我需要一些帮助来理解如何使用 C 中的条件变量来解决练习。这是一个小例子：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#define OKTOWRITE "/oktowrite"
#define MESSAGE "/message"
#define MUTEX "/lock"
int main(int argc, char** argv)
{
pthread_cond_t* condition;
pthread_mutex_t *mutex;
char* message;
int des_cond, des_msg, des_mutex;
int mode = S_IRWXU | S_IRWXG;
des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR | O_TRUNC, mode);
if (des_mutex < 0)
{
perror("failure on shm_open on des_mutex");
exit(1);
}
if (ftruncate(des_mutex, sizeof(pthread_mutex_t)) == -1)
{
perror("Error on ftruncate to sizeof pthread_cond_t\n");
exit(-1);
}
mutex = (pthread_mutex_t*) mmap(NULL, sizeof(pthread_mutex_t),
PROT_READ | PROT_WRITE, MAP_SHARED, des_mutex, 0);
if (mutex == MAP_FAILED )
{
perror("Error on mmap on mutex\n");
exit(1);
}
pthread_mutex_init(mutex, NULL );
des_cond = shm_open(OKTOWRITE, O_CREAT | O_RDWR | O_TRUNC, mode);
if (des_cond < 0)
{
perror("failure on shm_open on des_cond");
exit(1);
}
if (ftruncate(des_cond, sizeof(pthread_cond_t)) == -1)
{
perror("Error on ftruncate to sizeof pthread_cond_t\n");
exit(-1);
}
condition = (pthread_cond_t*) mmap(NULL, sizeof(pthread_cond_t),
PROT_READ | PROT_WRITE, MAP_SHARED, des_cond, 0);
if (condition == MAP_FAILED )
{
perror("Error on mmap on condition\n");
exit(1);
}
pthread_cond_init(condition, NULL );
if (!fork())
{
sleep(3);
pthread_mutex_lock(mutex);
pthread_cond_signal(condition);
pthread_mutex_unlock(mutex);
printf("son signaled\n");
exit(0);
}
else
{
printf("wait on condition\n");
pthread_mutex_lock(mutex);
pthread_cond_wait(condition, mutex);
pthread_mutex_unlock(mutex);
printf("Signaled by son process, wake up\n");
pthread_mutex_destroy(mutex);
pthread_cond_destroy(condition);
shm_unlink(OKTOWRITE);
shm_unlink(MESSAGE);
shm_unlink(MUTEX);
return 0;
}
}
问题是，即使在儿子发出信号之后，该进程的父亲仍然被锁定。一切都在共享内存中（使用shm_open and mmap）因此两个过程的条件应该相同。
在调用 wait 或 signal 之前锁定互斥锁可能会犯错误吗？
编辑：
感谢所有帮助过我的人。这是正确的代码，其中标记了关键部分：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#define OKTOWRITE "/condwrite"
#define MESSAGE "/msg"
#define MUTEX "/mutex_lock"
int main(int argc, char** argv) {
pthread_cond_t* condition;
pthread_mutex_t* mutex;
char* message;
int des_cond, des_msg, des_mutex;
int mode = S_IRWXU | S_IRWXG;
des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR | O_TRUNC, mode);
if (des_mutex < 0) {
perror("failure on shm_open on des_mutex");
exit(1);
}
if (ftruncate(des_mutex, sizeof(pthread_mutex_t)) == -1) {
perror("Error on ftruncate to sizeof pthread_cond_t\n");
exit(-1);
}
mutex = (pthread_mutex_t*) mmap(NULL, sizeof(pthread_mutex_t),
PROT_READ | PROT_WRITE, MAP_SHARED, des_mutex, 0);
if (mutex == MAP_FAILED ) {
perror("Error on mmap on mutex\n");
exit(1);
}
des_cond = shm_open(OKTOWRITE, O_CREAT | O_RDWR | O_TRUNC, mode);
if (des_cond < 0) {
perror("failure on shm_open on des_cond");
exit(1);
}
if (ftruncate(des_cond, sizeof(pthread_cond_t)) == -1) {
perror("Error on ftruncate to sizeof pthread_cond_t\n");
exit(-1);
}
condition = (pthread_cond_t*) mmap(NULL, sizeof(pthread_cond_t),
PROT_READ | PROT_WRITE, MAP_SHARED, des_cond, 0);
if (condition == MAP_FAILED ) {
perror("Error on mmap on condition\n");
exit(1);
}
/* HERE WE GO */
/**************************************/
/* set mutex shared between processes */
pthread_mutexattr_t mutexAttr;
pthread_mutexattr_setpshared(&mutexAttr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(mutex, &mutexAttr);
/* set condition shared between processes */
pthread_condattr_t condAttr;
pthread_condattr_setpshared(&condAttr, PTHREAD_PROCESS_SHARED);
pthread_cond_init(condition, &condAttr);
/*************************************/
if (!fork()) {
sleep(10);
pthread_mutex_lock(mutex);
pthread_cond_signal(condition);
printf("son signaled\n");
pthread_mutex_unlock(mutex);
exit(0);
}
else {
printf("father waits on condition\n");
pthread_mutex_lock(mutex);
pthread_cond_wait(condition, mutex);
pthread_mutex_unlock(mutex);
printf("Signaled by son process, wake up!!!!!!!!\n");
pthread_condattr_destroy(&condAttr);
pthread_mutexattr_destroy(&mutexAttr);
pthread_mutex_destroy(mutex);
pthread_cond_destroy(condition);
shm_unlink(OKTOWRITE);
shm_unlink(MESSAGE);
shm_unlink(MUTEX);
}
return 0;
}
