How does the behavior of 3000pc change if you delete lines 149 and 152 (the if s

ID: 3599384 • Letter: H

Question

How does the behavior of 3000pc change if you delete lines 149 and 152 (the if statement in wakeup_consumer())? Why? (Explain what the program does after this change and why it may be problematic.)

/* 3000pc.c */

8 #include <stdio.h>

9 #include <stdlib.h>

10 #include <unistd.h>

11 #include <sys/mman.h>

12 #include <errno.h>

13 #include <string.h>

14 #include <sys/types.h>

15 #include <sys/wait.h>

16 #include <semaphore.h>

17 #include <string.h>

18 #include <time.h>

20 #define QUEUESIZE 32

21 #define WORDSIZE 16

23 const int wordlist_size = 27;

24 const char *wordlist[] = {

25 "Alpha",

26 "Bravo",

27 "Charlie",

28 "Delta",

29 "Echo",

30 "Foxtrot",

31 "Golf",

32 "Hotel",

33 "India",

34 "Juliet",

35 "Kilo",

36 "Lima",

37 "Mike",

38 "November",

39 "Oscar",

40 "Papa",

41 "Quebec",

42 "Romeo",

43 "Sierra",

44 "Tango",

45 "Uniform",

46 "Victor",

47 "Whiskey",

48 "X-ray",

49 "Yankee",

50 "Zulu",

51 "Dash"

52 };

54 typedef struct entry {

55 char word[WORDSIZE];

56 sem_t lock;

57 } entry;

59 typedef struct shared {

60 int prod_waiting;

61 int con_waiting;

62 entry queue[QUEUESIZE];

63 int last_produced;

64 int last_consumed;

65 pid_t prod_pid;

66 pid_t con_pid;

67 int prod_count;

68 int con_count;

69 } shared;

72 void report_error(char *error)

73 {

74 fprintf(stderr, "Error: %s ", error);

75 }

77 void usage_exit(char *progname)

78 {

79 fprintf(stderr,

80 "Usage: %s <event count> <prod delay int> <con delay int> ",

81 progname);

82 exit(-1);

83 }

85 void producer_handler(int the_signal)

86 {

87 if (the_signal == SIGUSR1) {

88 fprintf(stderr, "Producer received SIGUSR1. ");

89 return;

91 } else {

92 fprintf(stderr, "Producer: No handler for for signal %d?! ",

93 the_signal);

94 return;

95 }

96 }

98 void consumer_handler(int the_signal)

99 {

100 if (the_signal == SIGUSR1) {

101 fprintf(stderr, "Consumer received SIGUSR1. ");

102 return;

103 } else {

104 fprintf(stderr, "Consumer: No handler for for signal %d?! ",

105 the_signal);

106 return;

107 }

108 }

109

110 void pick_word(char *word)

111 {

112 int pick;

113

114 pick = random() % wordlist_size;

115

116 strcpy(word, wordlist[pick]);

117 }

118

119 void wait_for_producer(shared *s)

120 {

121 struct timespec delay;

122

123 delay.tv_sec = 100;

124 delay.tv_nsec = 0;

125

126 s->con_waiting = 1;

127

128 while (s->con_waiting) {

129 nanosleep(&delay, NULL);

130 }

131 }

132

133 void wait_for_consumer(shared *s)

134 {

135 struct timespec delay;

136

137 delay.tv_sec = 100;

138 delay.tv_nsec = 0;

139

140 s->prod_waiting = 1;

141

142 while (s->prod_waiting) {

143 nanosleep(&delay, NULL);

144 }

145 }

146

147 void wakeup_consumer(shared *s)

148 {

149 if (s->con_waiting) {

150 s->con_waiting = 0;

151 kill(s->con_pid, SIGUSR1);

152 }

153 }

154

155 void wakeup_producer(shared *s)

156 {

157 if (s->prod_waiting) {

158 s->prod_waiting = 0;

159 kill(s->prod_pid, SIGUSR1);

160 }

161 }

162

163 void output_word(int c, char *w)

164 {

165 printf("Word %d: %s ", c, w);

166 }

167

168 int queue_word(char *word, shared *s)

169 {

170 entry *e;

171 int current, retval;

172

173 current = (s->last_produced + 1) % QUEUESIZE;

174

175 e = &s->queue[current];

176

177 sem_wait(&e->lock);

178

179 if (e->word[0] != '') {

180 /* consumer hasn't consumed this entry yet */

181 sem_post(&e->lock);

182 wait_for_consumer(s);

183 sem_wait(&e->lock);

184 }

185

186 if (e->word[0] != '') {

187 fprintf(stderr, "ERROR: No room for producer after waiting! ");

188 retval = -1;

189 goto done;

190 } else {

191 strncpy(e->word, word, WORDSIZE);

192 s->last_produced = current;

193 s->prod_count++;

194 wakeup_consumer(s);

195 retval = 0;

196 goto done;

197 }

198

199 done:

200 sem_post(&e->lock);

201 return retval;

202 }

203

204 int get_next_word(char *word, shared *s)

205 {

206 entry *e;

207 int current, retval;

208

209 current = (s->last_consumed + 1) % QUEUESIZE;

210

211 e = &s->queue[current];

212

213 sem_wait(&e->lock);

214

215 if (e->word[0] == '') {

216 /* producer hasn't filled in this entry yet */

217 sem_post(&e->lock);

218 wait_for_producer(s);

219 sem_wait(&e->lock);

220 }

221

222 if (e->word[0] == '') {

223 fprintf(stderr, "ERROR: Nothing for consumer after waiting! ");

224 retval = -1;

225 goto done;

226 } else {

227 strncpy(word, e->word, WORDSIZE);

228 e->word[0] = '';

229 s->last_consumed = current;

230 s->con_count++;

231 wakeup_producer(s);

232 retval = 0;

233 goto done;

234 }

235

236 done:

237 sem_post(&e->lock);

238 return retval;

239 }

240

241 void producer(shared *s, int event_count, int producer_delay_interval)

242 {

243 char word[WORDSIZE];

244 int i;

245 struct sigaction signal_handler_struct;

246

247 memset (&signal_handler_struct, 0, sizeof(signal_handler_struct));

248 signal_handler_struct.sa_handler = producer_handler;

249

250 if (sigaction(SIGUSR1, &signal_handler_struct, NULL)) {

251 fprintf(stderr, "Producer couldn't register SIGUSR1 handler. ");

252 }

253

254 for (i=0; i < event_count; i++) {

255 pick_word(word);

256 queue_word(word, s);

257 if (producer_delay_interval > 0) {

258 if (i % producer_delay_interval == 0) {

259 sleep(1);

260 }

261 }

262 }

263

264 printf("Producer finished. ");

265 exit(0);

266 }

267

268 void consumer(shared *s, int event_count, int consumer_delay_interval)

269 {

270 char word[WORDSIZE];

271 int i;

272 struct sigaction signal_handler_struct;

273

274 memset (&signal_handler_struct, 0, sizeof(signal_handler_struct));

275 signal_handler_struct.sa_handler = consumer_handler;

276

277 if (sigaction(SIGUSR1, &signal_handler_struct, NULL)) {

278 fprintf(stderr, "Consumer couldn't register SIGUSR1 handler. ");

279 }

280

281 for (i=0; i < event_count; i++) {

282 get_next_word(word, s);

283 output_word(s->con_count, word);

284 if (consumer_delay_interval > 0) {

285 if (i % consumer_delay_interval == 0) {

286 sleep(1);

287 }

288 }

289 }

290

291 printf("Consumer finished. ");

292 exit(0);

293 }

294

295 void init_shared(shared *s)

296 {

297 int i;

298

299 s->con_waiting = 0;

300 s->last_consumed = -1;

301

302 s->prod_waiting = 0;

303 s->last_produced = -1;

304

305 s->prod_pid = -1;

306 s->con_pid = -1;

307

308 s->prod_count = 0;

309 s->con_count = 0;

310

311 for (i=0; i<QUEUESIZE; i++) {

312 s->queue[i].word[0] = '';

313 /* semaphore is shared between processes,

314 and initial value is 1 (unlocked) */

315 sem_init(&s->queue[i].lock, 1, 1);

316 }

317 }

318

319 int main(int argc, char *argv[])

320 {

321 int pid, count, prod_interval, con_interval;

322

323 shared *s;

324

325 srandom(42);

326

327 if (argc < 4) {

328 if (argc < 1) {

329 report_error("no command line");

330 usage_exit(argv[0]);

331 } else {

332 report_error("Not enough arguments");

333 usage_exit(argv[0]);

334 }

335 }

336

337 count = atoi(argv[1]);

338 prod_interval = atoi(argv[2]);

339 con_interval = atoi(argv[3]);

340

341 s = (shared *) mmap(NULL, sizeof(shared),

342 PROT_READ|PROT_WRITE,

343 MAP_SHARED|MAP_ANONYMOUS, -1, 0);

344

345 if (s == MAP_FAILED) {

346 report_error(strerror(errno));

347 }

348

349 init_shared(s);

350

351 pid = fork();

352

353 if (pid) {

354 /* producer */

355 s->prod_pid = getpid();

356 producer(s, count, prod_interval);

357 } else {

358 /* consumer */

359 s->con_pid = getpid();

360 consumer(s, count, con_interval);

361 }

362

363 /* This line should never be reached */

364 return -1;

365 }

Explanation / Answer

#include <stdio.h>

9 #include <stdlib.h>

10 #include <unistd.h>

11 #include <sys/mman.h>

12 #include <errno.h>

13 #include <string.h>

14 #include <sys/types.h>

15 #include <sys/wait.h>

16 #include <semaphore.h>

17 #include <string.h>

18 #include <time.h>