编写一个C语言程序，实现一个简单的银行账户管理系统。用户可以选择存款，取款，查询余额，退出操作。要求使用选择结构来实现操作选择，并进行合法性检查（如取款不能超过余额等）

在C语言中，编写一个简单的银行账户管理系统可以利用函数和控制结构（如`switch-case`）。下面是一个基本的示例：

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

typedef struct {
    float balance;
} Account;

void deposit(Account *account, float amount) {
    if (amount > 0) {
        account->balance += amount;
        printf("Deposit successful. New balance is %.2f.\n", account->balance);
    } else {
        printf("Invalid deposit amount. Please enter a positive value.\n");
    }
}

void withdraw(Account *account, float amount) {
    if (amount > 0 && amount <= account->balance) {
        account->balance -= amount;
        printf("Withdrawal successful. New balance is %.2f.\n", account->balance);
    } else {
        printf("Invalid withdrawal request. Amount exceeds balance or not a positive number.\n");
    }
}

void check_balance(Account *account) {
    printf("Current balance: %.2f\n", account->balance);
}

bool exit_system() {
    char choice;
    printf("Do you want to exit? (y/n): ");
    scanf("%c", &choice);
    return choice == 'y';
}

int main() {
    Account account = {0};
    char user_choice;
    
    while (!exit_system()) {
        printf("\nBank Account Management System:\n");
        printf("1. Deposit\n");
        printf("2. Withdraw\n");
        printf("3. Check Balance\n");
        printf("4. Exit\n");
        scanf(" %d", &user_choice);

        switch (user_choice) {
            case 1:
                float deposit_amount;
                printf("Enter deposit amount: ");
                scanf("%f", &deposit_amount);
                deposit(&account, deposit_amount);
                break;
            case 2:
                float withdraw_amount;
                printf("Enter withdrawal amount: ");
                scanf("%f", &withdraw_amount);
                withdraw(&account, withdraw_amount);
                break;
            case 3:
                check_balance(&account);
                break;
            case 4:
                printf("Exiting the system...\n");
                break;
            default:
                printf("Invalid choice. Please try again.\n");
        }
    }

    return 0;
}

在这个程序中，我们首先定义了一个`Account`结构体表示银行账户，然后分别实现了存款、取款、查看余额和退出的操作。主循环通过`switch-case`结构处理用户的输入并相应地调用各个功能。