用C语言编写程序：通过键盘读取元素建立一个单链表；指定一个元素，在此元素之前插入一个新元素；指定一个元素，删除此元素

在C语言中，我们可以使用`stdio.h`库来进行键盘输入，`malloc`函数进行动态内存分配，以及定义链表节点结构来实现所需的功能。首先，让我们定义一个简单的链表节点结构：

#include <stdio.h>
#include <stdlib.h>

// 定义链表节点结构
typedef struct Node {
    int data;
    struct Node* next;
} Node;

// 创建链表
Node* createList() {
    Node* head = NULL;
    int input;

    printf("请输入元素，按回车结束：");
    while (scanf("%d", &input) == 1) {
        Node* newNode = (Node*)malloc(sizeof(Node));
        newNode->data = input;
        newNode->next = head;
        head = newNode;
    }
    return head;
}

// 在指定元素之前插入新元素
void insertBefore(Node** head, int targetValue, int newValue) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = newValue;

    if (*head == NULL) {
        newNode->next = *head;
        *head = newNode;
        return;
    }

    Node* current = *head;
    while (current->data != targetValue && current->next != NULL) {
        current = current->next;
    }

    if (current->next == NULL) {
        newNode->next = *head;
        *head = newNode;
    } else {
        newNode->next = current->next;
        current->next = newNode;
    }
}

// 删除指定元素
void deleteElement(Node** head, int targetValue) {
    if (*head == NULL) return;

    if ((*head)->data == targetValue) {
        Node* temp = *head;
        *head = (*head)->next;
        free(temp);
        return;
    }

    Node* current = *head;
    Node* previous = NULL;

    while (current != NULL && current->data != targetValue) {
        previous = current;
        current = current->next;
    }

    if (current != NULL) {
        previous->next = current->next;
        free(current);
    }
}

int main() {
    Node* list = createList();
    if (list == NULL) {
        printf("无法创建链表.\n");
        return 1;
    }

    printf("\n当前链表: ");
    Node* temp = list;
    while (temp != NULL) {
        printf("%d -> ", temp->data);
        temp = temp->next;
    }
    printf("NULL\n");

    int target = 0; // 假设你想在这个元素之前插入或删除
    int newValue = 5; // 新插入的值

    printf("要在哪个元素前插入新值?\n");
    scanf("%d", &target);

    insertBefore(&list, target, newValue);
    printf("插入新元素后的链表: ");
    temp = list;
    while (temp != NULL) {
        printf("%d -> ", temp->data);
        temp = temp->next;
    }
    printf("NULL\n");

    printf("要删除哪个元素?\n");
    scanf("%d", &target);

    deleteElement(&list, target);
    printf("删除元素后的链表: ");
    temp = list;
    while (temp != NULL) {
        printf("%d -> ", temp->data);
        temp = temp->next;
    }
    printf("NULL\n");

    return 0;
}

这个程序会提示用户输入一系列数字，然后允许他们在指定元素前插入新元素和删除指定元素。请注意，输入的`target`值需与已经存在的元素匹配才能成功操作。