为什么在函数参数中使用赋值运算符会失败？

Question

有人能指出为什么在下面的代码中，在传递给p_char SIGSEGV时使用带有“返回rvalue”样式的变量write()吗？

#include <iostream>
#include <cerrno>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <stdint.h>
#include <sstream>
#include <stdexcept>

#define ASSERT( EXPRESSION, SOCKETPAIR ) \
{ \
  if ( ! ( EXPRESSION ) ) \
  { \
    std::ostringstream oss; \
    oss << "Expression \"" << #EXPRESSION << "\" failed at line " << __LINE__ \
        << "; errno == " << errno << " (" << strerror( errno ) << ")"; \
    throw MyException( oss.str(), SOCKETPAIR ); \
  } \
}

class SocketPair
{
public:
  SocketPair()
  {
    memset( socket_, -1, sizeof( socket_ ) );
  }
  int* operator()() { return socket_; }
  int operator[]( const uint32_t idx )
  {
    if ( idx > 1 ) return -1;
    return socket_[ idx ];
  }
private:
  int socket_[ 2 ];
};

class MyException : public std::runtime_error
{
public:
  MyException( const std::string& what_arg, SocketPair& sp )
    : runtime_error( what_arg )
    , sp_( sp )
  {}
  SocketPair& sp_;
};

int main( int argc, char* argv[] )
{
  SocketPair sp;
  try
  {
    int result;

    errno = 0;
    result = socketpair( AF_LOCAL, SOCK_STREAM, 0, sp() );
    ASSERT( result == 0, sp );

    std::cout << "sp[0]==" << sp[0] << ", sp[1]==" << sp[1] << std::endl;

    const char* p_char;
    result = write( sp[ 1 ], ( p_char = "Hello?" ), strlen( p_char ) );
    ASSERT( result == strlen( p_char ), sp );
  }
  catch ( MyException& e )
  {
    std::cout << e.what() << std::endl;
    if ( e.sp_[ 0 ] >= 0 ) close( e.sp_[ 0 ] );
    if ( e.sp_[ 1 ] >= 0 ) close( e.sp_[ 1 ] );
    return 1;
  }

  close( sp[ 0 ] );
  close( sp[ 1 ] );
  return 0;
}

如果我改变以下两行..。

const char* p_char;
result = write( sp[ 1 ], ( p_char = "Hello?" ), strlen( p_char ) );

...to这个..。

const char* p_char = "Hello?";
result = write( sp[ 1 ], p_char, strlen( p_char ) );

...then程序没有SIGSEGV并优雅地退出。

编译测试gcc 4.8.3和4.9.2 (守则厨师)。

Answer 1

函数参数的计算顺序未定义(至少在C++14和更早版本中如此)。

所以在

result = write( sp[ 1 ], ( p_char = "Hello?" ), strlen( p_char ) );

strlen( p_char )可能在p_char = "Hello?"之前进行评估，从而导致灾难。