'Mutex' undeclared (first use in this function)

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I want to create a program that receives a number N as an argument and starts N threads each displaying one of the numbers from 1 to N synchronously, to produce an ordered sequence (12 ... N) *. Where, it is necessary to use one and the same function executed by all threads and pass the number to be displayed as an argument when starting the thread. Here is my code :

C#

#include<stdlib.h>
#include<pthread.h>
#include<sys/types.h>
#include<unistd.h>
//#include <pthread.h>   // pour les threads
#include <semaphore.h> // pour les semaphores
#include <fcntl.h>     // pour les flags O_CREAT, O_EXCL, ...


// Discussion : perte de cycles
// $ gcc -Wall synchro-valeur.c -lpthread
int N;
//sem_t mutex[10];
void* f0(int j) {

    for(int i = 0; i < 100; i++) {

        if(j < N-1){

            //printf("sdfsfsfsdsds: %d\n",j);

            sem_wait(&mutex[j]);
            //puts("lol");
            char k = (char)(j+1);
            puts(k+"\n");
            sem_post(&mutex[j+1]);
        }
        else{
            sem_wait(& mutex[N-1]);
            char k = (char)N;
            puts(k+"\n");
            sem_post(&mutex[0]);
        }



    }
    return NULL;
}



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

    N = atoi(argv[1]);

    sem_t mutex[N];



    unsigned int value[N];

    /*value[0]=1;

    for(int i =1;i<N;i++){
        value[i] = 0;
    }
    */

    sem_init(&mutex[0],0,1);
    for(int i =1;i<N;i++){
        sem_init(&mutex[i],0,0);
    }
    pthread_t pid[N];

    for (int j=0;j<N;j++){

        pthread_create(&pid[j],NULL,f0(j),NULL);

    }
    //puts("heyG");
    for (int i=0;i<N;i++){
        pthread_join(pid[i],0);
    }
    //puts("heyH");
    for (int i=0;i<N;i++){
        sem_destroy(&mutex[i]);
    }
    //puts("heyN");
    return EXIT_SUCCESS;
    //return 0;
}

I don't know how to resolve this error
error: ‘mutex’ undeclared (first use in this function) sem_wait(&mutex[j]);

What I have tried:

PLease, can anyone help me to fix this error
THank you

Posted 11-Oct-18 1:56am

Member 14016042

Updated 11-Oct-18 2:36am

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OriginalGriff · Answer 1 · 2018-10-11T02:28:00

mutex is declared as part of your main function:

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

    N = atoi(argv[1]);

    sem_t mutex[N];

which makes it a local variable - its scope is limited to the set of curly brackets which most recently enclose it - the main function itself.
And then you try to use it within the f0 function:

void* f0(int j) {

    for(int i = 0; i < 100; i++) {

        if(j < N-1){

            //printf("sdfsfsfsdsds: %d\n",j);

            sem_wait(&mutex[j]);

Because no global or local variable exists in the f0 function, the system rightly complains.

Possibly, you want to make mutex either global (by declaring it outside the body of any function), or pass it as a parameter to the f0 function along with j

KarstenK · Answer 2 · 2018-10-11T02:29:00

Solution 2

Really strange that the first error is at this line. The compiler should complain already on:

C++

sem_t mutex[N];

This code shuld do the job

C++

sem_t *mutex = malloc( sizeof( sem_t ) * N );
//at the ende
free( mutex );

Tip: set the warning level higher. This warnings are helping you to write good code :-)

Posted 11-Oct-18 2:29am

KarstenK

Updated 12-Oct-18 7:22am

v2

Comments

Member 14016044 11-Oct-18 8:43am

I did what you say
but I had this error

error: initializer element is not constant
sem_t *mutex = malloc( sizeof( sem_t ) * N );

KarstenK 12-Oct-18 13:23pm

I think you must write sem_t *mutex; at start of the function, because C needs all vars at the begin.

CHill60 · Answer 3 · 2018-10-11T02:36:00

Solution 4

If the code you have posted as a solution is genuinely the code causing the error then try changing

C++

sem_wait(& mutex[N-1]);

to

C++

sem_wait(&mutex[N-1]);

Posted 11-Oct-18 2:36am

CHill60

Comments

Member 14016044 11-Oct-18 8:39am

I still have the error

CHill60 11-Oct-18 9:24am

And the error is still "‘mutex’ undeclared (first use in this function) sem_wait(&mutex[j]);"?
Have you actually tried @OriginalGriff's suggestions?

Member 14016042 11-Oct-18 10:24am

oh I did not see that ! Thank you

Member 14016042 · Answer 4 · 2018-10-11T02:30:00

Sorry I copied the wrong code.

#include<stdio.h>
#include<stdlib.h>
#include<pthread.h>
#include<sys/types.h>
#include<unistd.h>
//#include <pthread.h>   // pour les threads
#include <semaphore.h> // pour les semaphores
#include <fcntl.h>     // pour les flags O_CREAT, O_EXCL, ...


// Discussion : perte de cycles
// $ gcc -Wall synchro-valeur.c -lpthread
int N;
sem_t mutex[N];
void* f0(int j) {
	//sem_t mutex[N];
    for(int i = 0; i < 100; i++) {
    	
    	if(j < N-1){
    		
    		//printf("sdfsfsfsdsds: %d\n",j);
    		
    		//sem_wait(&mutex[j]);
    		//puts("lol");
    		char k = (char)(j+1);
    		puts(k+"\n");
    		sem_post(&mutex[j+1]);
    	}
    	else{
    		sem_wait(& mutex[N-1]);
    		char k = (char)N;
    		puts(k+"\n");
    		sem_post(&mutex[0]);
    	}
    	
 

    }
    return NULL;
}



int main(int argc, char *argv[]) {
	
	N = atoi(argv[1]);
	
//	sem_t mutex[N];
	
	
	
	//unsigned int value[N];
	
	/*value[0]=1;

	for(int i =1;i<N;i++){
		value[i] = 0;
	}
	*/

	sem_init(&mutex[0],0,1);
	for(int i =1;i<N;i++){
		sem_init(&mutex[i],0,0);
	}
	pthread_t pid[N];
	
	for (int j=0;j<N;j++){
		
		pthread_create(&pid[j],NULL,f0(j),NULL);
		
	}
	//puts("heyG");
	for (int i=0;i<N;i++){
		pthread_join(pid[i],0);
	}
	//puts("heyH");
	for (int i=0;i<N;i++){
		sem_destroy(&mutex[i]);
	}
	//puts("heyN");
	return EXIT_SUCCESS;
    //return 0;
}