c++ - C++/gSoap 2.8.2 中奇怪的结构成员对齐

Question

我在 C++ 中遇到了一个奇怪的结构成员对齐问题。使用 gSoap 2.8.2 时会发生这种情况。 gSoap 定义了这个结构:

struct SOAP_STD_API soap
{ short state;          /* 0 = uninitialized, 1 = initialized, 2 = copy of another soap struct */
  short version;        /* 1 = SOAP1.1 and 2 = SOAP1.2 (set automatically from namespace URI in nsmap table) */
  soap_mode mode;
  soap_mode imode;
  soap_mode omode;
  const char *float_format; /* user-definable format string for floats (<1024 chars) */
  const char *double_format;    /* user-definable format string for doubles (<1024 chars) */
  const char *dime_id_format;   /* user-definable format string for integer DIME id (<SOAP_TAGLEN chars) */
  const char *http_version; /* HTTP version used "1.0" or "1.1" */
  const char *http_content; /* optional custom response content type (with SOAP_FILE) */
  const char *encodingStyle;    /* default = NULL which means that SOAP encoding is used */
  const char *actor;        /* SOAP-ENV:actor or role attribute value */
  const char *lang;     /* xml:lang attribute value of SOAP-ENV:Text */
  int recv_timeout;     /* when > 0, gives socket recv timeout in seconds, < 0 in usec */
  int send_timeout;     /* when > 0, gives socket send timeout in seconds, < 0 in usec */
  int connect_timeout;      /* when > 0, gives socket connect() timeout in seconds, < 0 in usec */
  int accept_timeout;       /* when > 0, gives socket accept() timeout in seconds, < 0 in usec */
  int socket_flags;     /* socket recv() and send() flags, e.g. set to MSG_NOSIGNAL to disable sigpipe */
  int connect_flags;        /* connect() SOL_SOCKET sockopt flags, e.g. set to SO_DEBUG to debug socket */
  int bind_flags;       /* bind() SOL_SOCKET sockopt flags, e.g. set to SO_REUSEADDR to enable reuse */
  int accept_flags;     /* accept() SOL_SOCKET sockopt flags */
  unsigned short linger_time;   /* linger time for SO_LINGER option */
  const struct Namespace *namespaces;   /* Pointer to global namespace mapping table */
  struct Namespace *local_namespaces;   /* Local namespace mapping table */
  struct soap_nlist *nlist; /* namespace stack */
  struct soap_blist *blist; /* block allocation stack */
  struct soap_clist *clist; /* class instance allocation list */
  void *alist;          /* memory allocation (malloc) list */
  struct soap_ilist *iht[SOAP_IDHASH];
  struct soap_plist *pht[SOAP_PTRHASH];
  struct soap_pblk *pblk;   /* plist block allocation */
  short pidx;           /* plist block allocation */
  struct SOAP_ENV__Header *header;
  struct SOAP_ENV__Fault *fault;
  int idnum;
  void *user;           /* for user to pass user-defined data */
  void *data[4];        /* extension data = {smdevp, mecevp, ...} */
  struct soap_plugin *plugins;  /* linked list of plug-in data */
  const char *userid;       /* HTTP Basic authorization userid */
  const char *passwd;       /* HTTP Basic authorization passwd */
  int (*fpost)(struct soap*, const char*, const char*, int, const char*, const char*, size_t);
  int (*fget)(struct soap*);    /* HTTP GET hook (not set by default) */
  int (*fput)(struct soap*);    /* HTTP PUT hook (handled as POST) */
  int (*fdel)(struct soap*);    /* HTTP DELETE hook (not set by default) */
  int (*fopt)(struct soap*);    /* HTTP OPTIONS hook (not set by default) */
  int (*fhead)(struct soap*);   /* HTTP HEAD hook (not set by default) */
  int (*fform)(struct soap*);   /* HTTP/HTML form handler for plugins */
  int (*fposthdr)(struct soap*, const char*, const char*);
  int (*fresponse)(struct soap*, int, size_t);
  int (*fparse)(struct soap*);
  int (*fparsehdr)(struct soap*, const char*, const char*);
  int (*fheader)(struct soap*);
  int (*fresolve)(struct soap*, const char*, struct in_addr* inaddr);
  int (*fconnect)(struct soap*, const char*, const char*, int);
  int (*fdisconnect)(struct soap*);
  int (*fclosesocket)(struct soap*, SOAP_SOCKET);
  int (*fshutdownsocket)(struct soap*, SOAP_SOCKET, int);
  SOAP_SOCKET (*fopen)(struct soap*, const char*, const char*, int);
  SOAP_SOCKET (*faccept)(struct soap*, SOAP_SOCKET, struct sockaddr*, int *n);
  int (*fclose)(struct soap*);
  int (*fsend)(struct soap*, const char*, size_t);
  size_t (*frecv)(struct soap*, char*, size_t);
  int (*fpoll)(struct soap*);
  int (*fselect)(struct soap*, SOAP_SOCKET, int, int, void*);
  void *fselect_param;
  void (*fseterror)(struct soap*, const char **c, const char **s);
  int (*fignore)(struct soap*, const char*);
  int (*fserveloop)(struct soap*);
  void *(*fplugin)(struct soap*, const char*);
  void *(*fmalloc)(struct soap*, size_t);
#ifndef WITH_LEANER
  int (*feltbegin)(struct soap*, const char*);
  int (*feltendin)(struct soap*, const char*, const char*);
  int (*feltbegout)(struct soap*, const char*);
  int (*feltendout)(struct soap*, const char*);
  int (*fprepareinitsend)(struct soap*);
  int (*fprepareinitrecv)(struct soap*);
  int (*fpreparesend)(struct soap*, const char*, size_t);
  int (*fpreparerecv)(struct soap*, const char*, size_t);
  int (*fpreparefinalsend)(struct soap*);
  int (*fpreparefinalrecv)(struct soap*);
  int filterstop;
  int (*ffiltersend)(struct soap*, const char**, size_t*);
  int (*ffilterrecv)(struct soap*, char*, size_t*, size_t);
  void *(*fdimereadopen)(struct soap*, void*, const char*, const char*, const char*);
  void *(*fdimewriteopen)(struct soap*, const char*, const char*, const char*);
  void (*fdimereadclose)(struct soap*, void*);
  void (*fdimewriteclose)(struct soap*, void*);
  size_t (*fdimeread)(struct soap*, void*, char*, size_t);
  int (*fdimewrite)(struct soap*, void*, const char*, size_t);
  void *(*fmimereadopen)(struct soap*, void*, const char*, const char*, const char*);
  void *(*fmimewriteopen)(struct soap*, void*, const char*, const char*, const char*, enum soap_mime_encoding);
  void (*fmimereadclose)(struct soap*, void*);
  void (*fmimewriteclose)(struct soap*, void*);
  size_t (*fmimeread)(struct soap*, void*, char*, size_t);
  int (*fmimewrite)(struct soap*, void*, const char*, size_t);
#endif
  SOAP_SOCKET master;
  SOAP_SOCKET socket;
#if defined(__cplusplus) && !defined(WITH_LEAN) && !defined(WITH_COMPAT)
  std::ostream *os;
  std::istream *is;
#else
  void *os;     /* preserve struct size */
  void *is;     /* preserve struct size */
#endif
#ifndef UNDER_CE
  int sendfd;       /* WinCE FD to send */
  int recvfd;       /* WinCE FD to receive */
#else
  FILE *sendfd;
  FILE *recvfd;
#endif
  size_t bufidx;    /* index in soap.buf[] */
  size_t buflen;    /* length of soap.buf[] content */
  soap_wchar ahead; /* parser lookahead */
  short cdata;      /* CDATA parser state */
  short body;       /* parsed XML element has a body or not */
  unsigned int level;   /* XML nesting level */
  size_t count;     /* message length counter */
  size_t length;    /* message length as set by HTTP header */
  char *labbuf;     /* look-aside buffer */
  size_t lablen;    /* look-aside buffer allocated length */
  size_t labidx;    /* look-aside buffer index to available part */
  char buf[SOAP_BUFLEN];/* send and receive buffer */
  char msgbuf[1024];    /* in/out buffer for HTTP/MIME headers >=1024 bytes */
  char tmpbuf[1024];    /* in/out buffer for HTTP/MIME headers, simpleType values, element and attribute tag names, and DIME must be >=1024 bytes */
  char tag[SOAP_TAGLEN];
  char id[SOAP_TAGLEN];
  char href[SOAP_TAGLEN];
  char type[SOAP_TAGLEN];
  char arrayType[SOAP_TAGLEN];
  char arraySize[SOAP_TAGLEN];
  char arrayOffset[SOAP_TAGLEN];
  short other;
  short position;
  int positions[SOAP_MAXDIMS];
  short root;
  struct soap_attribute *attributes;    /* attribute list */
  short encoding;   /* when set, output encodingStyle */
  short mustUnderstand; /* a mustUnderstand element was parsed or is output */
  short null;       /* parsed XML is xsi:nil */
  short ns;     /* when not set, output full xmlns bindings */
  short part;       /* SOAP part state (header or body) */
  short event;      /* engine events and states for use by plugins */
  short alloced;
  short peeked;
  size_t chunksize;
  size_t chunkbuflen;
  char endpoint[SOAP_TAGLEN];
  char path[SOAP_TAGLEN];
  char host[SOAP_TAGLEN];
  char *action;
  char *authrealm;      /* HTTP authentication realm */
  char *prolog;         /* XML declaration prolog */
  unsigned long ip;     /* IP number */
  int port;         /* port number */
  short keep_alive;     /* connection should be kept open */
  short tcp_keep_alive;     /* enable SO_KEEPALIVE */
  unsigned int tcp_keep_idle;   /* set TCP_KEEPIDLE */
  unsigned int tcp_keep_intvl;  /* set TCP_KEEPINTVL */
  unsigned int tcp_keep_cnt;    /* set TCP_KEEPCNT */
  unsigned int max_keep_alive;  /* maximum keep-alive session (default=100) */
  const char *proxy_http_version;/* HTTP version of proxy "1.0" or "1.1" */
  const char *proxy_host;   /* Proxy Server host name */
  int proxy_port;       /* Proxy Server port (default = 8080) */
  const char *proxy_userid; /* Proxy Authorization user name */
  const char *proxy_passwd; /* Proxy Authorization password */
  const char *proxy_from;   /* X-Forwarding-For header returned by proxy */
  int status;           /* -1 when request, else error code to be returned by server */
  int error;
  int errmode;
  int errnum;
#ifndef WITH_LEANER
  struct soap_dom_element *dom;
  struct soap_dime dime;
  struct soap_mime mime;
  struct soap_xlist *xlist;
#endif
#if !defined(WITH_LEAN) || defined(SOAP_DEBUG)
  const char *logfile[SOAP_MAXLOGS];
  FILE *fdebug[SOAP_MAXLOGS];
  struct soap_mlist *mht[SOAP_PTRHASH];
#endif
#ifndef WITH_LEAN
  const char *wsuid;        /* space-separated string of element tags */
  const char *c14nexclude;  /* space-separated string of prefixes */
  struct soap_cookie *cookies;
  const char *cookie_domain;
  const char *cookie_path;
  int cookie_max;
#endif
#ifndef WITH_NOIO
  int ipv6_multicast_if; /* in6addr->sin6_scope_id IPv6 value */
  char* ipv4_multicast_if; /* IP_MULTICAST_IF IPv4 setsockopt interface_addr */
  unsigned char ipv4_multicast_ttl; /* IP_MULTICAST_TTL value 0..255 */
#ifdef WITH_IPV6
  struct sockaddr_storage peer; /* IPv6: set by soap_accept and by UDP recv */
#else
  struct sockaddr_in peer;  /* IPv4: set by soap_connect/soap_accept and by UDP recv */
#endif
#endif
  size_t peerlen;
#if defined(WITH_OPENSSL)   /* OpenSSL */
  int (*fsslauth)(struct soap*);
  int (*fsslverify)(int, X509_STORE_CTX*);
  BIO *bio;
  SSL *ssl;
  SSL_CTX *ctx;
  SSL_SESSION *session;
  const char *dhfile;
  const char *randfile;
#elif defined(WITH_GNUTLS)      /* GNUTLS */
  int (*fsslauth)(struct soap*);
  void *fsslverify;
  gnutls_certificate_credentials_t xcred;   /* cert pointer */
  gnutls_anon_client_credentials_t acred;   /* anon pointer */
  gnutls_priority_t cache;          /* priority cache pointer */
  gnutls_session_t session;         /* session pointer */
  gnutls_dh_params_t dh_params;
  gnutls_rsa_params_t rsa_params;
#else               /* No SSL/TLS */
  void *fsslauth;       /* dummy members, to preserve struct size */
  void *fsslverify;
  void *bio;
  void *ssl;
  void *ctx;
  void *session;
  void *dh_params;
  void *rsa_params;
#endif
  unsigned short ssl_flags;
  const char *keyfile;
  const char *password;
  const char *cafile;
  const char *capath;
  const char *crlfile;
  char session_host[SOAP_TAGLEN];
  int session_port;
#ifdef WITH_C_LOCALE
  locale_t c_locale;        /* set to C locale by default */
#else
  void *c_locale;
#endif
#ifdef WITH_ZLIB
  z_stream *d_stream;       /* decompression stream */
  uLong z_crc;          /* internal gzip crc */
#else
  void *d_stream;       /* dummy members, to preserve struct size */
  soap_int32 z_crc;
#endif
  const char *z_dict;       /* support for zlib static dictionaries */
  unsigned int z_dict_len;
  short zlib_state;     /* SOAP_ZLIB_NONE, SOAP_ZLIB_DEFLATE, or SOAP_ZLIB_INFLATE */
  short zlib_in;        /* SOAP_ZLIB_NONE, SOAP_ZLIB_DEFLATE, or SOAP_ZLIB_GZIP */
  short zlib_out;       /* SOAP_ZLIB_NONE, SOAP_ZLIB_DEFLATE, or SOAP_ZLIB_GZIP */
  char *z_buf;          /* buffer */
  size_t z_buflen;
  unsigned short z_level;   /* compression level to be used (0=none, 1=fast to 9=best) */
  float z_ratio_in;     /* detected compression ratio compressed_length/length of inbound message */
  float z_ratio_out;        /* detected compression ratio compressed_length/length of outbound message */
#ifdef WMW_RPM_IO       /* VxWorks */
  void *rpmreqid;
#endif
#ifdef __cplusplus
  soap();
  soap(soap_mode);
  soap(soap_mode, soap_mode);
  soap(const struct soap&);
  virtual ~soap();
#else
  void (*dummy)(void);
#endif
};

后来在图书馆中被广泛使用。我需要稍微修改一下行为，但即使是像

这样的简单行

struct soap *soap;
...
// soap->error is 401 up to this point
soap->error = 0;
// soap->error is still 401, at least according to VS2008 debugger

我做的第一件事是查看程序集:

soap->error = 0;
172A35D8  mov         eax,dword ptr [soap] 
172A35DB  mov         dword ptr [eax+191A4h],0 

而 &soap->error = 0x0cb3ead0 和 soap = 0x0cb25930，以及 0x0cb3ead0 - 0x0cb25930 = 0x191A0 少了 4 个字节。所以要么是调试器错了，要么是程序集错了，但是由于库可以正常工作，调试器上的疑虑很大。无论哪种方式，在这些情况下都不可能更改代码。

有什么建议可能导致这种情况吗？我的指针算法在某处有误吗？我试过了

#pragma pack(push)
#pragma pack(1)
...
#pragma pack(pop)

围绕结构定义，但这没有帮助。

Answer 1

四个字节的差异可能是由 C++ 编译器添加 vtable 引起的到结构的开头。要验证这种常量差异的存在，可以查看修改偏移量0处的元素时生成的代码(state)。