Tinyhttpd源码解析

简介

Tinyhttpd是一轻量级的web服务器，它由美国学生J. David Blackstone于1999年在学习网络课程时编写。源码不到500行，非常适合学习Web编程和Linux/Unix编程接口。

其源码见Tinyhttpd

分析

Tinyhttpd执行流程如下图：



主线程完成socket的创建、绑定、端口监听、创建子线程处理每一个请求。代码如下：

int main(void)
{
int server_sock = -1;
u_short port = 8000;
int client_sock = -1;
struct sockaddr_in client_name;
int client_name_len = sizeof(client_name);
pthread_t newthread;

/* 在指定端口上建立http服务 */
server_sock = startup(&port);
printf("httpd-socket:%d\n", server_sock);
printf("httpd running on port %d\n", port);

int count = 0;
while (1) {
/* accpet成功后，可获得客户端socket和sockaddr */
client_sock = accept(server_sock,
(struct sockaddr *)&client_name,
&client_name_len);
if (client_sock == -1)
error_die("accept");

/* accept_request(client_sock); */
if (pthread_create(&newthread , NULL, accept_request, client_sock) != 0)
perror("pthread_create");
//printf("\b%d", ++count); fflush(stdout); //统计连接请求数
}

close(server_sock);
return(0);
}

其中startup()函数完成socket的bind/listen。代码实现如下：

/**********************************************************************/
/* This function starts the process of listening for web connections
* on a specified port.  If the port is 0, then dynamically allocate a
* port and modify the original port variable to reflect the actual
* port.
* Parameters: pointer to variable containing the port to connect on
* Returns: the socket */
/**********************************************************************/
int startup(u_short *port)
{
int httpd = 0;
struct sockaddr_in name;

/* 建立socket */
httpd = socket(PF_INET, SOCK_STREAM, 0);
if (httpd == -1)
error_die("socket");
memset(&name, 0, sizeof(name));
name.sin_family = AF_INET;
name.sin_port = htons(*port);
name.sin_addr.s_addr = htonl(INADDR_ANY);

/* 绑定socket */
if (bind(httpd, (struct sockaddr *)&name, sizeof(name)) < 0)
error_die("bind");
if (*port == 0) {  /* if dynamically allocating a port */
int namelen = sizeof(name);
if (getsockname(httpd, (struct sockaddr *)&name, &namelen) == -1)
error_die("getsockname");
*port = ntohs(name.sin_port);
}
/* 监听socket */
if (listen(httpd, 5) < 0)
error_die("listen");

return(httpd);
}

子线程accep_request()处理连接请求，请求处理完成后关闭连接。当然这种多线程方式，不可能做到高并发。每来一个请求都会创建一个子线程来处理，处理完成后结束子线程，这必然会导致服务器有不少性能开销，一般用线程池技术。当然Web服务也可以使用多进程如nginx。多线程共享父进程的数据空间，可能导致问题蔓延。多进程有很好的隔离性。accep_request()代码如下：

/**********************************************************************/
/* A request has caused a call to accept() on the server port to
* return.  Process the request appropriately.
* Parameters: the socket connected to the client */
/**********************************************************************/
void accept_request(int client)
{
char buf[1024];
int numchars;
char method[255]; //请求方法
char url[255]; //URL
char path[512];
size_t i, j;
struct stat st;
int cgi = 0; /* becomes true if server decides this is a CGI program */
char *query_string = NULL;

/* 读取请求行 */
numchars = get_line(client, buf, sizeof(buf));
printf("Request-line:%s\n", buf); //debug
i = 0; j = 0;
/* 读取请求行中的请求方法至method */
while (!ISspace(buf[j]) && (i < sizeof(method) - 1)) {
method[i] = buf[j];
i++; j++;
}
method[i] = '\0';
printf("Method:%s ", method);

/* 仅支持GET和POST请求方法 */
if (strcasecmp(method, "GET") && strcasecmp(method, "POST")) {
unimplemented(client); //通知客户客，该请求方法未实现
return;
}

/* POST时开启CGI */
if (strcasecmp(method, "POST") == 0)
cgi = 1;

/* 读取请求行中URL */
i = 0;
while (ISspace(buf[j]) && (j < sizeof(buf)))
j++;
while (!ISspace(buf[j]) && (i < sizeof(url) - 1) && (j < sizeof(buf))) {
url[i] = buf[j];
i++; j++;
}
url[i] = '\0';
printf("URL:%s \n", url);

/* 处理GET方法 */
if (strcasecmp(method, "GET") == 0) {
query_string = url;
while ((*query_string != '?') && (*query_string != '\0'))
query_string++;
/* GET方法的特点，? 后面为参数 */
if (*query_string == '?') {
/* 开启CGI */
cgi = 1;
*query_string = '\0';
query_string++;
}
}

/* 格式化URL到path数组, html在htdocs中，从客户端发过来的URL至少是/ */
sprintf(path, "htdocs%s", url);
if (path[strlen(path) - 1] == '/')
/* 默认情况为index.html */
strcat(path, "index.html");

/* 根据路径找到对应的文件 */
if (stat(path, &st) == -1) {
while ((numchars > 0) && strcmp("\n", buf))  /* read & discard headers */
numchars = get_line(client, buf, sizeof(buf)); //从客户端中把请求读完
not_found(client);
}
else {
if ((st.st_mode & S_IFMT) == S_IFDIR) //文件是目录
strcat(path, "/index.html");
/* 判断文件是否具有可执行权限 */
if ((st.st_mode & S_IXUSR) || (st.st_mode & S_IXGRP) || (st.st_mode & S_IXOTH))
cgi = 1;
if (!cgi)
serve_file(client, path);
else
execute_cgi(client, path, method, query_string);
}
/* 关闭连接，假定不是keepalive */
close(client);
}

浏览器是按照http协议请求的格式给服务器发送请求的，这个函数完成了http协议请求解析。目前只支持GET和POST请求。其中get_line()函数并且不管原来是以\n还是\r\n结束，均转化为以\n再加\0字符结束。实现代码如下：

/**********************************************************************/
/* Get a line from a socket, whether the line ends in a newline,
* carriage return, or a CRLF combination.  Terminates the string read
* with a null character.  If no newline indicator is found before the
* end of the buffer, the string is terminated with a null.  If any of
* the above three line terminators is read, the last character of the
* string will be a linefeed and the string will be terminated with a
* null character.
* Parameters: the socket descriptor
*             the buffer to save the data in
*             the size of the buffer
* Returns: the number of bytes stored (excluding null) */
/**********************************************************************/
int get_line(int sock, char *buf, int size)
{
int i = 0;
char c = '\0';
int n;

/*把终止条件统一为\n换行符，标准化buf数组*/
while ((i < size - 1) && (c != '\n')) {
/* 每次只接收一个字符 */
n = recv(sock, &c, 1, 0);
/* DEBUG printf("%02X\n", c); */
if (n > 0) {
/*收到\r则继续接收下个字节，因为换行符可能是\r\n */
if (c == '\r') {
/*MSG_PEEK使下一次读取的内容和本次读取的内容相同，可认为接收窗口不滑动*/
n = recv(sock, &c, 1, MSG_PEEK); //提前偷窥下次内容
/* DEBUG printf("%02X\n", c); */
if ((n > 0) && (c == '\n'))
recv(sock, &c, 1, 0);
else
c = '\n';
}
buf[i] = c;
i++;
}
else
c = '\n';
}
buf[i] = '\0';

return(i);
}

get_line完后，就是开始解析第一行，判断是GET方法还是POST方法，目前只支持这两种。如果是POST，还是把cgi置1，表明要运行CGI程序；如果是GET方法且附带以?开头的参数时，也认为是执行CGI程序。
获取URL得到文件在服务上的访问路径，获取访问文件的属性，并判断文件是否具有可执行权限。如果有可执行权限，则认为是要执行CGI程序。否则访问静态文件，访问方法为serve_file()，其实现如下：

/**********************************************************************/
/* Send a regular file to the client.  Use headers, and report
* errors to client if they occur.
* Parameters: a pointer to a file structure produced from the socket
*              file descriptor
*             the name of the file to serve */
/**********************************************************************/
void serve_file(int client, const char *filename)
{
FILE *resource = NULL;
int numchars = 1;
char buf[1024];

/* 读取并丢弃headers */
buf[0] = 'A'; buf[1] = '\0';
while ((numchars > 0) && strcmp("\n", buf))  /* read & discard headers */
numchars = get_line(client, buf, sizeof(buf));

/* 打开serve的文件 */
resource = fopen(filename, "r");
if (resource == NULL)
not_found(client);
else {
/* 写http头 */
headers(client, filename);
/* 复制文件 */
cat(client, resource);
}
fclose(resource);
}

其中在服务上读取要访问的文件，并构造http响应格式，以响应客户端。其中，headers()完成响应头的构造，并把响应头信息发送给客户端。cat()函数则负责把访问的文件发送给客户端。
headers()代码如下:

/**********************************************************************/
/* Return the informational HTTP headers about a file. */
/* Parameters: the socket to print the headers on
*             the name of the file */
/**********************************************************************/
void headers(int client, const char *filename)
{
char buf[1024];
(void)filename;  /* could use filename to determine file type */

/* 正常响应客户端 */
strcpy(buf, "HTTP/1.0 200 OK\r\n");
send(client, buf, strlen(buf), 0);
strcpy(buf, SERVER_STRING);
send(client, buf, strlen(buf), 0);
sprintf(buf, "Content-Type: text/html\r\n");
send(client, buf, strlen(buf), 0);
strcpy(buf, "\r\n"); // 协议规定头部于实体的空行
send(client, buf, strlen(buf), 0);
}

cat()代码如下：

/**********************************************************************/
/* Put the entire contents of a file out on a socket.  This function
* is named after the UNIX "cat" command, because it might have been
* easier just to do something like pipe, fork, and exec("cat").
* Parameters: the client socket descriptor
*             FILE pointer for the file to cat */
/**********************************************************************/
void cat(int client, FILE *resource)
{
char buf[1024];

/* 读取文件内容发送给客户端 */
fgets(buf, sizeof(buf), resource);
while (!feof(resource)) {
send(client, buf, strlen(buf), 0);
fgets(buf, sizeof(buf), resource);
}
}

执行CGI程序的代码如下：

/**********************************************************************/
/* Execute a CGI script.  Will need to set environment variables as
* appropriate.
* Parameters: client socket descriptor
*             path to the CGI script */
/**********************************************************************/
void execute_cgi(int client, const char *path,
const char *method, const char *query_string)
{
char buf[1024];
int cgi_output[2];
int cgi_input[2];
pid_t pid;
int status;
int i;
char c;
int numchars = 1;
int content_length = -1;

buf[0] = 'A'; buf[1] = '\0';
if (strcasecmp(method, "GET") == 0) /* GET */
/* 读取并丢弃请求headers */
while ((numchars > 0) && strcmp("\n", buf))  /* read & discard headers */
numchars = get_line(client, buf, sizeof(buf));
else {  /* POST */
numchars = get_line(client, buf, sizeof(buf));
while ((numchars > 0) && strcmp("\n", buf)) {
buf[15] = '\0';
if (strcasecmp(buf, "Content-Length:") == 0)
content_length = atoi(&(buf[16]));
numchars = get_line(client, buf, sizeof(buf));
}
if (content_length == -1) {
bad_request(client);
return;
}
}

/* 正确，HTTP状态码200 */
sprintf(buf, "HTTP/1.0 200 OK\r\n");
send(client, buf, strlen(buf), 0);

/* 创建两个管道，管道半双工, cig_output[0]读, cig_output[1]写 */
if (pipe(cgi_output) < 0) {
cannot_execute(client);
return;
}
if (pipe(cgi_input) < 0) {
cannot_execute(client);
return;
}

if ( (pid = fork()) < 0 ) {
cannot_execute(client);
return;
}
if (pid == 0) { /* child: CGI script */
char meth_env[255];
char query_env[255];
char length_env[255];

/* 把STDOUT重定向到cgi_output的写入端 */
dup2(cgi_output[1], 1);
/* 把STDIN重定向到cgi_input的读取端 */
dup2(cgi_input[0], 0);

/* 关闭cgi_input的写入端和cgi_output的读取端, 把半双工变单工通信 */
close(cgi_output[0]);
close(cgi_input[1]);

/* 设置request_method的环境变量 */
sprintf(meth_env, "REQUEST_METHOD=%s", method);
putenv(meth_env);

if (strcasecmp(method, "GET") == 0) {
sprintf(query_env, "QUERY_STRING=%s", query_string);
putenv(query_env);
}
else { /* POST */
/* 设置content_length的环境变量 */
sprintf(length_env, "CONTENT_LENGTH=%d", content_length);
putenv(length_env);
}
/* 用execl运行cgi程序 */
execl(path, path, NULL);
exit(0);
}
else {    /* parent */
/* 关闭cgi_input的读取端和cgi_output的写入端 */
close(cgi_output[1]);
close(cgi_input[0]);
/* 接收POST方法过来的数据 */
if (strcasecmp(method, "POST") == 0)
for (i = 0; i < content_length; i++) {
recv(client, &c, 1, 0);
/*把POST数据写入cgi_input，现在重定向到STDIN */
write(cgi_input[1], &c, 1);
}
/* 读取cgi_output的管道输出到客户端，该管道输入是STDOUT */
while (read(cgi_output[0], &c, 1) > 0)
send(client, &c, 1, 0);

close(cgi_output[0]);
close(cgi_input[1]);
waitpid(pid, &status, 0);
}
}