Mutex Deadlock (pthread_mutex) - c

Question

I can't understand why in this code I get a deadlock.
I have defined this mutexes:
mutex3: = 0 (LOCKED)
mutex2: = 1 (UNLOCKED)
I have 2 processes: Father and son. Each has N threads that I pass by argument. The child's threads contend with the father's threads a "resource" (typical producer / consumer problem).
Thanks in advance!
I suppose the problem is here:
void * func_padre(void * n)
{
while(1)
{
pthread_mutex_lock(&mutex2);
printf("Scrivi messaggio: ");
fflush(stdout);
scanf("%[^\n]",(char *)addr_mem);
getchar();
pthread_mutex_unlock(&mutex3);
}
}
void * func_figlio(void * n)
{
while(1)
{
pthread_mutex_lock(&mutex3);
write(fd,addr_mem,4096);
lseek(fd,0,SEEK_SET);
memset(addr_mem,0,4096);
pthread_mutex_unlock(&mutex2);
}
}
CODE:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>
void * addr_mem;
pthread_mutex_t mutex2,mutex3;
int fd;
void * func_padre(void * n)
{
while(1)
{
pthread_mutex_lock(&mutex2);
printf("Scrivi messaggio: ");
fflush(stdout);
scanf("%[^\n]",(char *)addr_mem);
getchar();
pthread_mutex_unlock(&mutex3);
}
}
void * func_figlio(void * n)
{
while(1)
{
pthread_mutex_lock(&mutex3);
write(fd,addr_mem,4096);
lseek(fd,0,SEEK_SET);
memset(addr_mem,0,4096);
pthread_mutex_unlock(&mutex2);
}
}
int main(int argc, char*argv[])
{
if(argc<2)
{
printf("POCHI PARAMETRI\n");
fflush(stdout);
exit(-1);
}
int val=strtol(argv[2],0,10);
fd = open(argv[1],O_CREAT|O_RDWR,0666);
lseek(fd,0,SEEK_SET);//USELESS
///SHARED MEMORY
int id_mem;
id_mem = shmget(6543,4096,IPC_CREAT|0666);
addr_mem = shmat(id_mem,NULL,0);
/////////////////////
/// MUTEX
pthread_mutex_init(&mutex2,NULL);
pthread_mutex_init(&mutex3,NULL);
pthread_mutex_lock(&mutex3);
/////////////////////
pthread_t tid;
int i=0;
int pid;
pid = fork();
if(pid==0)
{
//CHILD
for(i=0; i<val; i++)
pthread_create(&tid,NULL,func_figlio,NULL);
while(1)
{
pthread_mutex_lock(&mutex3);
write(fd,addr_mem,4096);
memset(addr_mem,0,4096);
pthread_mutex_unlock(&mutex2);
}
}
else
{
//FATHER
for(i=0; i<val; i++)
pthread_create(&tid,NULL,func_padre,NULL);
while(1)
{
pthread_mutex_lock(&mutex2);
printf("Scrivi messaggio: ");
fflush(stdout);
scanf("%[^\n]",(char *)addr_mem);
getchar();
pthread_mutex_unlock(&mutex3);
}
}
}

Answer 1

Your problem is not a deadlock, it's just buggy code. A thread cannot unlock a mutex it did not lock. Your while loops all unlock mutexes they do not hold and never unlock the mutexes they do hold.

Answer 2

suggest:
have global variable that is cycled through a limited set of values, say: 0, 1, 0 ...
Use the mutex to stop other threads from progressing, I.E.
When a call (in thread) returns from locking the mutex,
then check
if the value in the global variable is for 'this' thread.
then
process the thread activities,
increment the global variable,
endif
endif
unlock the mutex
call nanosleep() to delay for a while to allow other threads to run