C语言之linux内核可变参实现printf,sprintf

悠悠 2022-05-31 08:13 454阅读 0赞

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昨天，我发表了一篇用可变参实现的fprintf函数,其实说实话还不完全是可变参实现的，因为用到了FILE \* 这样的指针，需要包含stdio.h这个头文件才能实现这个函数，今天我们就来看看，如何抛弃stdio.h，全0开始实现printf , sprintf ,当然，这段代码是我在linux内核里面获取的，再经过我本人修改，移植，在DevC++这个编译环境中通过测试。我们来看看代码:![生气][mad.gif]![生气][mad.gif]

**\[cpp\]** [view plain][] [copy][view plain]

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1.  \#include <stdarg.h>
2.  \#define NULL 0
3.  //如果字符串中为数字，则返回数字 
4.  **static****int** skip\_atoi(**const****char** \*\*s) 
5.  \{ 
6.  **int** i = 0; 
7.  
8.  **while** (isdigit(\*\*s)) 
9.   i = i \* 10 + \*((\*s)++) - '0'; 
10. **return** i; 
11. \} 
12. 
13. **static****inline****int** isdigit(**int** ch) 
14. \{ 
15. **return** (ch >= '0') && (ch <= '9'); //返回从字符中提取0-9的数字
16. \} 
17. 
18. \#define ZEROPAD 1 /\* pad with zero \*/
19. \#define SIGN 2 /\* unsigned/signed long \*/
20. \#define PLUS 4 /\* show plus \*/
21. \#define SPACE 8 /\* space if plus \*/
22. \#define LEFT 16 /\* left justified \*/
23. \#define SMALL 32 /\* Must be 32 == 0x20 \*/
24. \#define SPECIAL 64 /\* 0x \*/
25. 
26. //这个宏主要用来实现判断是要转化成什么进制数 
27. \#define \_\_do\_div(n, base) (\{ \\
28. **int** \_\_res; \\ 
29. \_\_res = ((unsigned **long**) n) % (unsigned) base; \\ 
30. n = ((unsigned **long**) n) / (unsigned) base; \\ 
31. \_\_res; \}) 
32. 
33. **static****char** \*number(**char** \*str, **long** num, **int** base, **int** size, **int** precision, 
34. **int** type) 
35. \{ 
36. /\*这个字符串数组存放着0-15这16个数字，到时候要用来进制转换\*/
37. **static****const****char** digits\[16\] = "0123456789ABCDEF"; 
38. 
39. **char** tmp\[66\]; 
40. **char** c, sign, locase; 
41. **int** i; 
42. 
43. /\*locase = 0 或者 0x20 , 产生与locase相同的数字或字母，也许字母是小写的\*/
44.  locase = (type & SMALL); 
45. **if** (type & LEFT) 
46.  type &= ~ZEROPAD; 
47. **if** (base < 2 || base > 36) 
48. **return** NULL; 
49.  c = (type & ZEROPAD) ? '0' : ' '; 
50.  sign = 0; 
51. **if** (type & SIGN) \{ 
52. **if** (num < 0) \{ 
53.  sign = '-'; 
54.  num = -num; 
55.  size--; 
56.  \} **else****if** (type & PLUS) \{ 
57.  sign = '+'; 
58.  size--; 
59.  \} **else****if** (type & SPACE) \{ 
60.  sign = ' '; 
61.  size--; 
62.  \} 
63.  \} 
64. //检测进制数，是要2进制还是要8进制还是16进制 
65. **if** (type & SPECIAL) \{ 
66. **if** (base == 16) 
67.  size -= 2; 
68. **else****if** (base == 8) 
69.  size--; 
70.  \} 
71.  i = 0; 
72. **if** (num == 0) 
73.  tmp\[i++\] = '0'; 
74. **else**
75. **while** (num != 0) 
76.  tmp\[i++\] = (digits\[\_\_do\_div(num, base)\] | locase); 
77. **if** (i > precision) 
78.  precision = i; 
79.  size -= precision; 
80. **if** (!(type & (ZEROPAD + LEFT))) 
81. **while** (size-- > 0) 
82.  \*str++ = ' '; 
83. **if** (sign) 
84.  \*str++ = sign; 
85. **if** (type & SPECIAL) \{ 
86. **if** (base == 8) 
87.  \*str++ = '0'; 
88. **else****if** (base == 16) \{ 
89.  \*str++ = '0'; 
90.  \*str++ = ('X' | locase); 
91.  \} 
92.  \} 
93. **if** (!(type & LEFT)) 
94. **while** (size-- > 0) 
95.  \*str++ = c; 
96. **while** (i < precision--) 
97.  \*str++ = '0'; 
98. **while** (i-- > 0) 
99.  \*str++ = tmp\[i\]; 
100. **while** (size-- > 0) 
101.  \*str++ = ' '; 
102. **return** str; 
103. \} 
104. 
105. **int** vsprintf(**char** \*buf, **const****char** \*fmt, **va\_list** args) 
106. \{ 
107. **int** len; 
108.  unsigned **long** num; 
109. **int** i, base; 
110. **char** \*str; 
111. **const****char** \*s; 
112. **int** flags; 
113. **int** field\_width; /\*位宽输出\*/
114. **int** precision; 
115. **int** qualifier; 
116. //这里判断，如果在字符串fmt中不存在%这个符号，那么字符串继续往后遍历 
117. **for** (str = buf; \*fmt; ++fmt) \{ 
118. **if** (\*fmt != '%') \{ 
119.  \*str++ = \*fmt; 
120. **continue**; 
121.  \} 
122. 
123. //程序设置标志位 
124.  flags = 0; 
125.  repeat: 
126.  \++fmt; /\* this also skips first '%' \*/
127. //格式控制 
128. **switch** (\*fmt) \{ 
129. **case**'-': 
130.  flags |= LEFT; 
131. **goto** repeat; 
132. **case**'+': 
133.  flags |= PLUS; 
134. **goto** repeat; 
135. **case**' ': 
136.  flags |= SPACE; 
137. **goto** repeat; 
138. **case**'\#': 
139.  flags |= SPECIAL; 
140. **goto** repeat; 
141. **case**'0': 
142.  flags |= ZEROPAD; 
143. **goto** repeat; 
144.  \} 
145. 
146. //获取宽度，这里主要是要实现printf的位宽机制 
147.  field\_width = -1; 
148. **if** (isdigit(\*fmt)) 
149.  field\_width = skip\_atoi(&fmt); 
150. **else****if** (\*fmt == '\*') \{ 
151.  \++fmt; 
152.  field\_width = va\_arg(args, **int**); 
153. **if** (field\_width < 0) \{ 
154.  field\_width = -field\_width; 
155.  flags |= LEFT; 
156.  \} 
157.  \} 
158. 
159.  precision = -1; 
160. **if** (\*fmt == '.') \{ 
161.  \++fmt; 
162. **if** (isdigit(\*fmt)) 
163.  precision = skip\_atoi(&fmt); 
164. **else****if** (\*fmt == '\*') \{ 
165.  \++fmt; 
166.  precision = va\_arg(args, **int**); 
167.  \} 
168. **if** (precision < 0) 
169.  precision = 0; 
170.  \} 
171. 
172. /\*得到的转换限定符\*/
173.  qualifier = -1; 
174. **if** (\*fmt == 'h' || \*fmt == 'l' || \*fmt == 'L') \{ 
175.  qualifier = \*fmt; 
176.  \++fmt; 
177.  \} 
178. 
179. /\*默认进制为10进制\*/
180.  base = 10; 
181. //以下主要是要实现printf的格式输出 例如：%d , %c , %u ...等等 
182. **switch** (\*fmt) \{ 
183. **case**'c': //以字符形式进行输出 
184. **if** (!(flags & LEFT)) 
185. **while** (--field\_width > 0) 
186.  \*str++ = ' '; 
187.  \*str++ = (unsigned **char**)va\_arg(args, **int**); 
188. **while** (--field\_width > 0) 
189.  \*str++ = ' '; 
190. **continue**; 
191. 
192. **case**'s': //以字符串形式进行输出 
193.  s = va\_arg(args, **char** \*); 
194.  len = strnlen(s, precision); 
195. 
196. **if** (!(flags & LEFT)) 
197. **while** (len < field\_width--) 
198.  \*str++ = ' '; 
199. **for** (i = 0; i < len; ++i) 
200.  \*str++ = \*s++; 
201. **while** (len < field\_width--) 
202.  \*str++ = ' '; 
203. **continue**; 
204. 
205. **case**'p': //以地址形式输出，也就是以16进制数输出 
206. **if** (field\_width == -1) \{ 
207.  field\_width = 2 \* **sizeof**(**void** \*); 
208.  flags |= ZEROPAD; 
209.  \} 
210.  str = number(str, 
211.  (unsigned **long**)va\_arg(args, **void** \*), 16, 
212.  field\_width, precision, flags); 
213. **continue**; 
214. 
215. **case**'n': 
216. **if** (qualifier == 'l') \{ 
217. **long** \*ip = va\_arg(args, **long** \*); 
218.  \*ip = (str - buf); 
219.  \} **else** \{ 
220. **int** \*ip = va\_arg(args, **int** \*); 
221.  \*ip = (str - buf); 
222.  \} 
223. **continue**; 
224. 
225. **case**'%': //这里表示字符串中存在%号这个字符 
226.  \*str++ = '%'; 
227. **continue**; 
228. 
229. /\* integer number formats - set up the flags and "break" \*/
230. **case**'o': //%o 表示8进制输出 
231.  base = 8; 
232. **break**; 
233. 
234. **case**'x': //%x或者%X 表示16进制输出 
235.  flags |= SMALL; 
236. **case**'X': 
237.  base = 16; 
238. **break**; 
239. 
240. **case**'d': //%d %i整形数输出，%u无符号整形 
241. **case**'i': 
242.  flags |= SIGN; 
243. **case**'u': 
244. **break**; 
245. 
246. **default**: 
247.  \*str++ = '%'; 
248. **if** (\*fmt) 
249.  \*str++ = \*fmt; 
250. **else**
251.  \--fmt; 
252. **continue**; 
253.  \} 
254. **if** (qualifier == 'l') //以无符号长整型输出 
255.  num = va\_arg(args, unsigned **long**); 
256. **else****if** (qualifier == 'h') \{ 
257.  num = (unsigned **short**)va\_arg(args, **int**); 
258. **if** (flags & SIGN) 
259.  num = (**short**)num; 
260.  \} **else****if** (flags & SIGN) 
261.  num = va\_arg(args, **int**); 
262. **else**
263.  num = va\_arg(args, unsigned **int**); 
264.  str = number(str, num, base, field\_width, precision, flags); 
265.  \} 
266.  \*str = '\\0'; //字符串遍历到有\\0的地方就停止 
267. **return** str - buf; 
268. \} 
269. //可变参形式实现sprintf 
270. **int** mysprintf(**char** \*buf, **const****char** \*fmt, ...) 
271. \{ 
272. **va\_list** args; 
273. **int** i; 
274.  va\_start(args, fmt); 
275. //将获取到的fmt格式字符串写入到buf这个缓存里去 
276.  i = vsprintf(buf, fmt, args); 
277. //释放args 
278.  va\_end(args); 
279. **return** i; 
280. \} 
281. 
282. //可变参形式进行实现myprintf 
283. **int** myprintf(**const****char** \*fmt, ...) 
284. \{ 
285. **char** printf\_buf\[1024\]; 
286. **va\_list** args; 
287. **int** printed; 
288.  va\_start(args, fmt); 
289.  printed = vsprintf(printf\_buf, fmt, args); 
290.  va\_end(args); 
291.  puts(printf\_buf); 
292. **return** printed; 
293. \} 
294. 
295. **int** main(**void**) 
296. \{ 
297.  myprintf("输出字符串：hello world!\\n") ; 
298. **static****int** sum , a = 3 , b = 4; 
299.  sum = a + b ; 
300.  myprintf("sum(十进制输出):%d\\n",sum); 
301.  myprintf("sum(16进制输出):%p\\n",sum); 
302. **char** buffer\[128\] = \{0\}; 
303. //将字符串存到一个数组buffer里去 
304.  mysprintf(buffer , "输出字符串：hello world！\\n"); 
305. //以字符串格式输出这个buffer的内容 
306.  myprintf("buffer:%s\\n",buffer); 
307. **return** 0 ; 
308. \}

运行结果:

看完代码就知道了，我们这个程序没有包含stdio.h这个头文件，一样也就实现了printf和sprintf这两个函数。这样的话，以后如果要自己实现一个printf函数，这份代码就可以作为一个文件来进行调用了，哈哈!![大笑][laugh.gif]

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转自：http://blog.csdn.net/morixinguan/article/details/50725779

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