#include <pthread.h>
int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t *attr,
int *pshared);
The pthread_rwlockattr_getpshared() function retrieves the current setting of the process shared attribute from the read/write lock attributes object. The process shared attribute indicates whether the read/write lock that is created using the read/write lock attributes object can be shared between threads in separate processes (PTHREAD_PROCESS_SHARED) or shared only between threads within the same process (PTHREAD_PROCESS_PRIVATE).
Even if the read/write lock in storage is accessible from two separate processes, it cannot be used from both processes unless the process shared attribute is PTHREAD_PROCESS_SHARED.
The default pshared attribute for read/write lock attributes objects is PTHREAD_PROCESS_PRIVATE.
None.
If pthread_rwlockattr_getpshared() was not successful, the error condition returned usually indicates one of the following errors. Under some conditions, the value returned could indicate an error other than those listed here.
The value specified for the argument is not correct.
Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.
#define _MULTI_THREADED
#include <pthread.h>
#include <stdio.h>
#include <spawn.h>
#include <sys/wait.h>
#include <unistd.h>
#include <sys/shm.h>
#include "check.h"
typedef struct {
int protectedResource;
pthread_rwlock_t rwlock;
} shared_data_t;
extern char **environ;
shared_data_t *sharedMem=NULL;
pid_t childPid=0;
int childStatus=-99;
int shmid=0;
/* Change this path to be the path to where you create this example program */
#define MYPATH "/QSYS.LIB/QP0WTEST.LIB/TPRWLSH0.PGM"
#define NTHREADSTHISJOB 2
#define NTHREADSTOTAL 4
void parentSetup(void);
void childSetup(void);
void parentCleanup(void);
void childCleanup(void);
void *childReaderThreadFunc(void *parm)
{
int rc;
int retries = 5;
while (retries--) {
rc = pthread_rwlock_rdlock(&sharedMem->rwlock);
checkResults("pthread_rwlock_rdlock()\n", rc);
/* Under protection of the shared read lock, read the resource */
printf("CHILD READER - current protectedResource = %d\n",
sharedMem->protectedResource);
sleep(1);
printf("CHILD READER - unlock\n");
rc = pthread_rwlock_unlock(&sharedMem->rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
}
return NULL;
}
void *parentWriterThreadFunc(void *parm)
{
int rc;
rc = pthread_rwlock_wrlock(&sharedMem->rwlock);
checkResults("pthread_rwlock_rdlock()\n", rc);
/* Under protection of the exclusive write lock, write the resource */
++sharedMem->protectedResource;
printf("PARENT WRITER - current protectedResource = %d\n",
sharedMem->protectedResource);
sleep(5);
printf("PARENT WRITER - unlock\n");
rc = pthread_rwlock_unlock(&sharedMem->rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
return NULL;
}
int main(int argc, char **argv)
{
int rc=0;
int i;
pthread_t threadid[NTHREADSTHISJOB];
int parentJob=0;
void *status=NULL;
/* If we run this from the QSHELL interpreter on the system, we want */
/* it to be line buffered even if we run it in batch so the output between*/
/* parent and child is intermixed. */
setvbuf(stdout,NULL,_IOLBF,4096);
/* Determine if we are running in the parent or child */
if (argc != 1 && argc != 2) {
printf("Incorrect usage\n");
printf("Pass no parameters to run as the parent testcase\n");
printf("Pass one parameter `ASCHILD' to run as the child testcase\n");
exit(1);
}
if (argc == 1) {
parentJob = 1;
}
else {
if (strcmp(argv[1], "ASCHILD")) {
printf("Incorrect usage\n");
printf("Pass no parameters to run as the parent testcase\n");
printf("Pass one parameter `ASCHILD' to run as the child testcase\n");
exit(1);
}
parentJob = 0;
}
/* PARENT *****************************************************************/
if (parentJob) {
printf("PARENT - Enter Testcase - %s\n", argv[0]);
parentSetup();
printf("PARENT - Create %d threads\n", NTHREADSTHISJOB);
for (i=0; i<NTHREADSTHISJOB; ++i) {
rc = pthread_create(&threadid[i], NULL, parentWriterThreadFunc, NULL);
checkResults("pthread_create()\n", rc);
}
for (i=0; i<NTHREADSTHISJOB; ++i) {
rc = pthread_join(threadid[i], NULL);
checkResults("pthread_create()\n", rc);
if (status != NULL) {
printf("PARENT - Got a bad status from a thread, "
"%.8x %.8x %.8x %.8x\n", status);
exit(1);
}
}
parentCleanup();
printf("PARENT - Main completed\n");
exit(0);
}
/* CHILD *****************************************************************/
{
printf("CHILD - Enter Testcase - %s\n", argv[0]);
childSetup();
printf("CHILD - Create %d threads\n", NTHREADSTHISJOB);
for (i=0; i<NTHREADSTHISJOB; ++i) {
rc = pthread_create(&threadid[i], NULL, childReaderThreadFunc, NULL);
checkResults("pthread_create()\n", rc);
}
/* The parent will wake up all of these threads using the */
/* pshared condition variable. We will just join to them... */
printf("CHILD - Joining to all threads\n");
for (i=0; i<NTHREADSTHISJOB; ++i) {
rc = pthread_join(threadid[i], &status);
checkResults("pthread_join()\n", rc);
if (status != NULL) {
printf("CHILD - Got a bad status from a thread, "
"%.8x %.8x %.8x %.8x\n", status);
exit(1);
}
}
/* After all the threads are awake, the parent will destroy */
/* the read/write lock. Do not use it anymore */
childCleanup();
printf("CHILD - Main completed\n");
}
return 0;
}
/***************************************************************/
/* This function initializes the shared memory for the job, */
/* sets up the environment variable indicating where the shared*/
/* memory is, and spawns the child job. */
/* */
/* It creates and initializes the shared memory segment, and */
/* It initializes the following global variables in this */
/* job. */
/* sharedMem */
/* childPid */
/* shmid */
/* */
/* If any of this setup/initialization fails, it will exit(1) */
/* and terminate the test. */
/* */
/***************************************************************/
void parentSetup(void)
{
int rc;
/***************************************************************/
/* Create shared memory for shared_data_t above */
/* attach the shared memory */
/* set the static/global sharedMem pointer to it */
/***************************************************************/
printf("PARENT - Create the shared memory segment\n");
rc = shmget(IPC_PRIVATE, sizeof(shared_data_t), 0666);
if (rc == -1) {
printf("PARENT - Failed to create a shared memory segment\n");
exit(1);
}
shmid = rc;
printf("PARENT - Attach the shared memory\n");
sharedMem = shmat(shmid, NULL, 0);
if (sharedMem == NULL) {
shmctl(shmid, IPC_RMID, NULL);
printf("PARENT - Failed to attach shared memory\n");
exit(1);
}
/***************************************************************/
/* Initialize the read/write lock and other shared memory data */
/***************************************************************/
{
pthread_rwlockattr_t rwlattr;
printf("PARENT - Init shared memory and read/write lock\n");
memset(sharedMem, 0, sizeof(shared_data_t));
/* Process Shared Read/Write lock */
rc = pthread_rwlockattr_init(&rwlattr);
checkResults("pthread_rwlockattr_init()\n", rc);
rc = pthread_rwlockattr_setpshared(&rwlattr, PTHREAD_PROCESS_SHARED);
checkResults("pthread_rwlockattr_setpshared()\n", rc);
rc = pthread_rwlock_init(&sharedMem->rwlock, &rwlattr);
checkResults("pthread_rwlock_init()\n", rc);
}
/**************************************************************/
/* Set and environment variable so that the child can inherit */
/* it and know the shared memory ID */
/**************************************************************/
{
char shmIdEnvVar[128];
sprintf(shmIdEnvVar, "TPRWLSH0_SHMID=%d\n", shmid);
rc = putenv(shmIdEnvVar);
if (rc) {
printf("PARENT - Failed to store env var %s, errno=%d\n",
shmIdEnvVar, errno);
exit(1);
}
printf("PARENT - Stored shared memory id of %d\n", shmid);
}
/**************************************************/
/* Spawn the child job */
/**************************************************/
{
inheritance_t in;
char *av[3] = {NULL, NULL, NULL};
/* Allow thread creation in the spawned child */
memset(&in, 0, sizeof(in));
in.flags = SPAWN_SETTHREAD_NP;
/* Set up the arguments to pass to spawn based on the */
/* arguments passed in */
av[0] = MYPATH;
av[1] = "ASCHILD";
av[2] = NULL;
/* Spawn the child that was specified, inheriting all */
/* of the environment variables. */
childPid = spawn(MYPATH, 0, NULL, &in, av, environ);
if (childPid == -1) {
/* spawn failure */
printf("PARENT - spawn() failed, errno=%d\n", errno);
exit(1);
}
printf("PARENT - spawn() success, [PID=%d]\n", childPid);
}
return;
}
/***************************************************************/
/* This function attaches the shared memory for the child job, */
/* It uses the environment variable indicating where the shared*/
/* memory is. */
/* */
/* If any of this setup/initialization fails, it will exit(1) */
/* and terminate the test. */
/* */
/* It initializes the following global variables: */
/* sharedMem */
/* shmid */
/***************************************************************/
void childSetup(void)
{
int rc;
printf("CHILD - Child setup\n");
/**************************************************************/
/* Set and environment variable so that the child can inherit */
/* it and know the shared memory ID */
/**************************************************************/
{
char *shmIdEnvVar;
shmIdEnvVar = getenv("TPRWLSH0_SHMID");
if (shmIdEnvVar == NULL) {
printf("CHILD - Failed to get env var \"TPRWLSH0_SHMID\", errno=%d\n",
errno);
exit(1);
}
shmid = atoi(shmIdEnvVar);
printf("CHILD - Got shared memory id of %d\n", shmid);
}
/***************************************************************/
/* Create shared memory for shared_data_t above */
/* attach the shared memory */
/* set the static/global sharedMem pointer to it */
/***************************************************************/
printf("CHILD - Attach the shared memory\n");
sharedMem = shmat(shmid, NULL, 0);
if (sharedMem == NULL) {
shmctl(shmid, IPC_RMID, NULL);
printf("CHILD - Failed to attach shared memory\n");
exit(1);
}
return;
}
/***************************************************************/
/* wait for child to complete and get return code. */
/* Destroy read/write lock in shared memory */
/* detach and remove shared memory */
/* set the child's return code in global storage */
/* */
/* If this function fails, it will call exit(1) */
/* */
/* This function sets the following global variables: */
/* sharedMem */
/* childStatus */
/* shmid */
/***************************************************************/
void parentCleanup(void)
{
int status=0;
int rc;
int waitedPid=0;
/* Even though there is no thread left in the child using the */
/* contents of the shared memory, before we destroy the */
/* read/write lock in that shared memory, we will wait for the */
/* child job to complete, we know for 100% certainty that no */
/* threads in the child are using it then, because the child */
/* is terminated. */
printf("PARENT - Parent cleanup\n");
/* Wait for the child to complete */
waitedPid = waitpid(childPid,&status,0);
if (rc == -1) {
printf("PARENT - waitpid failed, errno=%d\n", errno);
exit(1);
}
childStatus = status;
/* Cleanup resources */
rc = pthread_rwlock_destroy(&sharedMem->rwlock);
checkResults("pthread_rwlock_destroy()\n", rc);
/* Detach/Remove shared memory */
rc = shmdt(sharedMem);
if (rc) {
printf("PARENT - Failed to detach shared memory, errno=%d\n", errno);
exit(1);
}
rc = shmctl(shmid, IPC_RMID, NULL);
if (rc) {
printf("PARENT - Failed to remove shared memory id=%d, errno=%d\n",
shmid, errno);
exit(1);
}
shmid = 0;
return;
}
/***************************************************************/
/* Detach the shared memory */
/* At this point, there is no serialization, so the contents */
/* of the shared memory should not be used. */
/* */
/* If this function fails, it will call exit(1) */
/* */
/* This function sets the following global variables: */
/* sharedMem */
/***************************************************************/
void childCleanup(void)
{
int rc;
printf("CHILD - Child cleanup\n");
rc = shmdt(sharedMem);
sharedMem = NULL;
if (rc) {
printf("CHILD - Failed to detach shared memory, errno=%d\n", errno);
exit(1);
}
return;
}
This example was run under the IBM® i QShell Interpreter. In the QShell Interpreter, a program gets descriptors 0, 1, 2 as the standard files, the parent and child I/O is directed to the console. When run in the QShell Interpreter, the output shows the intermixed output from both parent and child processes, and gives a feeling for the time sequence of operations occurring in each job.
The QShell interpreter allows you to run multithreaded programs as if they were interactive. See the QShell documentation for a description of the QIBM_MULTI_THREADED shell variable which allows you to start multithreaded programs.
The QShell Interpreter is option 30 of Base IBM i.
PARENT - Enter Testcase - QP0WTEST/TPRWLSH0 PARENT - Create the shared memory segment PARENT - Attach the shared memory PARENT - Init shared memory and read/write lock PARENT - Stored shared memory id of 7 PARENT - spawn() success, [PID=584] PARENT - Create 2 threads PARENT WRITER - current protectedResource = 1 CHILD - Enter Testcase - QP0WTEST/TPRWLSH0 CHILD - Child setup CHILD - Got shared memory id of 7 CHILD - Attach the shared memory CHILD - Create 2 threads CHILD - Joining to all threads PARENT WRITER - unlock PARENT WRITER - current protectedResource = 2 PARENT WRITER - unlock CHILD READER - current protectedResource = 2 CHILD READER - current protectedResource = 2 PARENT - Parent cleanup CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - current protectedResource = 2 CHILD READER - unlock CHILD READER - unlock CHILD - Child cleanup CHILD - Main completed PARENT - Main completed
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