c 死锁避免银行家算法

银行家算法是一种死锁避免算法，它通过动态地分配系统资源来避免死锁的发生。该算法基于一个简单的假设：每个进程在开始执行之前，必须向系统声明它最多需要多少资源，以及在执行过程中最多需要多少资源。系统在分配资源时，必须遵循以下原则：

1. 如果请求的资源数量大于系统当前可用的资源数量，则该请求必须等待，直到有足够的资源可用为止。
2. 如果请求的资源数量小于等于系统当前可用的资源数量，但是分配给该请求后，系统将处于不安全状态，则该请求也必须等待，直到有足够的资源可用为止。
3. 如果请求的资源数量小于等于系统当前可用的资源数量，并且分配给该请求后，系统仍然处于安全状态，则可以将资源分配给该请求。

下面是一个简单的C语言实现银行家算法的例子：

#include <stdio.h>
#include <stdbool.h>

#define MAX_PROCESSES 10
#define MAX_RESOURCES 10

int available[MAX_RESOURCES];
int maximum[MAX_PROCESSES][MAX_RESOURCES];
int allocation[MAX_PROCESSES][MAX_RESOURCES];
int need[MAX_PROCESSES][MAX_RESOURCES];bool finish[MAX_PROCESSES];

bool is_safe(int num_processes, int num_resources) {
    int work[num_resources];
    for (int i = 0; i < num_resources; i++) {
        work[i] = available[i];
    }
    bool finish[num_processes];
    for (int i = 0; i < num_processes; i++) {
        finish[i] = false;
    }
    int count = 0;
    while (count < num_processes) {
        bool found = false;
        for (int i = 0; i < num_processes; i++) {
            if (!finish[i]) {
                bool enough_resources = true;
                for (int j = 0; j < num_resources; j++) {
                    if (need[i][j] > work[j]) {
                        enough_resources = false;
                        break;
                    }
                }
                if (enough_resources) {
                    for (int j = 0; j < num_resources; j++) {
                        work[j] += allocation[i][j];
                    }
                    finish[i] = true;
                    found = true;
                    count++;
                }
            }
        }
        if (!found) {
            return false;
        }
    }
    return true;
}

bool request_resources(int pid, int num_resources, int request[]) {
    for (int i = 0; i < num_resources; i++) {
        if (request[i] > need[pid][i] || request[i] > available[i]) {
            return false;
        }
    }
    for (int i = 0; i < num_resources; i++) {
        available[i] -= request[i];
        allocation[pid][i] += request[i];
        need[pid][i] -= request[i];
    }
    if (is_safe(MAX_PROCESSES, MAX_RESOURCES)) {
        return true;
    } else {
        for (int i = 0; i < num_resources; i++) {
            available[i] += request[i];
            allocation[pid][i] -= request[i];
            need[pid][i] += request[i];
        }
        return false;
    }
}

int release_resources(int pid, int num_resources, int release[]) {
    for (int i = 0; i < num_resources; i++) {
        if (release[i] > allocation[pid][i]) {
            return -1;
        }
    }
    for (int i = 0; i < num_resources; i++) {
        available[i] += release[i];
        allocation[pid][i] -= release[i];
        need[pid][i] += release[i];
    }
    return 0;
}

int main() {
    int num_resources;
    printf("Enter the number of resources: ");
    scanf("%d", &num_resources);
    printf("Enter the available resources: ");
    for (int i = 0; i < num_resources; i++) {
        scanf("%d", &available[i]);
    }
    int num_processes;
    printf("Enter the number of processes: ");
    scanf("%d", &num_processes);
    printf("Enter the maximum resources for each process: ");
    for (int i = 0; i < num_processes; i++) {
        for (int j = 0; j < num_resources; j++) {
            scanf("%d", &maximum[i][j]);
            need[i][j] = maximum[i][j];
        }
    }
    while (true) {
        int pid;
        printf("Enter the process ID (0-%d) or -1 to exit: ", num_processes - 1);
        scanf("%d", &pid);
        if (pid == -1) {
            break;
        }
        if (pid < 0 || pid >= num_processes) {
            printf("Invalid process ID\n");
            continue;
        }
        int request[num_resources];
        printf("Enter the resource request: ");
        for (int i = 0; i < num_resources; i++) {
            scanf("%d", &request[i]);
        }
        if (request_resources(pid, num_resources, request)) {
            printf("Resource request granted\n");
        } else {
            printf("Resource request denied\n");
        }
    }
    return 0;
}