上个学期我参加了 C 语言的操作系统实习，其中第一个项目涉及制作一个线程包，然后编写一个多生产者-消费者程序来演示该功能。然而，在获得评分反馈后，我因“信号量的使用存在微妙的错误”和“程序假定抢占（例如使用 yield 更改控制）”而失分（我们从非抢占线程包开始，后来添加了抢占。请注意，评论和示例相互矛盾。我相信它也不假设，并且在两种环境中都可以使用）。
这一直困扰着我很长一段时间 - 课程人员有点不知所措，所以我不能问他们这学期有什么问题。我花了很长时间思考这个问题，但我看不到问题所在。如果有人可以查看并指出错误，或者向我保证实际上没有问题，我将非常感激。
我相信就线程包函数（微型线程和信号量）而言，语法应该是相当标准的，但如果有什么令人困惑的地方，请告诉我。
#include <stdio.h>
#include <stdlib.h>
#include "minithread.h"
#include "synch.h"
#define BUFFER_SIZE 16
#define MAXCOUNT 100
int buffer[BUFFER_SIZE];
int size, head, tail;
int count = 1;
int out = 0;
int toadd = 0;
int toremove = 0;
semaphore_t empty;
semaphore_t full;
semaphore_t count_lock; // Semaphore to keep a lock on the
// global variables for maintaining the counts
/* Method to handle the working of a student
* The ID of a student is the corresponding minithread_id */
int student(int total_burgers) {
int n, i;
semaphore_P(count_lock);
while ((out+toremove) < arg) {
n = genintrand(BUFFER_SIZE);
n = (n <= total_burgers - (out + toremove)) ? n : total_burgers - (out + toremove);
printf("Student %d wants to get %d burgers ...\n", minithread_id(), n);
toremove += n;
semaphore_V(count_lock);
for (i=0; i<n; i++) {
semaphore_P(empty);
out = buffer[tail];
printf("Student %d is taking burger %d.\n", minithread_id(), out);
tail = (tail + 1) % BUFFER_SIZE;
size--;
toremove--;
semaphore_V(full);
}
semaphore_P(count_lock);
}
semaphore_V(count_lock);
printf("Student %d is done.\n", minithread_id());
return 0;
}
/* Method to handle the working of a cook
* The ID of a cook is the corresponding minithread_id */
int cook(int total_burgers) {
int n, i;
printf("Creating Cook %d\n",minithread_id());
semaphore_P(count_lock);
while ((count+toadd) <= arg) {
n = genintrand(BUFFER_SIZE);
n = (n <= total_burgers - (count + toadd) + 1) ? n : total_burgers - (count + toadd) + 1;
printf("Cook %d wants to put %d burgers into the burger stack ...\n", minithread_id(),n);
toadd += n;
semaphore_V(count_lock);
for (i=0; i<n; i++) {
semaphore_P(full);
printf("Cook %d is putting burger %d into the burger stack.\n", minithread_id(), count);
buffer[head] = count++;
head = (head + 1) % BUFFER_SIZE;
size++;
toadd--;
semaphore_V(empty);
}
semaphore_P(count_lock);
}
semaphore_V(count_lock);
printf("Cook %d is done.\n", minithread_id());
return 0;
}
/* Method to create our multiple producers and consumers
* and start their respective threads by fork */
void starter(int* c){
int i;
for (i=0;i<c[2];i++){
minithread_fork(cook, c[0]);
}
for (i=0;i<c[1];i++){
minithread_fork(student, c[0]);
}
}
/* The arguments are passed as command line parameters
* argv[1] is the no of students
* argv[2] is the no of cooks */
void main(int argc, char *argv[]) {
int pass_args[3];
pass_args[0] = MAXCOUNT;
pass_args[1] = atoi(argv[1]);
pass_args[2] = atoi(argv[2]);
size = head = tail = 0;
empty = semaphore_create();
semaphore_initialize(empty, 0);
full = semaphore_create();
semaphore_initialize(full, BUFFER_SIZE);
count_lock = semaphore_create();
semaphore_initialize(count_lock, 1);
minithread_system_initialize(starter, pass_args);
}