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NAME
Cpool - LCG Pool inferface
SYNOPSIS
#include <Cpool_api.h>
int Cpool_create(int nbwanted, int * nbget);
int Cpool_assign(int poolid, void *(*startroutine)(void *), void *arg, int timeout);
int Cpool_next_index(int poolid);
int Cpool_next_index_timeout(int poolid, int timeout);
ERRORS
See Cthread corresponding section.
DESCRIPTION
(Please read the NOTE section)
Cpool is a layer built upon Cthread, the LCG Thread interface. It allows the user to
create dedicated pools, and then to assign to one of them a given routine to execute.
The created processes or threads will remain alive, unless the routines assigned to are
crashing, or explicitly calling an exit statement, like exit() or pthread_exit().
Typical use might be writing a server, with a bunch of pre-created processes or pools
(depending on the environment with which Cthread has been compiled), and assign to a given
pool a routine with the socket file descriptor as argument address.
In principle Cpool should be compiled with the same relevant flags with which Cthread has
been.
int Cpool_create(int nbwanted, int * nbget);
This method is creating a pool of nbwanted processes or threads. If the second argument,
nbget , is not NULL, its location will contain the number of effectively created threads
or processes.
Return value is the pool ID, a number greater or equal to zero, or -1 in case of error.
int Cpool_assign(int poolid, void *(*startroutine)(void *), void *arg, int timeout);
This method is assigning a routine to poolid as returned by Cpool_create, whose address is
startroutine , that have the same prototype as every typical routine in multithread
programming. This means that it returns a pointer, and it gets as entry a pointer
identified by the arg parameter. The last argument is a possible timeout , in seconds,
which will apply if it is greater than zero. If it is lower than zero, the assignment will
wait forever until a thread is available. If it is equal to zero, the method will return
immediately if no thread is available.
Return value is 0 if success, or -1 in case of error.
int Cpool_next_index(int poolid);
int Cpool_next_index_timeout(int poolid, int timeout);
Those methods returns that next available thread number that will be assigned if you ever
call Cpool_assign immediately after. If you specify a timeout lower or equal than zero,
then this is a blocking method until one thread is available at least. Those methods, so,
returns a number greater or equal than zero, and -1 if there is an error.
NOTE
Arguments passing in a non-thread environment
Since a forked process can only address its namespace data segment, the address of
the arguments, if any, valid in its parent, will not be directly accessible for the
child we are talking about.
This means that Cpool, in a non-thread environment, have to trace-back all the
memory allocation visible for the parent. Then, Cpool is not passing the address of
the arguments, but its content to the child through a child-parent communication,
monitored with a simple protocol.
There are four cases:
1.The address is NULL: nothing will be transmitted to the child
2.The address is exactly a pointer returned by malloc() or realloc():
the full malloced area will be tranmitted.
3.The address is somewhere in a memory allocate block: the remaining
memory block, e.g. starting from the address up to its end, will be
transmitted.
4.the address do not point to any memory allocated area: Cpool will
assume it is a pointer-like argument, probably to some static variables,
visible for all processes, and will transmit the content of the memory
pointed by the address, assuming it is coded on 64-bits.
In any case, the user is passing a pointer, and the routine will see a pointer,
pointing to a (hopefully, see the point 4., listed upper) same-content area.
Arguments design to work on both thread and non-thread environments
The thread and non-thread arguments can have conceptually a different design when
dealing with arguments;
In a thread environment, the routined passed to Cpool_assign, is sharing memory, so
is allowed to free() the argument, because memory is then shared. On the contrary,
in a non-thread environment, this may be a segmentation fault.
This means that it is recommended to use static variables, containing simple value,
like an integer (for example: a socket file descriptor), and not allocated memory.
If, neverthless, you persist to use free() in your routine, you can use the
following trick:
/* ------------------------ */
/* In the Caller Routine */
/* ------------------------ */
arg = malloc(...);
if (! Cpool_assign(...)) {
if (Cthread_environment() != CTHREAD_TRUE_THREAD) {
/* Non-Thread environment */
free(arg);
} else {
/* Thread environment */
/* ... do nothing */
}
} else {
/* In cany case it is OK */
free(arg);
}
/* ------------------------ */
/* In the Execution Routine */
/* ------------------------ */
void *routine(void *arg) {
./..
if (Cthread_environment() == CTHREAD_TRUE_THREAD) {
/* Thread environment */
free(arg);
} else {
/* Non-Thread environment */
/* ... do nothing */
}
./..
}
EXAMPLE
#include <Cpool_api.h>
#include <stdio.h>
#include <errno.h>
#define NPOOL 2
#define PROCS_PER_POOL 2
#define TIMEOUT 2
void *testit(void *);
int main() {
int pid;
int i, j;
int ipool[NPOOL];
int npool[NPOOL];
int *arg;
pid = getpid();
printf("... Defining %d pools with %d elements each\n",
NPOOL,PROCS_PER_POOL);
for (i=0; i < NPOOL; i++) {
if ((ipool[i] = Cpool_create(PROCS_PER_POOL,&(npool[i]))) < 0) {
printf("### Error No %d creating pool (%s)\n",
errno,strerror(errno));
} else {
printf("... Pool No %d created with %d processes\n",
ipool[i],npool[i]);
}
}
for (i=0; i < NPOOL; i++) {
/* Loop on the number of processes + 1 ... */
for (j=0; j <= npool[i]; j++) {
if ((arg = malloc(sizeof(int))) == NULL) {
printf("### Malloc error, errno = %d (%s)\n",
errno,strerror(errno));
continue;
}
*arg = i*10+j;
printf("... Assign to pool %d (timeout=%d) the %d-th routine 0x%x(%d)\n",
ipool[i],TIMEOUT,j+1,(unsigned int) testit,*arg);
if (Cpool_assign(ipool[i], testit, arg, TIMEOUT)) {
printf("### Can't assign to pool No %d (errno=%d [%s]) the %d-th routine\n",
ipool[i],errno,strerror(errno),j);
free(arg);
} else {
printf("... Okay for assign to pool No %d of the %d-th routine\n",
ipool[i],j);
If (Cthread_environment() != CTHREAD_TRUE_THREAD) {
/* Non-thread environment: the child is in principle not allowed */
/* to do free himself */
free(arg);
}
}
}
}
/* We wait enough time for our threads to terminate... */
sleep(TIMEOUT*NPOOL*PROCS_PER_POOL);
exit(EXIT_SUCCESS);
}
void *testit(void *arg) {
int caller_pid, my_pid;
my_pid = getpid();
caller_pid = (int) * (int *) arg;
if (Cthread_environment() == CTHREAD_TRUE_THREAD) {
/* Thread environment : we free the memory */
free(arg);
}
printf("... I am PID=%d called by pool %d, try No %d\n",
my_pid,caller_pid/10,caller_pid - 10*(caller_pid/10));
/*
* Wait up to the timeout + 1
*/
sleep(TIMEOUT*2);
return(NULL);
}
SEE ALSO
Cthread
AUTHOR
LCG Grid Deployment Team