使用互斥和pthread的C语言中的Reader Writer程序 [英] Reader Writer program in C using mutexes and pthreads
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问题描述
我陷入了C语言中的Reader/Writer问题.任何人都可以向我解释下面的代码中发生了什么.我不明白pthread_create(& tid,NULL,writer,NULL)行之后执行的流程.
I am stuck on a Reader/Writer problem in C. Can anybody explain me what is happening in the code below. I dont understand how the execution flows after the pthread_create(&tid,NULL,writer,NULL) line.
#include<pthread.h>
//#include<semaphore.h>
#include<stdio.h>
#include<stdlib.h>
pthread_mutex_t x,wsem;
pthread_t tid;
int readcount;
void intialize()
{
pthread_mutex_init(&x,NULL);
pthread_mutex_init(&wsem,NULL);
readcount=0;
}
void * reader (void * param)
{
int waittime;
waittime = rand() % 5;
printf("\nReader is trying to enter");
pthread_mutex_lock(&x);
readcount++;
if(readcount==1)
pthread_mutex_lock(&wsem);
printf("\n%d Reader is inside ",readcount);
pthread_mutex_unlock(&x);
sleep(waittime);
pthread_mutex_lock(&x);
readcount--;
if(readcount==0)
pthread_mutex_unlock(&wsem);
pthread_mutex_unlock(&x);
printf("\nReader is Leaving");
}
void * writer (void * param)
{
int waittime;
waittime=rand() % 3;
printf("\nWriter is trying to enter");
pthread_mutex_lock(&wsem);
printf("\nWrite has entered");
sleep(waittime);
pthread_mutex_unlock(&wsem);
printf("\nWriter is leaving");
sleep(30);
exit(0);
}
int main()
{
int n1,n2,i;
printf("\nEnter the no of readers: ");
scanf("%d",&n1);
printf("\nEnter the no of writers: ");
scanf("%d",&n2);
for(i=0;i<n1;i++)
pthread_create(&tid,NULL,reader,NULL);
for(i=0;i<n2;i++)
pthread_create(&tid,NULL,writer,NULL);
sleep(30);
exit(0);
}
推荐答案
如果您没有得到答案,则可以尝试以下代码.尝试与上面给出的代码进行比较.
If you havn't get your answer then you can try following code. Try to compare with above given code.
semaphore mutex = 1; // Controls access to the reader count
semaphore db = 1; // Controls access to the database
int reader_count; // The number of reading processes accessing the data
Reader()
{
while (TRUE) { // loop forever
down(&mutex); // gain access to reader_count
reader_count = reader_count + 1; // increment the reader_count
if (reader_count == 1)
down(&db); // if this is the first process to read the database,
// a down on db is executed to prevent access to the
// database by a writing process
up(&mutex); // allow other processes to access reader_count
read_db(); // read the database
down(&mutex); // gain access to reader_count
reader_count = reader_count - 1; // decrement reader_count
if (reader_count == 0)
up(&db); // if there are no more processes reading from the
// database, allow writing process to access the data
up(&mutex); // allow other processes to access reader_countuse_data();
// use the data read from the database (non-critical)
}
Writer()
{
while (TRUE) { // loop forever
create_data(); // create data to enter into database (non-critical)
down(&db); // gain access to the database
write_db(); // write information to the database
up(&db); // release exclusive access to the database
}
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