_i;
  char *argv_i[MAX_ARGS];
  
  
  while (1) {
    // Read string, for example: cmd -i -a
    fgets(cmd,MAX_C,stdin);
    cmd[strlen(cmd) - 1] = 0;
    if (strlen(cmd) <= 1) break;

    // Transform into argc_i, argv_i, to feed to getopt
    argc_i = 0;
    char *p2 = strtok(cmd, " ");
    while (p2 && argc_i < MAX_ARGS)
      {
    argv_i[(argc_i)++] = p2;
    p2 = strtok(NULL, " ");
      }
    argv_i[argc_i] = 0;


    // Reset getopt
    optind  = 1;
    int option_index = 0;

    
    while (1) {
      c = getopt(argc_i, argv_i, "ia");

      if (c == -1)
    break;

      switch (c) {
      case 'a':
    printf("  -- option a\n");
    break;

      case 'i':
    printf("  -- option i\n");
    break;

      case '?':
    printf("  -- option unknown\n");
    break;

      default:
    printf("?? getopt returned character code 0%o ??\n", c);
      }
    }

    printf("\n\n");

    if (optind < argc) {
      printf("non-option ARGV-elements: ");
      while (optind < argc)
    printf("%s ", argv[optind++]);
      printf("\n");
    }
  }

  exit(EXIT_SUCCESS);
}

Answer 1

这看起来像是有意偏离 getopt 的 POSIX 规范。

首先，我更改了您代码中的这一行:

argv_i[(argc_i)++] = p2;

对此:

argv_i[(argc_i)++] = strdup(p2);

并将此添加到 while (1) 循环的末尾:

for (i=0;i<argc_i;i++) {
    free(argv_i[i]);
}

并将其添加到顶部:

内存集(argv_i,0,sizeof(argv_i));

然后我运行了 valgrind:

==54748== Memcheck, a memory error detector
==54748== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==54748== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==54748== Command: ./x1
==54748== 
optind=1
cmd -ia
arg 0: cmd
arg 1: -ia
  -- option i
  -- option a


cmd -i -a
==54748== Invalid read of size 1
==54748==    at 0x4F192A1: _getopt_internal_r (in /usr/lib64/libc-2.17.so)
==54748==    by 0x4F1A56A: _getopt_internal (in /usr/lib64/libc-2.17.so)
==54748==    by 0x4F1A5B7: getopt (in /usr/lib64/libc-2.17.so)
==54748==    by 0x400A2E: main (x1.c:48)
==54748==  Address 0x5205093 is 3 bytes inside a block of size 4 free'd
==54748==    at 0x4C2AF9D: free (vg_replace_malloc.c:540)
==54748==    by 0x400B25: main (x1.c:89)
==54748==  Block was alloc'd at
==54748==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==54748==    by 0x4EC3AA9: strdup (in /usr/lib64/libc-2.17.so)
==54748==    by 0x400991: main (x1.c:31)
==54748== 
  -- option i
  -- option a

由于在测试程序中没有对 malloc 内存的引用，这意味着 getopt 实现保留了一个指向它的指针。

现在让我们看一下 https://github.com/lattera/glibc/blob/master/posix/getopt.c 中的几行包括 _getopt_internal_r:

/* Index in ARGV of the next element to be scanned.
   This is used for communication to and from the caller
   and for communication between successive calls to 'getopt'.
   On entry to 'getopt', zero means this is the first call; initialize.
   When 'getopt' returns -1, this is the index of the first of the
   non-option elements that the caller should itself scan.
   Otherwise, 'optind' communicates from one call to the next
   how much of ARGV has been scanned so far.  */

/* 1003.2 says this must be 1 before any call.  */
int optind = 1;
...
static struct _getopt_data getopt_data;
...
int
_getopt_internal_r (int argc, char **argv, const char *optstring,
            const struct option *longopts, int *longind,
            int long_only, struct _getopt_data *d, int posixly_correct)
{
  int print_errors = d->opterr;

  if (argc < 1)
    return -1;

  d->optarg = NULL;

  if (d->optind == 0 || !d->__initialized)
    optstring = _getopt_initialize (argc, argv, optstring, d, posixly_correct);

...

int
_getopt_internal (int argc, char **argv, const char *optstring,
          const struct option *longopts, int *longind, int long_only,
          int posixly_correct)
{
  int result;

  getopt_data.optind = optind;
  getopt_data.opterr = opterr;

  result = _getopt_internal_r (argc, argv, optstring, longopts,
                   longind, long_only, &getopt_data,
                   posixly_correct);

  optind = getopt_data.optind;
  optarg = getopt_data.optarg;
  optopt = getopt_data.optopt;

  return result;
}

#define GETOPT_ENTRY(NAME, POSIXLY_CORRECT)         \
  int                               \
  NAME (int argc, char *const *argv, const char *optstring) \
  {                             \
    return _getopt_internal (argc, (char **)argv, optstring,    \
                 0, 0, 0, POSIXLY_CORRECT);     \
  }

#ifdef _LIBC
GETOPT_ENTRY(getopt, 0)
GETOPT_ENTRY(__posix_getopt, 1)
#else
GETOPT_ENTRY(getopt, 1)
#endif

在这里我们可以看到 optind 被初始化为 1 和一个未初始化的内部静态结构。还有一条评论说 “在进入 'getopt' 时，零表示这是第一个调用；初始化。” 表示不同的初始化条件。

getopt 调用 getopt_internal，随后将 optind 分配给静态结构的相应成员，然后将其传递给 _getopt_internal_r 。 _getopt_internal_r 函数然后检查结构的 optind 成员是否为 0 或 __initialized 成员是否为 0，如果是，则初始化函数被调用，定义如下:

static const char *
_getopt_initialize (int argc _GL_UNUSED,
            char **argv _GL_UNUSED, const char *optstring,
            struct _getopt_data *d, int posixly_correct)
{
  /* Start processing options with ARGV-element 1 (since ARGV-element 0
     is the program name); the sequence of previously skipped
     non-option ARGV-elements is empty.  */
  if (d->optind == 0)
    d->optind = 1;

  d->__first_nonopt = d->__last_nonopt = d->optind;
  d->__nextchar = NULL;

  /* Determine how to handle the ordering of options and nonoptions.  */
  if (optstring[0] == '-')
    {
      d->__ordering = RETURN_IN_ORDER;
      ++optstring;
    }
  else if (optstring[0] == '+')
    {
      d->__ordering = REQUIRE_ORDER;
      ++optstring;
    }
  else if (posixly_correct || !!getenv ("POSIXLY_CORRECT"))
    d->__ordering = REQUIRE_ORDER;
  else
    d->__ordering = PERMUTE;

  d->__initialized = 1;
  return optstring;
}

我们可以在这里看到，如果 optind 成员当前为 0，则该成员将被设置为 1。另外，请特别注意这一行:

d->__nextchar = NULL;

如果我们再看看 https://github.com/lattera/glibc/blob/master/posix/getopt_int.h在定义此字段的位置，我们看到以下内容:

  /* The next char to be scanned in the option-element
     in which the last option character we returned was found.
     This allows us to pick up the scan where we left off.
     If this is zero, or a null string, it means resume the scan
     by advancing to the next ARGV-element.  */
  char *__nextchar;

所以这个变量跟踪处理停止的地方。但是回到 _getopt_internal_r

中的这些行

  if (d->optind == 0 || !d->__initialized)
    optstring = _getopt_initialize (argc, argv, optstring, d, posixly_correct);

只有当 optind 为 0 时才会发生重置。

因此在此实现中，将optind 设置为0 将重置处理。然而，这不是可移植的，因为 POSIX 指定这样做是未指定的行为。

这意味着要么将此代码更改为将 optind == 1 作为初始化条件，要么更新文档以反射(reflect)与 POSIX 标准的这种偏差并声明这是一种偏差.