用C语言编写程序：通过键盘读取元素建立一个线性表；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，我们可以使用数组、结构体以及指针来创建简单的线性表，并实现插入和删除操作。这里是一个基础示例，我们将使用动态内存分配创建线性表并实现功能：

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

// 定义线性表节点结构
typedef struct Node {
    int data;
    struct Node* next;
} Node;

// 创建线性表
Node* createList() {
    int input;
    printf("请输入元素的数量：");
    scanf("%d", &input);
    
    Node* head = (Node*)malloc(sizeof(Node)); // 动态分配头结点
    if (!head) {
        printf("内存分配失败!");
        return NULL;
    }
    head->data = 0; // 设置默认值
    head->next = NULL;

    Node* current = head;
    for (int i = 1; i < input; ++i) {
        Node* newNode = (Node*)malloc(sizeof(Node));
        if (!newNode) {
            free(current); // 如果前面分配失败，则释放当前节点
            return NULL;
        }
        printf("请输入第 %d 个元素：", i+1);
        scanf("%d", &newNode->data);
        newNode->next = current->next;
        current->next = newNode;
        current = newNode;
    }
    return head;
}

// 在指定元素前插入新元素
void insertBefore(Node** head, int target, int value) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    if (!newNode) {
        printf("内存分配失败!");
        return;
    }
    newNode->data = value;
    
    Node* current = *head;
    while (current != NULL && current->data != target) {
        current = current->next;
    }
    if (current == NULL) {
        printf("目标元素未找到!");
        return;
    }
    newNode->next = current->next;
    current->next = newNode;
}

// 删除指定元素
void deleteElement(Node** head, int target) {
    Node* current = *head;
    Node* previous = NULL;

    if (current != NULL && current->data == target) {
        *head = current->next;
        free(current);
        return;
    }

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

    if (current == NULL) {
        printf("目标元素未找到!");
        return;
    }
    previous->next = current->next;
    free(current);
}

int main() {
    Node* list = createList();
    if (list != NULL) {
        int action;
        printf("\n请选择操作：1 插入 2 删除\n");
        scanf("%d", &action);

        switch (action) {
            case 1:
                int position, new_value;
                printf("请输入要在哪一位元素前插入：");
                scanf("%d", &position);
                printf("请输入新插入的值：");
                scanf("%d", &new_value);
                insertBefore(&list, position, new_value);
                break;
            case 2:
                int target_to_delete;
                printf("请输入要删除的元素：");
                scanf("%d", &target_to_delete);
                deleteElement(&list, target_to_delete);
                break;
            default:
                printf("无效的操作选择!");
                break;
        }
    }

    return 0;
}