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Seg Fault - Can not access memory ?

I am trying to implement a webserver with boss-worker model thread
pool implementation -

I have a header declaration threadpool.h as -

typedef struct threadpool_work {
void (*routine) ();
void *arg;
struct threadpool_work *next;
} threadpool_work_t;

typedef struct threadpool {
/* Pool Characteristics */
int num_threads;
int max_queue_size;

int do_not_block_when_full;
pthread_t *threads;
int cur_queue_size;
threadpool_work_t *queue_head;
threadpool_work_t *queue_tail;

pthread_mutex_t queue_lock;
pthread_cond_t queue_not_empty;
pthread_cond_t queue_not_full;
pthread_cond_t queue_empty;

int queue_closed;
int shutdown;
} *threadpool_t;

void threadpool_init(threadpool_t *threadpoolp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full);

int threadpool_add_work(threadpool_t threadpool,
void *routine,
void *arg);

int threadpool_destroy(threadpool_t threadpoolp, int finish);

void threadpool_thread(threadpool_t threadpool);

In my server_threadp.c main method , I do following -

threadpool_t threadpool;

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct threadpool))) ==
NULL) {
perror("malloc");
exit(1);
}

threadpool_add_work function has following code -

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;
printf("Signal for pthread_cond_wait waiting on
queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
threadpool->queue_tail = workp;
}

For the first HTTP request, it will find threadpool->cur_queue_size ==
0 and hence will execute the IF block of the above code, but for any
subsequent request it will execute ELSE block above.

When I am sending 2nd request, ITS SEGFAULTING at
(threadpool->queue_tail)-next = workp;

I debugged it through gdb and when trying to print following -

(threadpool->queue_tail)-next

it throws - Can not access memory

Which i believe is because - Memory is not allocated to the structure
threadpool self element.

My Question is - When i allocate memory in the begining to threadpool
structure do i have to still allocate memory for any element which is
itself in this case. ? If so, how do i do that ?

Or is there something else which I am missing here ?

Thanks
Mahendra
Oct 17 '07 #1
5 2399
[You really did not need to post your question 3 times. Your posts
may not appear immediately, that's just the way it is. You may need
to wait a bit.]

Mahendra Kumar Kutare wrote:
I am trying to implement a webserver with boss-worker model thread
pool implementation -
My first reaction was, "oh no, another off-topic question," but in
fact yours is perfectly topical.

<snip>
/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct threadpool))) ==
NULL) {
In C, casting malloc is not required and may hide a problem.
the preferred way is 'type *p = malloc(*p);'. That way, even if type
changes, the statement will remain safe. In your case, you could do

if ((threadpool = malloc(sizeof(*threadpool))) == /*...*/
perror("malloc");
exit(1);
}

threadpool_add_work function has following code -

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;
What's workp? Is it initialized to a valid threadpool?
printf("Signal for pthread_cond_wait waiting on
queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
^^^^^^^
Because if it isn't - BANG!
threadpool->queue_tail = workp;
}

For the first HTTP request, it will find threadpool->cur_queue_size ==
0 and hence will execute the IF block of the above code, but for any
subsequent request it will execute ELSE block above.

When I am sending 2nd request, ITS SEGFAULTING at
(threadpool->queue_tail)-next = workp;

I debugged it through gdb and when trying to print following -

(threadpool->queue_tail)-next

it throws - Can not access memory
If it does, then the implementors obviously could not spell.
It should be "cannot" (one word).
Which i believe is because - Memory is not allocated to the structure
threadpool self element.
Not so much not allocated as not initialized, I believe.
My Question is - When i allocate memory in the begining to threadpool
structure do i have to still allocate memory for any element which is
itself in this case. ? If so, how do i do that ?
I am not sure what you mean by "which is itself", but in general,
allocating alone leaves the value uninitialized. If the value happens
to be a struct, then its members are unitialized. Accessing an
uninitialized member may just yield garbage if it happens to be a
number, but if it is a pointer, the consequences may be more dire,
as you have experienced.
Or is there something else which I am missing here ?
Probably initializing your workp. Since I do not know where it came
from, I cannot be sure.

Peter
Oct 18 '07 #2

Thanks for your response. Sorry for multiple postings.

Now, coming back to the problem -

Code for the main method is as below -

************************************************** ************************

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

threadpool_t threadpool;

// Code to create INET socket
// bind, accept and listening socket code

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct
threadpool))) == NULL) {
perror("malloc");
exit(1);
}

/* Initialize the thread pool */
threadpool_init(&threadpool, NUM_WORKER_THREADS, MAX_QUEUE_SIZE,
0);

while(1) {
/* Accepting connections from the client */
clilen = sizeof(cli_addr);
newsockfd = accept (sockfd, (struct sockaddr *)
&cli_addr, &clilen);
if (newsockfd < 0) {
error ("ERROR on accept");
}

threadpool_add_work((void *) threadpool,
(*accept_requests), (void *) newsockfd);
}

return 0;
}
************************************************** ************************

void threadpool_init(threadpool_t *threadpoolp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full) {

int i, rtn;
threadpool_t threadpool;

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct
threadpool))) == NULL) {
perror("malloc");
exit(1);
}

/* Initialize the fields */
threadpool->num_threads = num_worker_threads;
threadpool->max_queue_size = max_queue_size;
threadpool->do_not_block_when_full = do_not_block_when_full;

if ((threadpool->threads = (pthread_t *)
malloc(sizeof(pthread_t) *num_worker_threads)) == NULL) {
perror("malloc");
exit(1);
}

threadpool->cur_queue_size = 0;
threadpool->queue_head = NULL;
threadpool->queue_tail = NULL;

threadpool->queue_closed = 0;
threadpool->shutdown = 0;

if ((rtn = pthread_mutex_init(&(threadpool->queue_lock), NULL))
!= 0) {
fprintf (stderr, "pthread_mutex_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_not_empty),
NULL)) != 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_not_full),
NULL)) != 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_empty), NULL))
!= 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

/* Create threads */
for (i = 0; i != num_worker_threads; i++) {
if ((rtn = pthread_create(&(threadpool->threads[i]),
NULL, (void *) (*threadpool_thread),
(void *) threadpool)) != 0) {
fprintf (stderr, "pthread_create %d", rtn);
exit(1);
}
}

*threadpoolp = threadpool;

************************************************** ************************

void threadpool_thread(threadpool_t threadpool) {

threadpool_work_t *my_workp;

for(;;) {
pthread_mutex_lock(&(threadpool->queue_lock));

printf("Cur queue size:: %d\n",
threadpool->cur_queue_size);
printf("Max queue size:: %d\n",
threadpool->max_queue_size);
printf("Cur shutdown :: %d\n",
(!(threadpool->shutdown)));
while((threadpool->cur_queue_size == 0) &&
(!threadpool->shutdown)) {
printf("Waiting for pthread_cond_wait
queue_not_empty:: %d\n", threadpool->queue_not_empty);

pthread_cond_wait(&(threadpool->queue_not_empty),
&(threadpool->queue_lock));
}

if (threadpool->shutdown) {
pthread_mutex_unlock(&(threadpool->queue_lock));
pthread_exit(NULL);
}

my_workp = threadpool->queue_head;
threadpool->cur_queue_size--;
if (threadpool->cur_queue_size == 0) {
threadpool->queue_head = threadpool->queue_tail = NULL;
} else {
threadpool->queue_head = my_workp->next;
}

if ((!threadpool->do_not_block_when_full) &&
(threadpool->cur_queue_size =
(threadpool->max_queue_size -1))) {

pthread_cond_signal(&(threadpool->queue_not_full));
}

if (threadpool->cur_queue_size == 0) {
pthread_cond_signal(&(threadpool->queue_empty));
}

pthread_mutex_unlock(&(threadpool->queue_lock));
(*(my_workp->routine))(my_workp->arg);
free(my_workp);
}
}

************************************************** ************************
int threadpool_add_work(threadpool_t threadpool, void *routine, void
*arg) {

threadpool_work_t *workp;
pthread_mutex_lock(&threadpool->queue_lock);

if((threadpool->cur_queue_size == threadpool->max_queue_size) &&
threadpool->do_not_block_when_full) {
pthread_mutex_unlock(&threadpool->queue_lock);
return -1;
}

while((threadpool->cur_queue_size ==
threadpool->max_queue_size) &&
((threadpool->shutdown || threadpool->queue_closed))) {

printf("Waiting for pthread_cond_wait queue_not_full::
%d\n", threadpool->queue_not_full);
pthread_cond_wait(&threadpool->queue_not_full,
&threadpool->queue_lock);
}

if(threadpool->shutdown || threadpool->queue_closed) {
printf("Threadpool_add_work::Unlocking mutex:: %d\n",
threadpool->queue_lock);
pthread_mutex_unlock(&threadpool->queue_lock);
return -1;
}
/* Allocate work structure */
workp = (threadpool_work_t *) malloc(sizeof(threadpool_work_t));
workp->routine = routine;
workp->arg = arg;
workp->next = NULL;

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;
printf("Signal for pthread_cond_wait waiting on queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
threadpool->queue_tail = workp;
}
threadpool->cur_queue_size++;
pthread_mutex_unlock(&threadpool->queue_lock);
return 1;
}
************************************************** ************************

The header file threadpool.h looks as below -

************************************************** ************************
typedef struct threadpool_work {
void (*routine) ();
void *arg;
struct threadpool_work *next;
} threadpool_work_t;

typedef struct threadpool {
/* Pool Characteristics */
int num_threads;
int max_queue_size;

int do_not_block_when_full;
pthread_t *threads;
int cur_queue_size;
threadpool_work_t *queue_head;
threadpool_work_t *queue_tail;

pthread_mutex_t queue_lock;
pthread_cond_t queue_not_empty;
pthread_cond_t queue_not_full;
pthread_cond_t queue_empty;

int queue_closed;
int shutdown;
} *threadpool_t;

void threadpool_init(threadpool_t *threadpoolp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full);

int threadpool_add_work(threadpool_t threadpool,
void *routine,
void *arg);

int threadpool_destroy(threadpool_t threadpoolp, int finish);

void threadpool_thread(threadpool_t threadpool);
************************************************** ************************

Now as can be seen "workp" is allocated structure in the above code and
initialized the 'routine' to routine, 'arg' to arg and 'next' a pointer
to NULL respectively.
I am not sure what you mean by "which is itself", but in general,
allocating alone leaves the value uninitialized. If the value happens
to be a struct, then its members are unitialized. Accessing an
uninitialized member may just yield garbage if it happens to be a
number, but if it is a pointer, the consequences may be more dire,
as you have experienced.
Now i see your point that 'next' which itself is a pointer to struct and
its members are not initialized. But threadpool->queue_tail which has
struct element 'next' is NOT initialized to which I am trying to
allocate 'workp'.

This threadpool is passed in the declared in the beginning of the main
method and then allocated before calling thread_init(....).

So how do I initialize

(threadpool->queue_tail) which points to a structure
threadpool_work_t and whose element - 'next' is pointer to
threadpool_work_t ?

Thanks
Mahendra
Peter Pichler wrote:
[You really did not need to post your question 3 times. Your posts
may not appear immediately, that's just the way it is. You may need
to wait a bit.]

Mahendra Kumar Kutare wrote:
>I am trying to implement a webserver with boss-worker model thread
pool implementation -

My first reaction was, "oh no, another off-topic question," but in
fact yours is perfectly topical.

<snip>
>/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct threadpool))) ==
NULL) {

In C, casting malloc is not required and may hide a problem.
the preferred way is 'type *p = malloc(*p);'. That way, even if type
changes, the statement will remain safe. In your case, you could do

if ((threadpool = malloc(sizeof(*threadpool))) == /*...*/
> perror("malloc");
exit(1);
}

threadpool_add_work function has following code -

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;

What's workp? Is it initialized to a valid threadpool?
> printf("Signal for pthread_cond_wait waiting on
queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
^^^^^^^
Because if it isn't - BANG!
> threadpool->queue_tail = workp;
}

For the first HTTP request, it will find threadpool->cur_queue_size ==
0 and hence will execute the IF block of the above code, but for any
subsequent request it will execute ELSE block above.

When I am sending 2nd request, ITS SEGFAULTING at
(threadpool->queue_tail)-next = workp;

I debugged it through gdb and when trying to print following -

(threadpool->queue_tail)-next

it throws - Can not access memory

If it does, then the implementors obviously could not spell.
It should be "cannot" (one word).
>Which i believe is because - Memory is not allocated to the structure
threadpool self element.

Not so much not allocated as not initialized, I believe.
>My Question is - When i allocate memory in the begining to threadpool
structure do i have to still allocate memory for any element which is
itself in this case. ? If so, how do i do that ?

I am not sure what you mean by "which is itself", but in general,
allocating alone leaves the value uninitialized. If the value happens
to be a struct, then its members are unitialized. Accessing an
uninitialized member may just yield garbage if it happens to be a
number, but if it is a pointer, the consequences may be more dire,
as you have experienced.
>Or is there something else which I am missing here ?

Probably initializing your workp. Since I do not know where it came
from, I cannot be sure.

Peter
Oct 18 '07 #3
Mahendra Kumar Kutare wrote:
>
Thanks for your response. Sorry for multiple postings.

Now, coming back to the problem -
3 issues stand out for me:

1) You don't check the return value of malloc() when allocating
workp in threadpool_add_work(). Granted, you don't immediately die
touching it... but you should check.

2) As previously noted, make sure you've included stdlib.h and get
rid of the malloc() casting.

3) I think your problem is this line in threadpool_thread():
if ((!threadpool->do_not_block_when_full) &&
(threadpool->cur_queue_size =
(threadpool->max_queue_size -1))) {

If the threadpool is not marked as "do_not_block_when_full", you're
always setting the size of the circular queue to be the max - 1
[because I think this is a typo where you have "=" instead of "=="].
As a result, the queue will erroneously report it has elements when
you've in fact removed them all... hence you derefence the NULL pointer
at the tail of the queue and die.

Since your main() does mark the threadpool via threadpool_init with
0 for the do_not_block_when_full state... this seems consistent to me.

Don
Code for the main method is as below -

************************************************** ************************

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

threadpool_t threadpool;

// Code to create INET socket
// bind, accept and listening socket code

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct
threadpool))) == NULL) {
perror("malloc");
exit(1);
}

/* Initialize the thread pool */
threadpool_init(&threadpool, NUM_WORKER_THREADS, MAX_QUEUE_SIZE,
0);

while(1) {
/* Accepting connections from the client */
clilen = sizeof(cli_addr);
newsockfd = accept (sockfd, (struct sockaddr *)
&cli_addr, &clilen);
if (newsockfd < 0) {
error ("ERROR on accept");
}

threadpool_add_work((void *) threadpool,
(*accept_requests), (void *) newsockfd);
}

return 0;
}
************************************************** ************************

void threadpool_init(threadpool_t *threadpoolp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full) {

int i, rtn;
threadpool_t threadpool;

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct
threadpool))) == NULL) {
perror("malloc");
exit(1);
}

/* Initialize the fields */
threadpool->num_threads = num_worker_threads;
threadpool->max_queue_size = max_queue_size;
threadpool->do_not_block_when_full = do_not_block_when_full;

if ((threadpool->threads = (pthread_t *)
malloc(sizeof(pthread_t) *num_worker_threads)) == NULL) {
perror("malloc");
exit(1);
}

threadpool->cur_queue_size = 0;
threadpool->queue_head = NULL;
threadpool->queue_tail = NULL;

threadpool->queue_closed = 0;
threadpool->shutdown = 0;

if ((rtn = pthread_mutex_init(&(threadpool->queue_lock), NULL))
!= 0) {
fprintf (stderr, "pthread_mutex_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_not_empty),
NULL)) != 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_not_full),
NULL)) != 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_empty), NULL))
!= 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

/* Create threads */
for (i = 0; i != num_worker_threads; i++) {
if ((rtn = pthread_create(&(threadpool->threads[i]),
NULL, (void *) (*threadpool_thread),
(void *) threadpool)) != 0) {
fprintf (stderr, "pthread_create %d", rtn);
exit(1);
}
}

*threadpoolp = threadpool;

************************************************** ************************

void threadpool_thread(threadpool_t threadpool) {

threadpool_work_t *my_workp;

for(;;) {
pthread_mutex_lock(&(threadpool->queue_lock));

printf("Cur queue size:: %d\n",
threadpool->cur_queue_size);
printf("Max queue size:: %d\n",
threadpool->max_queue_size);
printf("Cur shutdown :: %d\n",
(!(threadpool->shutdown)));
while((threadpool->cur_queue_size == 0) &&
(!threadpool->shutdown)) {
printf("Waiting for pthread_cond_wait
queue_not_empty:: %d\n", threadpool->queue_not_empty);

pthread_cond_wait(&(threadpool->queue_not_empty),
&(threadpool->queue_lock));
}

if (threadpool->shutdown) {
pthread_mutex_unlock(&(threadpool->queue_lock));
pthread_exit(NULL);
}

my_workp = threadpool->queue_head;
threadpool->cur_queue_size--;
if (threadpool->cur_queue_size == 0) {
threadpool->queue_head = threadpool->queue_tail = NULL;
} else {
threadpool->queue_head = my_workp->next;
}

if ((!threadpool->do_not_block_when_full) &&
(threadpool->cur_queue_size =
(threadpool->max_queue_size -1))) {

pthread_cond_signal(&(threadpool->queue_not_full));
}

if (threadpool->cur_queue_size == 0) {
pthread_cond_signal(&(threadpool->queue_empty));
}

pthread_mutex_unlock(&(threadpool->queue_lock));
(*(my_workp->routine))(my_workp->arg);
free(my_workp);
}
}

************************************************** ************************
int threadpool_add_work(threadpool_t threadpool, void *routine, void
*arg) {

threadpool_work_t *workp;
pthread_mutex_lock(&threadpool->queue_lock);

if((threadpool->cur_queue_size == threadpool->max_queue_size) &&
threadpool->do_not_block_when_full) {
pthread_mutex_unlock(&threadpool->queue_lock);
return -1;
}

while((threadpool->cur_queue_size ==
threadpool->max_queue_size) &&
((threadpool->shutdown || threadpool->queue_closed))) {

printf("Waiting for pthread_cond_wait queue_not_full::
%d\n", threadpool->queue_not_full);
pthread_cond_wait(&threadpool->queue_not_full,
&threadpool->queue_lock);
}

if(threadpool->shutdown || threadpool->queue_closed) {
printf("Threadpool_add_work::Unlocking mutex:: %d\n",
threadpool->queue_lock);
pthread_mutex_unlock(&threadpool->queue_lock);
return -1;
}
> /* Allocate work structure */
workp = (threadpool_work_t *) malloc(sizeof(threadpool_work_t));
workp->routine = routine;
workp->arg = arg;
workp->next = NULL;

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;
printf("Signal for pthread_cond_wait waiting on
queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
threadpool->queue_tail = workp;
}

threadpool->cur_queue_size++;
pthread_mutex_unlock(&threadpool->queue_lock);
return 1;
}
************************************************** ************************

The header file threadpool.h looks as below -

************************************************** ************************
typedef struct threadpool_work {
void (*routine) ();
void *arg;
struct threadpool_work *next;
} threadpool_work_t;

typedef struct threadpool {
/* Pool Characteristics */
int num_threads;
int max_queue_size;

int do_not_block_when_full;
pthread_t *threads;
int cur_queue_size;
threadpool_work_t *queue_head;
threadpool_work_t *queue_tail;

pthread_mutex_t queue_lock;
pthread_cond_t queue_not_empty;
pthread_cond_t queue_not_full;
pthread_cond_t queue_empty;

int queue_closed;
int shutdown;
} *threadpool_t;

void threadpool_init(threadpool_t *threadpoolp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full);

int threadpool_add_work(threadpool_t threadpool,
void *routine,
void *arg);

int threadpool_destroy(threadpool_t threadpoolp, int finish);

void threadpool_thread(threadpool_t threadpool);
************************************************** ************************

Now as can be seen "workp" is allocated structure in the above code and
initialized the 'routine' to routine, 'arg' to arg and 'next' a pointer
to NULL respectively.
I am not sure what you mean by "which is itself", but in general,
allocating alone leaves the value uninitialized. If the value happens
to be a struct, then its members are unitialized. Accessing an
uninitialized member may just yield garbage if it happens to be a
number, but if it is a pointer, the consequences may be more dire,
as you have experienced.

Now i see your point that 'next' which itself is a pointer to struct and
its members are not initialized. But threadpool->queue_tail which has
struct element 'next' is NOT initialized to which I am trying to
allocate 'workp'.

This threadpool is passed in the declared in the beginning of the main
method and then allocated before calling thread_init(....).

So how do I initialize

(threadpool->queue_tail) which points to a structure
threadpool_work_t and whose element - 'next' is pointer to
threadpool_work_t ?

Thanks
Mahendra
Peter Pichler wrote:
>[You really did not need to post your question 3 times. Your posts
may not appear immediately, that's just the way it is. You may need
to wait a bit.]

Mahendra Kumar Kutare wrote:
>>I am trying to implement a webserver with boss-worker model thread
pool implementation -

My first reaction was, "oh no, another off-topic question," but in
fact yours is perfectly topical.

<snip>
>>/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct threadpool))) ==
NULL) {

In C, casting malloc is not required and may hide a problem.
the preferred way is 'type *p = malloc(*p);'. That way, even if type
changes, the statement will remain safe. In your case, you could do

if ((threadpool = malloc(sizeof(*threadpool))) == /*...*/
>> perror("malloc");
exit(1);
}

threadpool_add_work function has following code -

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;

What's workp? Is it initialized to a valid threadpool?
>> printf("Signal for pthread_cond_wait waiting on
queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
^^^^^^^
Because if it isn't - BANG!
>> threadpool->queue_tail = workp;
}

For the first HTTP request, it will find threadpool->cur_queue_size ==
0 and hence will execute the IF block of the above code, but for any
subsequent request it will execute ELSE block above.

When I am sending 2nd request, ITS SEGFAULTING at
(threadpool->queue_tail)-next = workp;

I debugged it through gdb and when trying to print following -

(threadpool->queue_tail)-next

it throws - Can not access memory

If it does, then the implementors obviously could not spell.
It should be "cannot" (one word).
>>Which i believe is because - Memory is not allocated to the structure
threadpool self element.

Not so much not allocated as not initialized, I believe.
>>My Question is - When i allocate memory in the begining to threadpool
structure do i have to still allocate memory for any element which is
itself in this case. ? If so, how do i do that ?

I am not sure what you mean by "which is itself", but in general,
allocating alone leaves the value uninitialized. If the value happens
to be a struct, then its members are unitialized. Accessing an
uninitialized member may just yield garbage if it happens to be a
number, but if it is a pointer, the consequences may be more dire,
as you have experienced.
>>Or is there something else which I am missing here ?

Probably initializing your workp. Since I do not know where it came
from, I cannot be sure.

Peter
Oct 18 '07 #4

Hi Don and Peter,

Thanks for your responses.

I have modified my code to reflect 1 and 2.

The biggest issue was as mentioned in point 3. Rather than comparing (==) the
code was equating (=) that made the code work well.

I executed the code with the above changes and able to read the static html
files from webserver client code using command - GET /index.html HTTP/1.1

I have a further doubt about as following -

A) With above changes, the code did not SEGFAULT and run well. The code to
process work is as below -

I would like to know any improvements which can be done on code below specially
'accept_requests' and 'process_request' method. This code is currently working fine.

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

//////////
Code to create socket, bind and listen for incoming connections
//////////
/* Keep waiting for connections */
while(1) {
/* Accepting connections from the client */
clilen = sizeof(cli_addr);
newsockfd = accept (sockfd, (struct sockaddr *) &cli_addr,
&clilen);
if (newsockfd < 0) {
error ("ERROR on accept");
}
/* Add work to the queue for threads to work upon */
threadpool_add_work((void *) threadp, (*accept_requests), (void
*) newsockfd);
}

//////////
}

void *accept_requests(void *params) {

int socket_fd = (int) params;
pthread_t tid = pthread_self();

/* Take the data from socket and pass it for processing */
process_requests(socket_fd);

printf("Done processing: %d\n", tid);
pthread_exit(NULL);
}

void process_requests(int socket_fd) {

int n, ret;
char buffer[256];
char line[100], method[100], path[100], protocol[100];
char *file;
FILE *fp;
size_t bytes_read;

/* Initializing the buffer */
bzero(buffer, 256);
/* Reading from socket */
n = read (socket_fd, buffer, 255);
if (n < 0) {
error("ERROR reading from socket");
}

printf("Client communication message:: %s\n", buffer);

ret = sscanf(buffer, "%[^ ] %[^ ] %[^ ]", method, path, protocol);
if (ret != 3) {
printf(" Bad Request : Can't parse request ");
}
file = &(path[1]);
fp = fopen(file, "r");
if (fp == NULL) {
error (" Could not open the file ");
} else {
bzero(buffer, 256);
bytes_read = fread(buffer, sizeof(buffer), 1, fp);

}

write(socket_fd, buffer, 256);
close(socket_fd);
}

B) I have typedefs in header file -

1) threadpool_t which is a pointer variable - pointer to threadpool structure.
>typedef struct threadpool {
/* Pool Characteristics */
int num_threads;
int max_queue_size;

int do_not_block_when_full;
pthread_t *threads;
int cur_queue_size;
threadpool_work_t *queue_head;
threadpool_work_t *queue_tail;

pthread_mutex_t queue_lock;
pthread_cond_t queue_not_empty;
pthread_cond_t queue_not_full;
pthread_cond_t queue_empty;

int queue_closed;
int shutdown;
} *threadpool_t;
2) I declare in my (current modified code) main method -
threadpool_t threadp;

which by header definition above =threadp is of type threadpool_t (typedef),
which is a pointer to struct threadpool.

This 'threadp' variable is malloced before threadpool_init is called as -

if ((threadp = malloc(sizeof(threadp))) == NULL) {
perror("malloc");
exit(1);
}

Hence to threadpool_init function that has following signature -

void threadpool_init(threadpool_t *threadp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full);

I pass arg1 as '&threadp' - i.e. I am passing the address of memory location of
'threadp' - i.e. address of memory location of struct-type 'threadpool'

But, inside 'threadpool_init' method there is local 'threadP' variable of type
'threadpool_t' as -
threadpool_t threadP;

Here again, I malloced the threadP of structure type 'threadpool_t' as -

if ((threadP = malloc(sizeof(*threadP))) == NULL) {
perror("malloc");
exit(1);
}

It can be seen that malloc() assignment for 'threadP' has <b>'*threadP'</bas
parameter for sizeof() method. However compare this with malloc assignment of
'threadp' and it can seen that <b>'threadp'</bis passed as parameter for
sizeof() method. So essentially once I am passing pointer variable and in
another case just the variable of type 'threadpool_t'.

I tried passing just the variable of type 'threadpool_t' i.e. 'threadP' in
sizeof() function of malloc code above but that causes program to hang indefinitely.

I am not sure, as why is this difference of behaviour in mallocing ? Why can not
the 2nd mallocing work as following -

if ((threadP = malloc(sizeof(threadP))) == NULL) {
perror("malloc");
exit(1);
}

It should be noticed that that finally in 'threadpool_init' function 'threadP'
is assigned to '*threadp' as -

====>>>*threadp = threadP; // In the last line of 'threadpool_init' function

Appreciate your help on this so far.

Thanks
Mahendra

Don Morris wrote:
Mahendra Kumar Kutare wrote:
>>
Thanks for your response. Sorry for multiple postings.

Now, coming back to the problem -

3 issues stand out for me:

1) You don't check the return value of malloc() when allocating
workp in threadpool_add_work(). Granted, you don't immediately die
touching it... but you should check.

2) As previously noted, make sure you've included stdlib.h and get
rid of the malloc() casting.

3) I think your problem is this line in threadpool_thread():
if ((!threadpool->do_not_block_when_full) &&
(threadpool->cur_queue_size =
(threadpool->max_queue_size -1))) {

If the threadpool is not marked as "do_not_block_when_full", you're
always setting the size of the circular queue to be the max - 1
[because I think this is a typo where you have "=" instead of "=="].
As a result, the queue will erroneously report it has elements when
you've in fact removed them all... hence you derefence the NULL pointer
at the tail of the queue and die.

Since your main() does mark the threadpool via threadpool_init with
0 for the do_not_block_when_full state... this seems consistent to me.

Don
>Code for the main method is as below -

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

threadpool_t threadpool;
// Code to create INET socket
// bind, accept and listening socket code

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct
threadpool))) == NULL) {
perror("malloc");
exit(1);
}

/* Initialize the thread pool */
threadpool_init(&threadpool, NUM_WORKER_THREADS, MAX_QUEUE_SIZE,
0);

while(1) {
/* Accepting connections from the client */
clilen = sizeof(cli_addr);
newsockfd = accept (sockfd, (struct sockaddr *)
&cli_addr, &clilen);
if (newsockfd < 0) {
error ("ERROR on accept");
}

threadpool_add_work((void *) threadpool,
(*accept_requests), (void *) newsockfd);
}

return 0;
}
************************************************* *************************
void threadpool_init(threadpool_t *threadpoolp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full) {

int i, rtn;
threadpool_t threadpool;

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct
threadpool))) == NULL) {
perror("malloc");
exit(1);
}

/* Initialize the fields */
threadpool->num_threads = num_worker_threads;
threadpool->max_queue_size = max_queue_size;
threadpool->do_not_block_when_full = do_not_block_when_full;

if ((threadpool->threads = (pthread_t *)
malloc(sizeof(pthread_t) *num_worker_threads)) == NULL) {
perror("malloc");
exit(1);
}

threadpool->cur_queue_size = 0;
threadpool->queue_head = NULL;
threadpool->queue_tail = NULL;

threadpool->queue_closed = 0;
threadpool->shutdown = 0;

if ((rtn = pthread_mutex_init(&(threadpool->queue_lock), NULL))
!= 0) {
fprintf (stderr, "pthread_mutex_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_not_empty),
NULL)) != 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_not_full),
NULL)) != 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

if ((rtn = pthread_cond_init(&(threadpool->queue_empty), NULL))
!= 0) {
fprintf (stderr, "pthread_cond_init %s", strerror(rtn));
exit(1);
}

/* Create threads */
for (i = 0; i != num_worker_threads; i++) {
if ((rtn = pthread_create(&(threadpool->threads[i]),
NULL, (void *) (*threadpool_thread),
(void *) threadpool)) != 0) {
fprintf (stderr, "pthread_create %d", rtn);
exit(1);
}
}

*threadpoolp = threadpool;

************************************************* *************************
void threadpool_thread(threadpool_t threadpool) {

threadpool_work_t *my_workp;

for(;;) {
pthread_mutex_lock(&(threadpool->queue_lock));

printf("Cur queue size:: %d\n",
threadpool->cur_queue_size);
printf("Max queue size:: %d\n",
threadpool->max_queue_size);
printf("Cur shutdown :: %d\n",
(!(threadpool->shutdown)));
while((threadpool->cur_queue_size == 0) &&
(!threadpool->shutdown)) {
printf("Waiting for pthread_cond_wait
queue_not_empty:: %d\n", threadpool->queue_not_empty);

pthread_cond_wait(&(threadpool->queue_not_empty),
&(threadpool->queue_lock));
}

if (threadpool->shutdown) {
pthread_mutex_unlock(&(threadpool->queue_lock));
pthread_exit(NULL);
}

my_workp = threadpool->queue_head;
threadpool->cur_queue_size--;
if (threadpool->cur_queue_size == 0) {
threadpool->queue_head = threadpool->queue_tail = NULL;
} else {
threadpool->queue_head = my_workp->next;
}

if ((!threadpool->do_not_block_when_full) &&
(threadpool->cur_queue_size =
(threadpool->max_queue_size -1))) {

pthread_cond_signal(&(threadpool->queue_not_full));
}

if (threadpool->cur_queue_size == 0) {
pthread_cond_signal(&(threadpool->queue_empty));
}

pthread_mutex_unlock(&(threadpool->queue_lock));
(*(my_workp->routine))(my_workp->arg);
free(my_workp);
}
}

************************************************* *************************

int threadpool_add_work(threadpool_t threadpool, void *routine, void
*arg) {

threadpool_work_t *workp;
pthread_mutex_lock(&threadpool->queue_lock);

if((threadpool->cur_queue_size == threadpool->max_queue_size) &&
threadpool->do_not_block_when_full) {
pthread_mutex_unlock(&threadpool->queue_lock);
return -1;
}

while((threadpool->cur_queue_size ==
threadpool->max_queue_size) &&
((threadpool->shutdown || threadpool->queue_closed))) {

printf("Waiting for pthread_cond_wait queue_not_full::
%d\n", threadpool->queue_not_full);
pthread_cond_wait(&threadpool->queue_not_full,
&threadpool->queue_lock);
}

if(threadpool->shutdown || threadpool->queue_closed) {
printf("Threadpool_add_work::Unlocking mutex:: %d\n",
threadpool->queue_lock);
pthread_mutex_unlock(&threadpool->queue_lock);
return -1;
}
>> /* Allocate work structure */
workp = (threadpool_work_t *) malloc(sizeof(threadpool_work_t));
workp->routine = routine;
workp->arg = arg;
workp->next = NULL;

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;
printf("Signal for pthread_cond_wait waiting on
queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
threadpool->queue_tail = workp;
}

threadpool->cur_queue_size++;
pthread_mutex_unlock(&threadpool->queue_lock);
return 1;
}
************************************************* *************************
The header file threadpool.h looks as below -

************************************************* *************************

typedef struct threadpool_work {
void (*routine) ();
void *arg;
struct threadpool_work *next;
} threadpool_work_t;

typedef struct threadpool {
/* Pool Characteristics */
int num_threads;
int max_queue_size;

int do_not_block_when_full;
pthread_t *threads;
int cur_queue_size;
threadpool_work_t *queue_head;
threadpool_work_t *queue_tail;

pthread_mutex_t queue_lock;
pthread_cond_t queue_not_empty;
pthread_cond_t queue_not_full;
pthread_cond_t queue_empty;

int queue_closed;
int shutdown;
} *threadpool_t;

void threadpool_init(threadpool_t *threadpoolp,
int num_worker_threads,
int max_queue_size,
int do_not_block_when_full);

int threadpool_add_work(threadpool_t threadpool,
void *routine,
void *arg);

int threadpool_destroy(threadpool_t threadpoolp, int finish);

void threadpool_thread(threadpool_t threadpool);
************************************************* *************************
Now as can be seen "workp" is allocated structure in the above code
and initialized the 'routine' to routine, 'arg' to arg and 'next' a
pointer to NULL respectively.
> I am not sure what you mean by "which is itself", but in general,
allocating alone leaves the value uninitialized. If the value happens
to be a struct, then its members are unitialized. Accessing an
uninitialized member may just yield garbage if it happens to be a
number, but if it is a pointer, the consequences may be more dire,
as you have experienced.

Now i see your point that 'next' which itself is a pointer to struct
and its members are not initialized. But threadpool->queue_tail which
has struct element 'next' is NOT initialized to which I am trying to
allocate 'workp'.

This threadpool is passed in the declared in the beginning of the main
method and then allocated before calling thread_init(....).

So how do I initialize

(threadpool->queue_tail) which points to a structure
threadpool_work_t and whose element - 'next' is pointer to
threadpool_work_t ?

Thanks
Mahendra
Peter Pichler wrote:
>>[You really did not need to post your question 3 times. Your posts
may not appear immediately, that's just the way it is. You may need
to wait a bit.]

Mahendra Kumar Kutare wrote:

I am trying to implement a webserver with boss-worker model thread
pool implementation -

My first reaction was, "oh no, another off-topic question," but in
fact yours is perfectly topical.

<snip>

/* Allocate a pool data structure */
if ((threadpool = (threadpool_t) malloc(sizeof(struct threadpool))) ==
NULL) {

In C, casting malloc is not required and may hide a problem.
the preferred way is 'type *p = malloc(*p);'. That way, even if type
changes, the statement will remain safe. In your case, you could do

if ((threadpool = malloc(sizeof(*threadpool))) == /*...*/

perror("malloc");
exit(1);
}

threadpool_add_work function has following code -

if (threadpool->cur_queue_size == 0) {
threadpool->queue_tail = threadpool->queue_head =workp;

What's workp? Is it initialized to a valid threadpool?

printf("Signal for pthread_cond_wait waiting on
queue_not_empty:: %d\n", threadpool->queue_not_empty);
pthread_cond_signal(&threadpool->queue_not_empty);
} else {
(threadpool->queue_tail)-next = workp;
^^^^^^^
Because if it isn't - BANG!

threadpool->queue_tail = workp;
}

For the first HTTP request, it will find threadpool->cur_queue_size ==
0 and hence will execute the IF block of the above code, but for any
subsequent request it will execute ELSE block above.

When I am sending 2nd request, ITS SEGFAULTING at
(threadpool->queue_tail)-next = workp;

I debugged it through gdb and when trying to print following -

(threadpool->queue_tail)-next

it throws - Can not access memory

If it does, then the implementors obviously could not spell.
It should be "cannot" (one word).

Which i believe is because - Memory is not allocated to the structure
threadpool self element.

Not so much not allocated as not initialized, I believe.

My Question is - When i allocate memory in the begining to threadpool
structure do i have to still allocate memory for any element which is
itself in this case. ? If so, how do i do that ?

I am not sure what you mean by "which is itself", but in general,
allocating alone leaves the value uninitialized. If the value happens
to be a struct, then its members are unitialized. Accessing an
uninitialized member may just yield garbage if it happens to be a
number, but if it is a pointer, the consequences may be more dire,
as you have experienced.

Or is there something else which I am missing here ?

Probably initializing your workp. Since I do not know where it came
from, I cannot be sure.

Peter
Oct 19 '07 #5
"Peter Pichler" <us****@pichler.co.ukschrieb im Newsbeitrag
news:47**********@mk-nntp-2.news.uk.tiscali.com...
Mahendra Kumar Kutare wrote:
>I have a further doubt about as following -

Where I live, a "doubt" is an expressed disbelief: "I doubt I can drink
all night as I used to when I was young." "I have doubts about usefulness
of gets()."

Did you mean "question" or "uncertainty"?
This seems to be a common expression in India and should mean what you
guessed it might.

Bye, Jojo
Oct 20 '07 #6

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