C++ I/O 重定向方法(定向到串口或Socket)(二)

上面的代码用于将控制台上的输入写入到套接字中。

用于输入的streambuf

basic_streambuf中输入相关的操作有sgetc，sbumpc，sgetn，sungetc和sputbackc。其中sungetc和sputbackc用于回退字符，这个功能不常用到，而且也不太可能在套接字上回退字符，因此这里省略对回退字符的介绍，关于这方面的内容可以参考《C++标准程序库》。

sgetc和sbumpc都用于读取一个字符，区别是后者会将读取位置向后移动一个位置，而前者不会改变读取位置。如果没有提供缓冲区，或者缓冲区的内容已经读完，那么sgetc会调用underflow方法，而sbumpc会调用uflow方法，从外部设备读取更多数据。uflow的默认行为是调用underflow，然后移动缓冲区的读取指针，如果没有提供缓冲区，则必须同时重写underflow和uflow。sgetn用于读取多个字符，它会调用xsgetn，而xsgetn的默认行为是依次调用sbumpc，如果为了改善读取多个字符的性能，可以重写xsgetn方法。

basic_streambuf的源码中：

1. int_type
2. sputc(char_type __c)
3. {
4. int_type __ret;
5. if (__builtin_expect(this->pptr() < this->epptr(), true))
6. {
7. *this->pptr() = __c;
8. this->pbump(1);
9. __ret = traits_type::to_int_type(__c);
10. }
11. else
12. __ret = this->overflow(traits_type::to_int_type(__c));
13. return __ret;
14. }
16. /**
17. * @brief Multiple character insertion.
18. * @param s A buffer area.
19. * @param n Maximum number of characters to write.
20. * @return The number of characters written.
21. *
22. * Writes @a s[0] through @a s[n-1] to the output sequence, as if
23. * by @c sputc(). Stops when either @a n characters have been
24. * copied, or when @c sputc() would return @c traits::eof().
25. *
26. * It is expected that derived classes provide a more efficient
27. * implementation by overriding this definition.
28. */
29. virtual streamsize
30. xsputn(const char_type* __s, streamsize __n);
33. /**
34. * @brief Consumes data from the buffer; writes to the
35. * controlled sequence.
36. * @param c An additional character to consume.
37. * @return eof() to indicate failure, something else (usually
38. * @a c, or not_eof())
39. *
40. * Informally, this function is called when the output buffer
41. * is full (or does not exist, as buffering need not actually
42. * be done). If a buffer exists, it is @a consumed, with
43. * some effect on the controlled sequence.
44. * (Typically, the buffer is written out to the sequence
45. * verbatim.) In either case, the character @a c is also
46. * written out, if @a c is not @c eof().
47. *
48. * For a formal definition of this function, see a good text
49. * such as Langer & Kreft, or [27.5.2.4.5]/3-7.
50. *
51. * A functioning output streambuf can be created by overriding only
52. * this function (no buffer area will be used).
53. *
54. * @note Base class version does nothing, returns eof().
55. */
56. virtual int_type
57. overflow(int_type /* __c */ = traits_type::eof())
58. { return traits_type::eof(); }
63. /**
64. * @brief Synchronizes the buffer arrays with the controlled sequences.
65. * @return -1 on failure.
66. *
67. * Each derived class provides its own appropriate behavior,
68. * including the definition of @a failure.
69. * @note Base class version does nothing, returns zero.
70. */
71. virtual int
72. sync() { return 0; }
75. /**
76. * @brief Entry point for all single-character output functions.
77. * @param s A buffer read area.
78. * @param n A count.
79. *
80. * One of two public output functions.
81. *
82. *
83. * Returns xsputn(s,n). The effect is to write @a s[0] through
84. * @a s[n-1] to the output sequence, if possible.
85. */
86. streamsize
87. sputn(const char_type* __s, streamsize __n)
88. { return this->xsputn(__s, __n); }
92. /**
93. * @brief Synchronizes the buffer arrays with the controlled sequences.
94. * @return -1 on failure.
95. *
96. * Each derived class provides its own appropriate behavior,
97. * including the definition of @a failure.
98. * @note Base class version does nothing, returns zero.
99. */
100. virtual int
101. sync() { return 0; }
     
     

     无缓冲方式

     首先来看下无缓冲方式的输入实现，如