how to synchronize process using mutex with multithreading in c - c

Question

HI I am writing a program which write the string in file, process 1 will write a small letters in file and process 2 write a capital letters in same file. i implemented a program using threading, process 1 must run first, after that run the process 2 . program as follows.
/******************
* Header Files
******************/
#include <stdio.h>
#include <pthread.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
pthread_mutex_t lock;
void* write_p1()
{
if((pthread_mutex_lock(&lock)) != 0)
{
printf("pthread_mutex_lock() failed\n");
exit(1);
}
int index=0; /*used in for loop*/
int flag=0; /*flag used in capital letter and space detecion*/
FILE *fp=NULL; /*file pointer used to store open file*/
char str[100]; /*to store user string*/
printf("*****Hi, I am Process 1*****\n");
/*open the sample.txt file for write*/
fp=fopen("sample.txt","w");
if(fp==NULL)
{
printf("Not able to open file\n");
exit(1);
}
printf("Enter small letters\n");
fgets(str,100,stdin);
/*capital letter and space detection*/
if((strlen(str)>0)&&(str[strlen(str)-1]=='\n'))
{
str[strlen(str)-1]='\0';
}
for(index=0;str[index]!='\0';index++)
{
if(islower(str[index]) || str[index] == ' ')
{
flag=0;
}
else
{
printf("Enter Small Letters\n");
exit(1);
}
}
if(flag==0)
{
fprintf(fp,"%s",str);
}
/*close the file*/
fclose(fp);
printf("Entered string: %s\n",str);
if((pthread_mutex_unlock(&lock)) != 0)
{
printf("pthread_mutex_unlock() failed\n");
exit(1);
}
printf("\n\n");
}
void* write_p2()
{
if((pthread_mutex_lock(&lock)) != 0)
{
printf("pthread_mutex_lock() failed\n");
exit(1);
}
int index=0; /*used in for loop*/
int flag=0; /*flag used in small letter and space detecion*/
FILE *fp=NULL; /*file pointer used to store open file*/
char str[100]; /*to store user string*/
printf("*****Hi, I am Process 2*****\n");
/*open the sample.txt file for write*/
fp=fopen("sample.txt","a");
if(fp==NULL)
{
printf("Not able to open file\n");
exit(1);
}
printf("Enter Capital letters\n");
fgets(str,100,stdin);
/*capital letter and space detection*/
if((strlen(str)>0)&&(str[strlen(str)-1]=='\n'))
{
str[strlen(str)-1]='\0';
}
for(index=0;str[index]!='\0';index++)
{
if(isupper(str[index]) || str[index] == ' ')
{
flag=0;
}
else
{
printf("Enter capital Letters\n");
exit(1);
}
}
if(flag==0)
{
fprintf(fp,"%s",str);
}
/*close the file*/
fclose(fp);
printf("Entered string: %s\n",str);
if((pthread_mutex_unlock(&lock)) != 0)
{
printf("pthread_mutex_unlock() failed\n");
exit(1);
}
printf("\n\n");
}
int main(void)
{
/*initialized semaphore*/
if((pthread_mutex_init(&lock,NULL)) != 0)
{
printf("pthread_mutex_init() failed\n");
exit(1);
}
/*create a two thread*/
pthread_t t1=0,t2=0;
pthread_create(&t1,NULL,write_p1,NULL);
pthread_create(&t2,NULL,write_p2,NULL);
/*this function wait for thread to terminate*/
pthread_join(t1,NULL);
pthread_join(t2,NULL);
/*destroy the semaphore*/
if((pthread_mutex_destroy(&lock)) != 0)
{
printf("pthread_mutex_destroy() failed\n");
exit(1);
}
return 0;
}
Now This program is working but sometime it will run first thread 2(process 2) or some time thread 1(process 1) , i need a solution for that, it must run thread 1(process 1) first after that thread 2(process 2), so what changes should i do?

Answer 1

I am wring below program for you. You can do it by using conditional variable. Implement same procedure in your code. How will it work? To start the controlling process, we allowing thread1 first. In the main thread (i.e. main function; every program has one main thread, in C/C++ this main thread is created automatically by the operating system once the control passes to the main method/function via the kernel) we are calling pthread_cond_signal(&cond1);. Once this function is called from the main thread, thread1, which was waiting on cond1, will be released and it will start executing further. Once it finishes its final task, it will call pthread_cond_signal(&cond2);. Now, the thread which was waiting on condition cond2, i.e. thread2, will be released and it will start to execute its final stage
#include<pthread.h>
pthread_cond_t cond1 = PTHREAD_COND_INITIALIZER;
pthread_cond_t cond2 = PTHREAD_COND_INITIALIZER;
pthread_mutex_t lock1 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t lock2 = PTHREAD_MUTEX_INITIALIZER;
int TRUE = 1;
void * threadMethod1(void *arg)
{
printf("In thread1\n");
do{
pthread_mutex_lock(&lock1);
//Add your business logic(parallel execution codes) here
pthread_cond_wait(&cond1, &lock1);
printf("I am thread1 generating the final report and inserting into file \n");
pthread_cond_signal(&cond2);/* Now allow 2nd thread to process */
pthread_mutex_unlock(&lock1);
}while(TRUE);
pthread_exit(NULL);
}
void * threadMethod2(void *arg)
{
printf("In thread2\n");
do
{
pthread_mutex_lock(&lock2);
//Add your business logic(parallel execution codes) here
pthread_cond_wait(&cond2, &lock2);
printf("I am thread2 generating the final report and inserting into a file \n");
pthread_cond_signal(&cond1);
pthread_mutex_unlock(&lock2);
}while(TRUE);
pthread_exit(NULL);
}
int main(void)
{
pthread_t tid1, tid2;
int i = 0;
printf("Before creating the threads\n");
if( pthread_create(&tid1, NULL, threadMethod1, NULL) != 0 )
printf("Failed to create thread1\n");
if( pthread_create(&tid2, NULL, threadMethod2, NULL) != 0 )
printf("Failed to create thread2\n");
pthread_cond_signal(&cond1);/* Now allow first thread to process first */
sleep(1);
TRUE = 0;/* Stop all the thread */
//sleep(3);
pthread_join(tid1,NULL);
pthread_join(tid2,NULL);
exit(0);
}