我写了一个包含两个进程的程序。第一个 一个包含两组semaphores，并创建子进程，读取共享内存段的所有数据并打印出来。
在 第二 一，子进程使用计算函数计算数据，当所有数据计算完毕后返回0。它通过共享内存段将这些数据传送给父进程。
要写入数据。
在第一旗语上，子进程做P，父进程做V。
在第2个信标上，子代做V，父代做P。
但是由于我是新手，还在硬着头皮去理解，似乎我做错了什么，因为它没有按照必须的方式工作。
下面是我的代码。
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/shm.h>
#include <errno.h>
#include <sys/wait.h>
int sum =0;
int compute(int data){
sum += data;
return sum;
}
int main(){
int i;
int shm_id;
int data;
pid_t pid;
key_t shm_key;
sem_t *sem;
// unsigned int sem_value =2;
shm_key = ftok("/dev/null", 65);
shm_id = shmget(shm_id, sizeof(int), 0644 | IPC_CREAT);
if (shm_id < 0){
perror("shmgget");
exit(EXIT_FAILURE);
}
// data = shmat(shm_id, NULL, 0);
sem = sem_open("semaphore", O_CREAT | O_EXCL, 0644, 2);
for (i = 0; i < 2; i++){
pid = fork();
if (pid < 0)
{
perror("fork");
sem_unlink("semaphore");
sem_close(sem);
exit(EXIT_FAILURE);
}
else if (pid == 0)
{
break;
}
}
if (pid == 0)
{
puts("Enter the data:");
scanf("%d", &data);
//child process
sem_wait(sem);
printf("Child - %d is in critical section\n", i);
sleep(1);
puts("Enter the data:");
scanf("%d", &data);
// *shrd_value += data;
printf("Child - %d: new value of data = %d\n", i, data);
printf("Child - %d: sum of whole data by far = %d\n", i, compute(data));
sem_post(sem);
exit(EXIT_SUCCESS);
}
else if (pid > 0)
{
//parent process
while (pid = waitpid(-1, NULL, 0))
{
if (errno == ECHILD)
{
break;
}
}
puts("All children exited");
shmdt(&data);
shmctl(shm_id, IPC_RMID, 0);
sem_unlink("semaphore");
sem_close(sem);
exit(0);
}
}
输出：
Enter the data:
Enter the data:
2
Child - 0 is in critical section
1Enter the data:
Child - 1 is in critical section
Enter the data:
3
Child - 0: new value of data = 3
Child - 0: sum of whole data by far = 3
2
Child - 1: new value of data = 2
Child - 1: sum of whole data by far = 2
All children exited
我还修改了它们向共享内存写入的方式：它们直接在shmat调用给出的地址上写入，而你的代码中缺少这个地址.我修正了一些bug，并简化了代码（删除了数组--特别是在进入critial部分前后添加了详细的日志）。
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/shm.h>
#include <errno.h>
#include <sys/wait.h>
int main(){
int i;
int shm_id;
pid_t pid;
int *addr;
int data;
pid_t current_pid;
key_t shm_key;
sem_t *sem;
shm_key = ftok("/dev/null", 65);
shm_id = shmget(shm_key, sizeof(int), 0644 | IPC_CREAT);
if (shm_id < 0){
perror("shmget");
exit(EXIT_FAILURE);
}
sem_unlink("semaphore");
sem = sem_open("semaphore", O_CREAT, 0644, 1);
if (sem == SEM_FAILED) {
perror("sem_open");
exit(EXIT_FAILURE);
}
addr = (int *) shmat(shm_id, (void *) 0, 0);
if (addr == (void *) -1) {
perror("shmat");
exit(EXIT_FAILURE);
}
*addr = 0;
for (i = 0; i < 2; i++){
pid = fork();
if (pid < 0)
{
perror("fork");
sem_close(sem);
sem_unlink("semaphore");
exit(EXIT_FAILURE);
}
}
if (pid == 0)
{
current_pid = getpid();
printf("Child %d: waiting for critical section \n", current_pid);
sem_wait(sem);
printf("Child %d: enters in critical section \n", current_pid);
printf("child %d: Enter the data:\n", current_pid);
scanf("%d", &data);
printf("Child %d: new value of data = %d\n", current_pid, data);
printf("Child %d: sum of whole data so far = %d\n", current_pid, *addr += data);
sem_post(sem);
printf("Child %d exits from critical section\n", current_pid);
exit(EXIT_SUCCESS);
}
else if (pid > 0)
{
//parent process
while (pid = waitpid(-1, NULL, 0))
{
if (errno == ECHILD)
{
break;
}
}
puts("All children exited");
shmdt(addr);
shmctl(shm_id, IPC_RMID, 0);
sem_close(sem);
sem_unlink("semaphore");
exit(0);
}
exit(0);
}
请注意，semaphore初始值必须为1，才能对2个进程进行真正的critical部分。
我还删除了睡眠调用，我们可以看到其中一个进程在等待。
Child 22514: waiting for critical section
Child 22514: enters in critical section
child 22514: Enter the data:
Child 22515: waiting for critical section
333
Child 22514: new value of data = 333
Child 22514: sum of whole data so far = 333
Child 22514 exits from critical section
Child 22515: enters in critical section
child 22515: Enter the data:
666
Child 22515: new value of data = 666
Child 22515: sum of whole data so far = 999
Child 22515 exits from critical section
All children exited
All children exited
以下是有生产者和消费者进程的代码：
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h> // O_CREAT, O_EXEC
#include <errno.h> // errno, ECHILD
#include <unistd.h>
#include <sys/shm.h> // shmat(), IPC_RMID
#include <sys/wait.h>
#include <semaphore.h> // sem_open(), sem_destroy(), sem_wait()...
#include <sys/types.h> // key_t, sem_t, pid_t
#include <pthread.h>
#define BUFF 10
typedef struct data{
int buff[BUFF];
int size;
int index;
}DATA;
int main(int argc, char const *argv[])
{
sem_t *full, *empty, *access;
key_t shm_key;
int shm_id;
full = sem_open ("fullname", O_CREAT , 0644, 15);
empty = sem_open ("empty", O_CREAT , 0644, 0);
access = sem_open ("access", O_CREAT , 0644, 1);
if (argc!=2)
{
exit(1);
}
int value=atoi(argv[1]);
//initialize a shared variable in shared memory
shm_key = ftok("/dev/null", 60);
shm_id = shmget(shm_key, sizeof(DATA), 0);
// shared memory error check
if (shm_id < 0){
shm_id = shmget(shm_key, sizeof(DATA), 0644 | IPC_CREAT);
DATA *data = (DATA*) shmat (shm_id, NULL, 0);
data->size=0;
data->index=0; //index
}
printf("Shared memory id: %d\n",shm_id );
printf("Checking buffer...,\n");
//If in the buffer have free space then will wait for consumer to consume the data\n"
sem_wait(empty);
printf("\nLocking buffer to produce data\n");
sem_wait(access);
//initialize a shared variable in shared memory
shm_key = ftok("/dev/null", 60);
shm_id = shmget(shm_id, sizeof(DATA), 0644 | IPC_CREAT);
// shared memory error check
if (shm_id < 0){
perror("semaphore");
exit(EXIT_FAILURE);
}
//Shared variable
DATA *data = (DATA*) shmat (shm_id, NULL, 0);
int index=(data->size + data->index) % 15;
data->buff[index]=value;
data->size++;
printf("%d is located in %d on the buffer\n",value,index );
//consusming
// attach data to shared memory
index=data->index;
value=data->buff[index];
printf("%d now consumed\n",value );
data->size--;
data->index++;
sem_post(access);
sem_post(full);
return 0;
}