用c语言编写以head为头指针的带头结点的单链表，并且将结点x 插入第一个结点值为key的结点之前。

下面是用C语言实现带头结点的单链表，并且在第一个值为key的结点之前插入结点x的代码：

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

typedef struct Node {
    int data;
    struct Node *next;
} Node, *LinkList;

LinkList initList() {
    LinkList head = (LinkList) malloc(sizeof(Node));
    head->next = NULL;
    return head;
}

void insertNode(LinkList head, int key, int x) {
    Node *p = head->next;
    Node *pre = head;
    while (p != NULL && p->data != key) {
        pre = p;
        p = p->next;
    }
    if (p == NULL) {
        printf("No node with value %d\n", key);
        return;
    }
    Node *newNode = (Node *) malloc(sizeof(Node));
    newNode->data = x;
    newNode->next = pre->next;
    pre->next = newNode;
}

void printList(LinkList head) {
    Node *p = head->next;
    while (p != NULL) {
        printf("%d ", p->data);
        p = p->next;
    }
    printf("\n");
}

int main() {
    LinkList head = initList();
    for (int i = 1; i <= 5; i++) {
        Node *newNode = (Node *) malloc(sizeof(Node));
        newNode->data = i;
        newNode->next = head->next;
        head->next = newNode;
    }
    printf("Original list: ");
    printList(head);
    int key = 3, x = 6;
    insertNode(head, key, x);
    printf("After inserting %d before the first node with value %d: ", x, key);
    printList(head);
    return 0;
}

解释一下代码：

首先定义了一个`Node`结构体，表示链表中的一个结点。其中`data`表示结点的数据，`next`表示指向下一个结点的指针。`LinkList`是指向`Node`结构体的指针，表示整个链表。

typedef struct Node {
    int data;
    struct Node *next;
} Node, *LinkList;

`initList()`函数用来初始化链表，创建一个头结点并返回头指针。

LinkList initList() {
    LinkList head = (LinkList) malloc(sizeof(Node));
    head->next = NULL;
    return head;
}

`insertNode()`函数用来插入结点x，它首先遍历链表，找到第一个值为key的结点，并且记录该结点和它的前一个结点。然后创建一个新的结点，并且把它插入到前一个结点和当前结点之间。

void insertNode(LinkList head, int key, int x) {
    Node *p = head->next;
    Node *pre = head;
    while (p != NULL && p->data != key) {
        pre = p;
        p = p->next;
    }
    if (p == NULL) {
        printf("No node with value %d\n", key);
        return;
    }
    Node *newNode = (Node *) malloc(sizeof(Node));
    newNode->data = x;
    newNode->next = pre->next;
    pre->next = newNode;
}

`printList()`函数用来打印整个链表的值。

void printList(LinkList head) {
    Node *p = head->next;
    while (p != NULL) {
        printf("%d ", p->data);
        p = p->next;
    }
    printf("\n");
}

在`main()`函数中，我们首先初始化链表，然后插入一些结点，打印链表，插入新的结点，再打印链表。

int main() {
    LinkList head = initList();
    for (int i = 1; i <= 5; i++) {
        Node *newNode = (Node *) malloc(sizeof(Node));
        newNode->data = i;
        newNode->next = head->next;
        head->next = newNode;
    }
    printf("Original list: ");
    printList(head);
    int key = 3, x = 6;
    insertNode(head, key, x);
    printf("After inserting %d before the first node with value %d: ", x, key);
    printList(head);
    return 0;
}

输出结果为：

Original list: 5 4 3 2 1 
After inserting 6 before the first node with value 3: 5 4 6 3 2 1