BOA 调用 CGI 原理

• 环境：
  arm7 i.mx6ul
  源码 boa-0.94.13

boa 移植以及怎么使用CGI网上有很多示例，但是找不到原理相关的。今天项目中有关用到，就看了下源码。

首先我们用放在’cgi-bin/’ 目录下的动xxx.cgi文件是一个可执行文件，可以使用./xxx.cgi来开始执行，其实是可以正常跑的。cgi编译的输出一般是用标准输出来实现，如下语句

fprintf(cgiOut, "		<div class=\"nav_m\">\n");

如果不传入环境变量，那么标准输出就到了登入的控制台。

那么boa是怎么调用我们编译的CGI进程的呢？源码cgi.c中init_cgi中可以发现行管的，调用入口我加了注释

/*
 * Name: init_cgi
 *
 * Description: Called for GET/POST requests that refer to ScriptAlias
 * directories or application/x-httpd-cgi files.  Ties stdout to socket,
 * stdin to data if POST, and execs CGI.
 * stderr remains tied to our log file; is this good?
 *
 * Returns:
 * 0 - error or NPH, either way the socket is closed
 * 1 - success
 */

int init_cgi(request * req)
{
    int child_pid;
    int pipes[2];
    int use_pipes = 0;

    SQUASH_KA(req);

    if (req->is_cgi) {
        if (complete_env(req) == 0) {
            return 0;
        }
    }
#ifdef FASCIST_LOGGING
    {
        int i;
        for (i = 0; i < req->cgi_env_index; ++i)
            fprintf(stderr, "%s - environment variable for cgi: \"%s\"\n",
                    __FILE__, req->cgi_env[i]);
    }
#endif

    if (req->is_cgi == CGI || 1) {
        use_pipes = 1;
        if (pipe(pipes) == -1) {
            log_error_time();
            perror("pipe");
            return 0;
        }

        /* set the read end of the socket to non-blocking */
        if (set_nonblock_fd(pipes[0]) == -1) {
            log_error_time();
            perror("cgi-fcntl");
            close(pipes[0]);
            close(pipes[1]);
            return 0;
        }
    }

    child_pid = fork();
    switch(child_pid) {
    case -1:
        /* fork unsuccessful */
        log_error_time();
        perror("fork");

        if (use_pipes) {
            close(pipes[0]);
            close(pipes[1]);
        }
        send_r_error(req);
        /* FIXME: There is aproblem here. send_r_error would work
           for NPH and CGI, but not for GUNZIP.  Fix that. */
        /* i'd like to send_r_error, but.... */
        return 0;
        break;
    case 0:
        /* child */
        if (req->is_cgi == CGI || req->is_cgi == NPH) {
            char *foo = strdup(req->pathname);
            char *c;

            if (!foo) {
                WARN("unable to strdup pathname for req->pathname");
                _exit(1);
            }
            c = strrchr(foo, '/');
            if (c) {
                ++c;
                *c = '\0';
            } else {
                /* we have a serious problem */
                log_error_time();
                perror("chdir");
                if (use_pipes)
                    close(pipes[1]);
                _exit(1);
            }
            if (chdir(foo) != 0) {
                log_error_time();
                perror("chdir");
                if (use_pipes)
                    close(pipes[1]);
                _exit(1);
            }
        }
        if (use_pipes) {
            close(pipes[0]);
            /* tie cgi's STDOUT to it's write end of pipe */
            if (dup2(pipes[1], STDOUT_FILENO) == -1) {
                log_error_time();
                perror("dup2 - pipes");
                close(pipes[1]);
                _exit(1);
            }
            close(pipes[1]);
            if (set_block_fd(STDOUT_FILENO) == -1) {
                log_error_time();
                perror("cgi-fcntl");
                _exit(1);
            }
        } else {
            /* tie stdout to socket */
            if (dup2(req->fd, STDOUT_FILENO) == -1) {
                log_error_time();
                perror("dup2 - fd");
                _exit(1);
            }
            /* Switch socket flags back to blocking */
            if (set_block_fd(req->fd) == -1) {
                log_error_time();
                perror("cgi-fcntl");
                _exit(1);
            }
        }
        /* tie post_data_fd to POST stdin */
        if (req->method == M_POST) { /* tie stdin to file */
            lseek(req->post_data_fd, SEEK_SET, 0);
            dup2(req->post_data_fd, STDIN_FILENO);
            close(req->post_data_fd);
        }
        /* Close access log, so CGI program can't scribble
         * where it shouldn't
         */
        close_access_log();

        /*
         * tie STDERR to cgi_log_fd
         * cgi_log_fd will automatically close, close-on-exec rocks!
         * if we don't tied STDERR (current log_error) to cgi_log_fd,
         *  then we ought to close it.
         */
        if (!cgi_log_fd)
            dup2(devnullfd, STDERR_FILENO);
        else
            dup2(cgi_log_fd, STDERR_FILENO);

        if (req->is_cgi) {					// 如果是CGI的请求，则创建新进程
            char *aargv[CGI_ARGC_MAX + 1];
            create_argv(req, aargv);
            execve(req->pathname, aargv, req->cgi_env);  // execve 是把环境变量和进程名字，以及进程参数都传入到要执行性的进程。可以在上面看到fork语句，父程序是不受新建程序影响，cgi程序跑完就结束了
        } else {
            if (req->pathname[strlen(req->pathname) - 1] == '/')
                execl(dirmaker, dirmaker, req->pathname, req->request_uri,
                      NULL);
#ifdef GUNZIP
            else
                execl(GUNZIP, GUNZIP, "--stdout", "--decompress",
                      req->pathname, NULL);
#endif
        }
        /* execve failed */
        WARN(req->pathname);
        _exit(1);
        break;

    default:
        /* parent */
        /* if here, fork was successful */
        if (verbose_cgi_logs) {
            log_error_time();
            fprintf(stderr, "Forked child \"%s\" pid %d\n",
                    req->pathname, child_pid);
        }

        if (req->method == M_POST) {
            close(req->post_data_fd); /* child closed it too */
            req->post_data_fd = 0;
        }

        /* NPH, GUNZIP, etc... all go straight to the fd */
        if (!use_pipes)
            return 0;

        close(pipes[1]);
        req->data_fd = pipes[0];

        req->status = PIPE_READ;
        if (req->is_cgi == CGI) {
            req->cgi_status = CGI_PARSE; /* got to parse cgi header */
            /* for cgi_header... I get half the buffer! */
            req->header_line = req->header_end =
                (req->buffer + BUFFER_SIZE / 2);
        } else {
            req->cgi_status = CGI_BUFFER;
            /* I get all the buffer! */
            req->header_line = req->header_end = req->buffer;
        }

        /* reset req->filepos for logging (it's used in pipe.c) */
        /* still don't know why req->filesize might be reset though */
        req->filepos = 0;
        break;
    }

    return 1;
}