Producer/Consumer W/ mutual exclusion

[pantherman34]

Edit** new code in post 4

[MK27]

it compiles with no errors but i get a segmentation fault (core dumped) and have no idea why.

Right now is the time in your life when it is good to learn to use gdb to debug a bit:

getting a segfault using pointers

Scroll down to post #7. This will take you about 2 minutes, then you will know WHY you get a seg fault -- or at least, you will know exactly where it happened. And you will be able to figure out WHY very easily forevermore!!!

The other option is you can wait for me or someone else to copy paste your code, compile it, and run it in a debugger...but that seems a little silly, doesn't it?

[pantherman34]

that is very useful, thanks for the info. this is what it said

$ gdb ./a.exe
GNU gdb 6.8.0.20080328-cvs (cygwin-special)
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-cygwin"...
(gdb) run
Starting program: /home/brandon/a.exe
[New thread 4068.0xa10]
[New thread 4068.0x794]
USAGE:./main.out <INT> <INT> <INT>
      2 [main] a 4068 exception::handle: Exception: STATUS_ACCESS_VIOLATION
    369 [main] a 4068 open_stackdumpfile: Dumping stack trace to a.exe.stackdump


Program received signal SIGSEGV, Segmentation fault.
0x6110a694 in _strtol_r () from /usr/bin/cygwin1.dll
(gdb) bt
#0 0x6110a694 in _strtol_r () from /usr/bin/cygwin1.dll
#1 0x7c859f5c in OutputDebugStringA ()
      from /cygdrive/c/WINDOWS/system32/kernel32.dll
#2 0x40010006 in ?? ()
#3 0x00000000 in ?? ()

[MK27]

This is where you want to use "bt" or "backtrace" as the next command -- that's in that post too

Actually, I just compiled and ran your code here, I dunno if this might be a factor?

[root/media/sda6/root/C] gcc test.c -lpthread
[root/media/sda6/root/C] ./a.out
USAGE:./main.out <INT> <INT> <INT>
Segmentation fault
[root/media/sda6/root/C] ./a.out 1 2 3
Exit the program

I'm pretty sure it is the issue, since when I did the backtrace the fault is on line 144:

   int mainSleepTime = atoi(argv[1]); /* Time in seconds for main to sleep */

Here's a good gdb tutorial:

RMS's gdb Tutorial

To run in gdb with args, use:

(gdb) run 1 2 3

[grumpy]

If the program is run with argc < 4, then argv[1] through argv[3] do not exist.

Consider what that means for this code in main() if the program is run with less than 3 command line arguments.

 if(argc != 4) {
      fprintf(stderr, "USAGE:./main.out <INT> <INT> <INT>\n");
   }

   int mainSleepTime = atoi(argv[1]); /* Time in seconds for main to sleep */
   int numProd = atoi(argv[2]); /* Number of producer threads */
   int numCons = atoi(argv[3]); /* Number of consumer threads */

The simplest fix is probably to terminate the program if argc is not equal to 4.

[pantherman34]

Ok. Here is new code, it was just easier to start over. I am not confused over the "idea" of this producer/consumer problem, but im still confused about threads and semaphores and stuff. this particular program had 4 threads that read or consume from a shared variable. It has 1 thread that writes or produces to that shared variable. All the thread loops need to loop indefinitly which im sure i havent done. and on each loop iteration it requirs 1 second on computation without access to the shared variable.

And the data output i have in main needs to print report every second. My code has no errors, but it is exiting a 1. i definitely need some help! i tried my best to comment the code as to what is going on.

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>

#define BUFF_SIZE   100	/* total number of slots */
#define NP          1		/* total number of producers */
#define NC          4		/* total number of consumers */
#define NITERS      4		/* number of items produced/consumed */

typedef struct {
    int buf[BUFF_SIZE];   /* shared var */
    int in;         	  /* buf[in%BUFF_SIZE] is the first empty slot */
    int out;        	  /* buf[out%BUFF_SIZE] is the first full slot */
    sem_t full;     	  /* keep track of the number of full spots */
    sem_t empty;    	  /* keep track of the number of empty spots */
    sem_t mutex;    	  /* enforce mutual exclusion to shared data */
} sbuf_t;

sbuf_t shared;

/* function prototypes */
void *Producer(void *arg);
void *Consumer(void *arg);

/* increments and decrements */
int produced, consumed;

int main()
{
   /* producer/consumer threads */
    pthread_t idP, idC;
    
    /* loop control */
    int index;
    
    /* temp variables to comp the consumes and produces per hour */
    int temp1, temp2;

    /* keeps track of seconds */
    int j = 1;

    /* creates semaphores */
    sem_init(&shared.full, 0, 0);
    sem_init(&shared.empty, 0, BUFF_SIZE);
    sem_inti(&shared.mutex, 0, 1);
    

    /* creates producer thread */
    for (index = 0; index < NP; index++)
    {  
       /* Create 1 producer */
       pthread_create(&idP, NULL, Producer, (void*)index);
    }

    /* creates 4 consumer threads */
    for (index = 0; index < NC; index++)
    {
       pthread_create(&idC, NULL, Consumer, (void*)index);
    }
    

    while(1)
    {
       sleep(1);
       sem_wait(&shared.mutex);
       temp1 = (consumed * 3600) / j;
       temp2 = (produced * 3600) / j;
       printf("Statistics after %d seconds: ", j);
       sem_post(&shared.mutex);
       printf("So many consumed per hour: %d\n", temp1);
       printf("So many produced per hour: %d\n", temp2);
       printf("Ratio of consumed/produced per hour: %d", temp1 / temp2);
       j++;
    }
       

    sem_destroy(&shared.mutex); // destroy semaphore
    pthread_exit(NULL);
}
void *Producer(void *arg)
{
    int data, index;

    index = (int)arg;

    printf("Thread %d in line to  produce\n", index);

    for (produced=0; produced < NITERS; produced++) {

        /* Produce item */
        data = produced;
        printf("[P%d] Producing %d ...\n", index, data); fflush(stdout);

       data=produce_item(shared.buf);

        /* If there are no empty slots, wait */
        sem_wait(&shared.empty);
        /* If another thread uses the buffer, wait */
        sem_wait(&shared.mutex);
        /* put new item in buffer */
        shared.buf[shared.in] = data;
        shared.in = (shared.in+1)%BUFF_SIZE;
        /* leave the critical region */
        sem_post(&shared.mutex);
        /* Increment the number of full slots */
        sem_post(&shared.full);

        /* Interleave  producer and consumer execution */
        if (produced % 2 == 1) sleep(1);
    }
    return NULL;
}

void *Consumer(void *arg)
{
   int data, index;

    index = (int)arg;

    int consumed;
    printf("[C%d] Consuming %d ...\n", index, data); fflush(stdout);

    for (consumed=0; consumed < NITERS; consumed++) {

        data = consumed;

        /* If there are no full slots, wait */
        sem_wait(&shared.full);
        /* If another thread uses the buffer, wait */
        sem_wait(&shared.mutex);
        shared.buf[shared.out] = data;
        shared.out = (shared.out+1)%BUFF_SIZE;
        /* Release the buffer */
        sem_post(&shared.mutex);
        /* Increment the number of empty slots */
        sem_post(&shared.empty);

        /* Interleave  producer and consumer execution */
        if (consumed % 2 == 1) sleep(1);
    }
    return NULL;
}

[Codeplug]

>> My code has no errors, ...
>> sem_inti

Producer/Consumer using mutexs C - CodeGuru Forums

You seem to be in a hurry - when is it due?

gg

[pantherman34]

ha yea i caught that mistype in the debug, i thought it was just a warning. and no not in a hurry, just ready to get it finished!

[Codeplug]

http://www.csc.villanova.edu/~mdamian/threads/PC.htm

>> but it is exiting a 1. i definitely need some help!
So what exactly is the output you receive - and why it's not what you expected?

gg

[pantherman34]

Well actually, i have it working now, and it is giving expected results. the ratio isnt working though (dont know why, its very simple unless i have something wrong). it keeps giving me a ratio of 0. the ratio should be .75

here is some up'd code

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>

#define BUFF_SIZE   100	/* total number of slots */
#define NP          2		/* total number of producers - 1*/
#define NC          5		/* total number of consumers - 1 */
#define NITERS      4		/* number of items produced/consumed */

typedef struct {
    int buf[BUFF_SIZE];   /* shared var */
    int in;         	  /* buf[in%BUFF_SIZE] is the first empty slot */
    int out;        	  /* buf[out%BUFF_SIZE] is the first full slot */
    sem_t full;     	  /* keep track of the number of full spots */
    sem_t empty;    	  /* keep track of the number of empty spots */
    sem_t mutex;    	  /* enforce mutual exclusion to shared data */
} sbuf_t;

sbuf_t shared;

/* function prototypes */
void *Producer(void *arg);
void *Consumer(void *arg);

/* increments and decrements */
int produced, consumed;

int main()
{
   /* producer/consumer threads */
    pthread_t idP, idC;
    
    /* loop control */
    int index;
    
    /* temp variables to comp the consumes and produces per hour */
    int temp1, temp2;
    int ratio = temp1 / temp2;

    /* keeps track of seconds */
    int j = 1;

    /* creates semaphores */
    sem_init(&shared.full, 0, 0);
    sem_init(&shared.empty, 0, BUFF_SIZE);
    sem_init(&shared.mutex, 0, 1);
    

    /* creates producer thread */
    for (index = 1; index < NP; index++)
    {  
       /* Create 1 producer */
       pthread_create(&idP, NULL, Producer, (void*)index);
    }

    /* creates 4 consumer threads */
    for (index = 1; index < NC; index++)
    {
       pthread_create(&idC, NULL, Consumer, (void*)index);
    }
    

    while(1)
    {
       sleep(1);
       sem_wait(&shared.mutex);
       temp1 = (consumed * 3600) / j;
       temp2 = (produced * 3600) / j;
       printf("\n\nStatistics after %d seconds: \n\n", j);
       sem_post(&shared.mutex);
       printf("So many consumed per hour: %d\n", temp1);
       printf("So many produced per hour: %d\n", temp2);
       printf("Ratio of consumed/produced per hour: %d\n", ratio);
       j++;
    }

    sem_destroy(&shared.mutex); // destroy semaphore
    pthread_exit(NULL);
}
void *Producer(void *arg)
{
    int data, index;

    index = (int)arg;

    printf("Thread %d in line to  produce\n\n", index);

    for (produced=0; produced < NITERS; produced++) {

        /* Produce item */
        data = produced;
        printf("[P%d] Producing %d ...\n\n", index, data); fflush(stdout);

      // data=produce_item(shared.buf);

        /* If there are no empty slots, wait */
        sem_wait(&shared.empty);
        /* If another thread uses the buffer, wait */
        sem_wait(&shared.mutex);
        /* put new item in buffer */
        shared.buf[shared.in] = data;
        shared.in = (shared.in+1)%BUFF_SIZE;
        /* leave the critical region */
        sem_post(&shared.mutex);
        /* Increment the number of full slots */
        sem_post(&shared.full);

        /* Interleave  producer and consumer execution */
        if (produced % 2 == 1) sleep(1);
    }
    return NULL;
}

void *Consumer(void *arg)
{
   int data, index;

    index = (int)arg;

    printf("[C%d] Consuming %d ...\n", index, data); fflush(stdout);

    for (consumed=0; consumed < NITERS; consumed++) {

        data = consumed;

        /* If there are no full slots, wait */
        sem_wait(&shared.full);
        /* If another thread uses the buffer, wait */
        sem_wait(&shared.mutex);
        shared.buf[shared.out] = data;
        shared.out = (shared.out+1)%BUFF_SIZE;
        /* Release the buffer */
        sem_post(&shared.mutex);
        /* Increment the number of empty slots */
        sem_post(&shared.empty);

        /* Interleave  producer and consumer execution */
        if (consumed % 2 == 1) sleep(1);
    }
    return NULL;
}

[Codeplug]

First, you have unsynchronized access to "produced" and "consumed". Use local variables for the for loop and put the "total produced" and "total consumed" as members of sbuf_t - and only access them while mutex is held.

You calculate temp1 and temp2 in the while loop, but not "ratio". You're also performing integer arithmetic (3/2 = 1).
Change temp1, temp2, and ratio to float:

temp1 = (consumed * 3600.0f) / j;
temp2 = (produced * 3600.0f) / j;
ratio = temp1 / temp2;

Then use %f instead of %d in printf.

>> the ratio should be .75
So for a given point in time, there should be 3 consumed for every 4 produced? Why is that?

With 4 threads consuming and only 1 producing, production shouldn't out-pace consumption. So the ratio should pretty much stay at 1.

gg