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如果您比較有耐心，建議從頭至尾讀完這篇文章。如果您只想快速應(yīng)用 C 語言的 HTTP GET 連接功能，可以直接跳到文末拷貝源代碼去使用。
1、HTTP 連接的流程
HTTP 連接都是建立在 TCP 連接之上的。這里我們不討論 TCP 的三次握手四次揮手過程。我們只單純地來分析下一個 HTTP 連接的過程應(yīng)該是怎樣的。
首先，我們需要創(chuàng)建一個 TCP 的 Socket 。后面我們的網(wǎng)絡(luò)連接操作都是基于這個 Socket 來構(gòu)建的。
其次，我們需要來組裝一下 HTTP 請求。就是封裝一個 GET 請求，表明一下我們想要連接哪個服務(wù)器的哪些資源。當(dāng)然，其實第 1 步和第 2 步的順序并不重要。
第三步，我們需要發(fā)送 HTTP GET 請求了，將前面封裝好的請求信息通過前面創(chuàng)建好的 TCP 通道發(fā)送出去。
第四步，讀取服務(wù)端的返回結(jié)果。
2、代碼實操
1、創(chuàng)建 Socket
int sockfd;
//創(chuàng)建套接字
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) {
printf("socket failed!!!\n");
exit(0);
}沒什么好說的，照著做就好了。
2、封裝 HTTP 請求
char request[512] = {0};
memset(request, 0, 512);
strcat(request, "GET ");
strcat(request, "/index.html");
strcat(request, " HTTP/1.1\n");
strcat(request, "Host: ");
strcat(request, "192.168.221.30");
strcat(request, "\nContent-Type: text/html\n");
strcat(request, "Content-Length: 0\n");
strcat(request, "\r\n");上面加粗標(biāo)灰底的部分分別是要訪問的資源路徑以及服務(wù)器地址。其中服務(wù)器地址并不是很重要，但是上面的資源路徑一定不能錯！
3、發(fā)起 HTTP 請求
struct sockaddr_in servaddr; int writeRet;
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(80);
if (inet_pton(AF_INET, "192.168.221.30", &servaddr.sin_addr) <= 0 ){
printf("inet_pton error!\n");
exit(0);
} if (connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0){
printf("connect error!\n");
exit(0);
}
writeRet = write(sockfd, request, strlen(request)); if (writeRet < 0) {
printf("make http error:%d,%s\n", errno, strerror(errno));
exit(0);
}上面加粗標(biāo)灰底的地址就很重要了，一定不能填錯，而且只能填 IP 地址。那個部分的 inet_pton 函數(shù)的作用是將字符串形式的 IPV4 地址轉(zhuǎn)換成二進(jìn)制形式的。如果是 IPV6 地址，則要用 inet_ntop 函數(shù)。
4、讀取返回結(jié)果
struct timeval tv; int selectRet = 0;
fd_set t_set1;
sleep(2);
tv.tv_sec= 0;
tv.tv_usec= 0;
FD_ZERO(&t_set1);
FD_SET(sockfd, &t_set1);
selectRet = select(sockfd + 1, &t_set1, NULL, NULL, &tv); if (selectRet < 0) {
close(sockfd);
printf("select failed!\n"); return;
} if (selectRet > 0){ char buf[4096] = {0}; int readLen = 0;
memset(buf, 0, 4096);
readLen = read(sockfd, buf, 4095); // read once only!
printf("readLen:%d\n", readLen);
printf("\n\n%s\n\n", buf);
}上示代碼最后會將讀取的結(jié)果保存到 buf 數(shù)組中并打印出來。
5、關(guān)閉 Socket
close(sockfd);
printf("Bye!\n");最后，用完 HTTP 通信以后，一定不要忘記關(guān)閉剛才打開的資源。
筆者這邊通過 nginx 搭建了一個模擬服務(wù)器，與這份代碼調(diào)試，一切正常。可以在控制臺上得到如下回復(fù)
hello world-------------GET /index.html HTTP/1.1Host: 192.168.221.30Content-Type: text/html
Content-Length: 0>>>>>>> success with 90 byte(s) <<<<<<<readLen:850HTTP/1.1 200 OK
Server: nginx/1.16.0Date: Wed, 08 May 2019 06:13:27 GMT
Content-Type: text/html
Content-Length: 612Last-Modified: Tue, 23 Apr 2019 13:09:07 GMT
Connection: keep-alive
ETag: "5cbf0e73-264"Accept-Ranges: bytes<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
body {
width: 35em;
margin: 0 auto;
font-family: Tahoma, Verdana, Arial, sans-serif;
}</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>
<p>For online documentation and support please refer to<a href="http://nginx.org/">nginx.org</a>.<br/>Commercial support is available at<a href="http://nginx.com/">nginx.com</a>.</p>
<p><em>Thank you for using nginx.</em></p>
</body>
</html>Bye!
再接下來，就是根據(jù)自己的實際需要，去做業(yè)務(wù)層的處理了。
3、完整代碼
#include <stdio.h>#include <stdlib.h>#include <sys/socket.h>#include <string.h>#include <netinet/in.h>#include <errno.h>#include "http.h"void main()
{
printf("hello world\n");
// step 1 , create socket
int sockfd;
//創(chuàng)建套接字
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) {
printf("socket failed!!!\n");
exit(0);
}
// step 2, package the http request
char request[512] = {0};
memset(request, 0, 512);
strcat(request, "GET ");
strcat(request, "/index.htm");
strcat(request, " HTTP/1.1\n");
strcat(request, "Host: ");
strcat(request, "www.baidu.com");
strcat(request, "\nContent-Type: text/html\n");
strcat(request, "Content-Length: 0\n");
strcat(request, "\r\n");
printf("-------------\n%s\n",request);
// step 3, connect and send http request.
struct sockaddr_in servaddr; int writeRet;
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(80);
if (inet_pton(AF_INET, "192.168.221.30", &servaddr.sin_addr) <= 0 ){
printf("inet_pton error!\n");
exit(0);
} if (connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0){
printf("connect error!\n");
exit(0);
}
writeRet = write(sockfd, request, strlen(request)); if (writeRet < 0) {
printf("make http error:%d,%s\n", errno, strerror(errno));
exit(0);
}
printf(">>>>>>> success with %d byte(s) <<<<<<<\n", writeRet);
// step 4, read response
struct timeval tv; int selectRet = 0;
fd_set t_set1;
sleep(2);
tv.tv_sec= 0;
tv.tv_usec= 0;
FD_ZERO(&t_set1);
FD_SET(sockfd, &t_set1);
selectRet = select(sockfd + 1, &t_set1, NULL, NULL, &tv); if (selectRet < 0) {
close(sockfd);
printf("select failed!\n"); return;
} if (selectRet > 0){ char buf[4096] = {0}; int readLen = 0;
memset(buf, 0, 4096);
readLen = read(sockfd, buf, 4095); // read once only!
printf("readLen:%d\n", readLen);
printf("\n\n%s\n\n", buf);
}
close(sockfd);
printf("Bye!\n");
}
