socket.c

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/* -*- c-file-style: "linux" -*-

   rsync -- fast file replication program

   Copyright (C) 1992-2001 by Andrew Tridgell <tridge@samba.org>
   Copyright (C) 2001, 2002 by Martin Pool <mbp@samba.org>

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

/**
 * @file socket.c
 *
 * Socket functions used in rsync.
 *
 * This file is now converted to use the new-style getaddrinfo()
 * interface, which supports IPv6 but is also supported on recent
 * IPv4-only machines.  On systems that don't have that interface, we
 * emulate it using the KAME implementation.
 **/

#include "rsync.h"
#include <netinet/in_systm.h>
#include <netinet/ip.h>

extern char *bind_address;
extern int default_af_hint;

#ifdef HAVE_SIGACTION
static struct sigaction sigact;
#endif

/**
 * Establish a proxy connection on an open socket to a web proxy by
 * using the CONNECT method. If proxy_user and proxy_pass are not NULL,
 * they are used to authenticate to the proxy using the "Basic"
 * proxy-authorization protocol
 **/
static int establish_proxy_connection(int fd, char *host, int port,
                                      char *proxy_user, char *proxy_pass)
{
        char *cp, buffer[1024];
        char *authhdr, authbuf[1024];
        int len;

        if (proxy_user && proxy_pass) {
                stringjoin(buffer, sizeof buffer,
                         proxy_user, ":", proxy_pass, NULL);
                len = strlen(buffer);

                if ((len*8 + 5) / 6 >= (int)sizeof authbuf - 3) {
                        rprintf(FERROR,
                                "authentication information is too long\n");
                        return -1;
                }

                base64_encode(buffer, len, authbuf, 1);
                authhdr = "\r\nProxy-Authorization: Basic ";
        } else {
                *authbuf = '\0';
                authhdr = "";
        }

        snprintf(buffer, sizeof buffer, "CONNECT %s:%d HTTP/1.0%s%s\r\n\r\n",
                 host, port, authhdr, authbuf);
        len = strlen(buffer);
        if (write(fd, buffer, len) != len) {
                rsyserr(FERROR, errno, "failed to write to proxy");
                return -1;
        }

        for (cp = buffer; cp < &buffer[sizeof buffer - 1]; cp++) {
                if (read(fd, cp, 1) != 1) {
                        rsyserr(FERROR, errno, "failed to read from proxy");
                        return -1;
                }
                if (*cp == '\n')
                        break;
        }

        if (*cp != '\n')
                cp++;
        *cp-- = '\0';
        if (*cp == '\r')
                *cp = '\0';
        if (strncmp(buffer, "HTTP/", 5) != 0) {
                rprintf(FERROR, "bad response from proxy -- %s\n",
                        buffer);
                return -1;
        }
        for (cp = &buffer[5]; isdigit(*(uchar*)cp) || *cp == '.'; cp++) {}
        while (*cp == ' ')
                cp++;
        if (*cp != '2') {
                rprintf(FERROR, "bad response from proxy -- %s\n",
                        buffer);
                return -1;
        }
        /* throw away the rest of the HTTP header */
        while (1) {
                for (cp = buffer; cp < &buffer[sizeof buffer - 1]; cp++) {
                        if (read(fd, cp, 1) != 1) {
                                rsyserr(FERROR, errno,
                                        "failed to read from proxy");
                                return -1;
                        }
                        if (*cp == '\n')
                                break;
                }
                if (cp > buffer && *cp == '\n')
                        cp--;
                if (cp == buffer && (*cp == '\n' || *cp == '\r'))
                        break;
        }
        return 0;
}


/**
 * Try to set the local address for a newly-created socket.  Return -1
 * if this fails.
 **/
int try_bind_local(int s, int ai_family, int ai_socktype,
                   const char *bind_addr)
{
        int error;
        struct addrinfo bhints, *bres_all, *r;

        memset(&bhints, 0, sizeof bhints);
        bhints.ai_family = ai_family;
        bhints.ai_socktype = ai_socktype;
        bhints.ai_flags = AI_PASSIVE;
        if ((error = getaddrinfo(bind_addr, NULL, &bhints, &bres_all))) {
                rprintf(FERROR, RSYNC_NAME ": getaddrinfo %s: %s\n",
                        bind_addr, gai_strerror(error));
                return -1;
        }

        for (r = bres_all; r; r = r->ai_next) {
                if (bind(s, r->ai_addr, r->ai_addrlen) == -1)
                        continue;
                freeaddrinfo(bres_all);
                return s;
        }

        /* no error message; there might be some problem that allows
         * creation of the socket but not binding, perhaps if the
         * machine has no ipv6 address of this name. */
        freeaddrinfo(bres_all);
        return -1;
}


/**
 * Open a socket to a tcp remote host with the specified port .
 *
 * Based on code from Warren.  Proxy support by Stephen Rothwell.
 * getaddrinfo() rewrite contributed by KAME.net.
 *
 * Now that we support IPv6 we need to look up the remote machine's
 * address first, using @p af_hint to set a preference for the type
 * of address.  Then depending on whether it has v4 or v6 addresses we
 * try to open a connection.
 *
 * The loop allows for machines with some addresses which may not be
 * reachable, perhaps because we can't e.g. route ipv6 to that network
 * but we can get ip4 packets through.
 *
 * @param bind_addr Local address to use.  Normally NULL to bind
 * the wildcard address.
 *
 * @param af_hint Address family, e.g. AF_INET or AF_INET6.
 **/
int open_socket_out(char *host, int port, const char *bind_addr,
                    int af_hint)
{
        int type = SOCK_STREAM;
        int error, s;
        struct addrinfo hints, *res0, *res;
        char portbuf[10];
        char *h, *cp;
        int proxied = 0;
        char buffer[1024];
        char *proxy_user = NULL, *proxy_pass = NULL;

        /* if we have a RSYNC_PROXY env variable then redirect our
         * connetcion via a web proxy at the given address. */
        h = getenv("RSYNC_PROXY");
        proxied = h != NULL && *h != '\0';

        if (proxied) {
                strlcpy(buffer, h, sizeof buffer);

                /* Is the USER:PASS@ prefix present? */
                if ((cp = strrchr(buffer, '@')) != NULL) {
                        *cp++ = '\0';
                        /* The remainder is the HOST:PORT part. */
                        h = cp;

                        if ((cp = strchr(buffer, ':')) == NULL) {
                                rprintf(FERROR,
                                        "invalid proxy specification: should be USER:PASS@HOST:PORT\n");
                                return -1;
                        }
                        *cp++ = '\0';

                        proxy_user = buffer;
                        proxy_pass = cp;
                } else {
                        /* The whole buffer is the HOST:PORT part. */
                        h = buffer;
                }

                if ((cp = strchr(h, ':')) == NULL) {
                        rprintf(FERROR,
                                "invalid proxy specification: should be HOST:PORT\n");
                        return -1;
                }
                *cp++ = '\0';
                strlcpy(portbuf, cp, sizeof portbuf);
                if (verbose >= 2) {
                        rprintf(FINFO, "connection via http proxy %s port %s\n",
                                h, portbuf);
                }
        } else {
                snprintf(portbuf, sizeof portbuf, "%d", port);
                h = host;
        }

        memset(&hints, 0, sizeof hints);
        hints.ai_family = af_hint;
        hints.ai_socktype = type;
        error = getaddrinfo(h, portbuf, &hints, &res0);
        if (error) {
                rprintf(FERROR, RSYNC_NAME ": getaddrinfo: %s %s: %s\n",
                        h, portbuf, gai_strerror(error));
                return -1;
        }

        s = -1;
        /* Try to connect to all addresses for this machine until we get
         * through.  It might e.g. be multi-homed, or have both IPv4 and IPv6
         * addresses.  We need to create a socket for each record, since the
         * address record tells us what protocol to use to try to connect. */
        for (res = res0; res; res = res->ai_next) {
                s = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
                if (s < 0)
                        continue;

                if (bind_addr
                 && try_bind_local(s, res->ai_family, type,
                                   bind_addr) == -1) {
                        close(s);
                        s = -1;
                        continue;
                }
                if (connect(s, res->ai_addr, res->ai_addrlen) < 0) {
                        close(s);
                        s = -1;
                        continue;
                }
                if (proxied
                 && establish_proxy_connection(s, host, port,
                                               proxy_user, proxy_pass) != 0) {
                        close(s);
                        s = -1;
                        continue;
                }
                break;
        }
        freeaddrinfo(res0);
        if (s < 0) {
                rsyserr(FERROR, errno, "failed to connect to %s", h);
                return -1;
        }
        return s;
}


/**
 * Open an outgoing socket, but allow for it to be intercepted by
 * $RSYNC_CONNECT_PROG, which will execute a program across a TCP
 * socketpair rather than really opening a socket.
 *
 * We use this primarily in testing to detect TCP flow bugs, but not
 * cause security problems by really opening remote connections.
 *
 * This is based on the Samba LIBSMB_PROG feature.
 *
 * @param bind_addr Local address to use.  Normally NULL to get the stack default.
 **/
int open_socket_out_wrapped(char *host, int port, const char *bind_addr,
                            int af_hint)
{
        char *prog = getenv("RSYNC_CONNECT_PROG");

        if (verbose >= 2) {
                rprintf(FINFO, "%sopening tcp connection to %s port %d\n",
                        prog ? "Using RSYNC_CONNECT_PROG instead of " : "",
                        host, port);
        }
        if (prog)
                return sock_exec(prog);
        return open_socket_out(host, port, bind_addr, af_hint);
}



/**
 * Open one or more sockets for incoming data using the specified type,
 * port, and address.
 *
 * The getaddrinfo() call may return several address results, e.g. for
 * the machine's IPv4 and IPv6 name.
 *
 * We return an array of file-descriptors to the sockets, with a trailing
 * -1 value to indicate the end of the list.
 *
 * @param bind_addr Local address to bind, or NULL to allow it to
 * default.
 **/
static int *open_socket_in(int type, int port, const char *bind_addr,
                           int af_hint)
{
        int one = 1;
        int s, *socks, maxs, i, ecnt;
        struct addrinfo hints, *all_ai, *resp;
        char portbuf[10], **errmsgs;
        int error;

        memset(&hints, 0, sizeof hints);
        hints.ai_family = af_hint;
        hints.ai_socktype = type;
        hints.ai_flags = AI_PASSIVE;
        snprintf(portbuf, sizeof portbuf, "%d", port);
        error = getaddrinfo(bind_addr, portbuf, &hints, &all_ai);
        if (error) {
                rprintf(FERROR, RSYNC_NAME ": getaddrinfo: bind address %s: %s\n",
                        bind_addr, gai_strerror(error));
                return NULL;
        }

        /* Count max number of sockets we might open. */
        for (maxs = 0, resp = all_ai; resp; resp = resp->ai_next, maxs++) {}

        socks = new_array(int, maxs + 1);
        errmsgs = new_array(char *, maxs);
        if (!socks || !errmsgs)
                out_of_memory("open_socket_in");

        /* We may not be able to create the socket, if for example the
         * machine knows about IPv6 in the C library, but not in the
         * kernel. */
        for (resp = all_ai, i = ecnt = 0; resp; resp = resp->ai_next) {
                s = socket(resp->ai_family, resp->ai_socktype,
                           resp->ai_protocol);

                if (s == -1) {
                        int r = asprintf(&errmsgs[ecnt++],
                                "socket(%d,%d,%d) failed: %s\n",
                                (int)resp->ai_family, (int)resp->ai_socktype,
                                (int)resp->ai_protocol, strerror(errno));
                        if (r < 0)
                                out_of_memory("open_socket_in");
                        /* See if there's another address that will work... */
                        continue;
                }

                setsockopt(s, SOL_SOCKET, SO_REUSEADDR,
                           (char *)&one, sizeof one);

#ifdef IPV6_V6ONLY
                if (resp->ai_family == AF_INET6) {
                        if (setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY,
                                       (char *)&one, sizeof one) < 0
                            && default_af_hint != AF_INET6) {
                                close(s);
                                continue;
                        }
                }
#endif

                /* Now we've got a socket - we need to bind it. */
                if (bind(s, resp->ai_addr, resp->ai_addrlen) < 0) {
                        /* Nope, try another */
                        int r = asprintf(&errmsgs[ecnt++],
                                "bind() failed: %s (address-family %d)\n",
                                strerror(errno), (int)resp->ai_family);
                        if (r < 0)
                                out_of_memory("open_socket_in");
                        close(s);
                        continue;
                }

                socks[i++] = s;
        }
        socks[i] = -1;

        if (all_ai)
                freeaddrinfo(all_ai);

        /* Only output the socket()/bind() messages if we were totally
         * unsuccessful, or if the daemon is being run with -vv. */
        for (s = 0; s < ecnt; s++) {
                if (!i || verbose > 1)
                        rwrite(FLOG, errmsgs[s], strlen(errmsgs[s]));
                free(errmsgs[s]);
        }
        free(errmsgs);

        if (!i) {
                rprintf(FERROR,
                        "unable to bind any inbound sockets on port %d\n",
                        port);
                free(socks);
                return NULL;
        }
        return socks;
}


/*
 * Determine if a file descriptor is in fact a socket
 */
int is_a_socket(int fd)
{
        int v;
        socklen_t l = sizeof (int);

        /* Parameters to getsockopt, setsockopt etc are very
         * unstandardized across platforms, so don't be surprised if
         * there are compiler warnings on e.g. SCO OpenSwerver or AIX.
         * It seems they all eventually get the right idea.
         *
         * Debian says: ``The fifth argument of getsockopt and
         * setsockopt is in reality an int [*] (and this is what BSD
         * 4.* and libc4 and libc5 have).  Some POSIX confusion
         * resulted in the present socklen_t.  The draft standard has
         * not been adopted yet, but glibc2 already follows it and
         * also has socklen_t [*]. See also accept(2).''
         *
         * We now return to your regularly scheduled programming.  */
        return getsockopt(fd, SOL_SOCKET, SO_TYPE, (char *)&v, &l) == 0;
}


static RETSIGTYPE sigchld_handler(UNUSED(int val))
{
#ifdef WNOHANG
        while (waitpid(-1, NULL, WNOHANG) > 0) {}
#endif
#ifndef HAVE_SIGACTION
        signal(SIGCHLD, sigchld_handler);
#endif
}


void start_accept_loop(int port, int (*fn)(int, int))
{
        fd_set deffds;
        int *sp, maxfd, i;

#ifdef HAVE_SIGACTION
        sigact.sa_flags = SA_NOCLDSTOP;
#endif

        /* open an incoming socket */
        sp = open_socket_in(SOCK_STREAM, port, bind_address, default_af_hint);
        if (sp == NULL)
                exit_cleanup(RERR_SOCKETIO);

        /* ready to listen */
        FD_ZERO(&deffds);
        for (i = 0, maxfd = -1; sp[i] >= 0; i++) {
                if (listen(sp[i], 5) < 0) {
                        rsyserr(FERROR, errno, "listen() on socket failed");
#ifdef INET6
                        if (errno == EADDRINUSE && i > 0) {
                                rprintf(FINFO,
                                    "Try using --ipv4 or --ipv6 to avoid this listen() error.\n");
                        }
#endif
                        exit_cleanup(RERR_SOCKETIO);
                }
                FD_SET(sp[i], &deffds);
                if (maxfd < sp[i])
                        maxfd = sp[i];
        }

        /* now accept incoming connections - forking a new process
         * for each incoming connection */
        while (1) {
                fd_set fds;
                pid_t pid;
                int fd;
                struct sockaddr_storage addr;
                socklen_t addrlen = sizeof addr;

                /* close log file before the potentially very long select so
                 * file can be trimmed by another process instead of growing
                 * forever */
                logfile_close();

#ifdef FD_COPY
                FD_COPY(&deffds, &fds);
#else
                fds = deffds;
#endif

                if (select(maxfd + 1, &fds, NULL, NULL, NULL) != 1)
                        continue;

                for (i = 0, fd = -1; sp[i] >= 0; i++) {
                        if (FD_ISSET(sp[i], &fds)) {
                                fd = accept(sp[i], (struct sockaddr *)&addr,
                                            &addrlen);
                                break;
                        }
                }

                if (fd < 0)
                        continue;

                SIGACTION(SIGCHLD, sigchld_handler);

                if ((pid = fork()) == 0) {
                        int ret;
                        for (i = 0; sp[i] >= 0; i++)
                                close(sp[i]);
                        /* Re-open log file in child before possibly giving
                         * up privileges (see logfile_close() above). */
                        logfile_reopen();
                        ret = fn(fd, fd);
                        close_all();
                        _exit(ret);
                } else if (pid < 0) {
                        rsyserr(FERROR, errno,
                                "could not create child server process");
                        close(fd);
                        /* This might have happened because we're
                         * overloaded.  Sleep briefly before trying to
                         * accept again. */
                        sleep(2);
                } else {
                        /* Parent doesn't need this fd anymore. */
                        close(fd);
                }
        }
}


enum SOCK_OPT_TYPES {OPT_BOOL,OPT_INT,OPT_ON};

struct
{
  char *name;
  int level;
  int option;
  int value;
  int opttype;
} socket_options[] = {
  {"SO_KEEPALIVE",      SOL_SOCKET,    SO_KEEPALIVE,    0,                 OPT_BOOL},
  {"SO_REUSEADDR",      SOL_SOCKET,    SO_REUSEADDR,    0,                 OPT_BOOL},
  {"SO_BROADCAST",      SOL_SOCKET,    SO_BROADCAST,    0,                 OPT_BOOL},
#ifdef TCP_NODELAY
  {"TCP_NODELAY",       IPPROTO_TCP,   TCP_NODELAY,     0,                 OPT_BOOL},
#endif
#ifdef IPTOS_LOWDELAY
  {"IPTOS_LOWDELAY",    IPPROTO_IP,    IP_TOS,          IPTOS_LOWDELAY,    OPT_ON},
#endif
#ifdef IPTOS_THROUGHPUT
  {"IPTOS_THROUGHPUT",  IPPROTO_IP,    IP_TOS,          IPTOS_THROUGHPUT,  OPT_ON},
#endif
#ifdef SO_SNDBUF
  {"SO_SNDBUF",         SOL_SOCKET,    SO_SNDBUF,       0,                 OPT_INT},
#endif
#ifdef SO_RCVBUF
  {"SO_RCVBUF",         SOL_SOCKET,    SO_RCVBUF,       0,                 OPT_INT},
#endif
#ifdef SO_SNDLOWAT
  {"SO_SNDLOWAT",       SOL_SOCKET,    SO_SNDLOWAT,     0,                 OPT_INT},
#endif
#ifdef SO_RCVLOWAT
  {"SO_RCVLOWAT",       SOL_SOCKET,    SO_RCVLOWAT,     0,                 OPT_INT},
#endif
#ifdef SO_SNDTIMEO
  {"SO_SNDTIMEO",       SOL_SOCKET,    SO_SNDTIMEO,     0,                 OPT_INT},
#endif
#ifdef SO_RCVTIMEO
  {"SO_RCVTIMEO",       SOL_SOCKET,    SO_RCVTIMEO,     0,                 OPT_INT},
#endif
  {NULL,0,0,0,0}};



/**
 * Set user socket options
 **/
void set_socket_options(int fd, char *options)
{
        char *tok;

        if (!options || !*options)
                return;

        options = strdup(options);

        if (!options)
                out_of_memory("set_socket_options");

        for (tok = strtok(options, " \t,"); tok; tok = strtok(NULL," \t,")) {
                int ret=0,i;
                int value = 1;
                char *p;
                int got_value = 0;

                if ((p = strchr(tok,'='))) {
                        *p = 0;
                        value = atoi(p+1);
                        got_value = 1;
                }

                for (i = 0; socket_options[i].name; i++) {
                        if (strcmp(socket_options[i].name,tok)==0)
                                break;
                }

                if (!socket_options[i].name) {
                        rprintf(FERROR,"Unknown socket option %s\n",tok);
                        continue;
                }

                switch (socket_options[i].opttype) {
                case OPT_BOOL:
                case OPT_INT:
                        ret = setsockopt(fd,socket_options[i].level,
                                         socket_options[i].option,
                                         (char *)&value, sizeof (int));
                        break;

                case OPT_ON:
                        if (got_value)
                                rprintf(FERROR,"syntax error -- %s does not take a value\n",tok);

                        {
                                int on = socket_options[i].value;
                                ret = setsockopt(fd,socket_options[i].level,
                                                 socket_options[i].option,
                                                 (char *)&on, sizeof (int));
                        }
                        break;
                }

                if (ret != 0) {
                        rsyserr(FERROR, errno,
                                "failed to set socket option %s", tok);
                }
        }

        free(options);
}

/**
 * Become a daemon, discarding the controlling terminal
 **/
void become_daemon(void)
{
        int i;

        if (fork()) {
                _exit(0);
        }

        /* detach from the terminal */
#ifdef HAVE_SETSID
        setsid();
#elif defined TIOCNOTTY
        i = open("/dev/tty", O_RDWR);
        if (i >= 0) {
                ioctl(i, (int)TIOCNOTTY, (char *)0);
                close(i);
        }
#endif
        /* make sure that stdin, stdout an stderr don't stuff things
         * up (library functions, for example) */
        for (i = 0; i < 3; i++) {
                close(i);
                open("/dev/null", O_RDWR);
        }
}


/**
 * This is like socketpair but uses tcp. It is used by the Samba
 * regression test code.
 *
 * The function guarantees that nobody else can attach to the socket,
 * or if they do that this function fails and the socket gets closed
 * returns 0 on success, -1 on failure the resulting file descriptors
 * are symmetrical.
 **/
static int socketpair_tcp(int fd[2])
{
        int listener;
        struct sockaddr_in sock;
        struct sockaddr_in sock2;
        socklen_t socklen = sizeof sock;
        int connect_done = 0;

        fd[0] = fd[1] = listener = -1;

        memset(&sock, 0, sizeof sock);

        if ((listener = socket(PF_INET, SOCK_STREAM, 0)) == -1)
                goto failed;

        memset(&sock2, 0, sizeof sock2);
#ifdef HAVE_SOCKADDR_IN_LEN
        sock2.sin_len = sizeof sock2;
#endif
        sock2.sin_family = PF_INET;

        bind(listener, (struct sockaddr *)&sock2, sizeof sock2);

        if (listen(listener, 1) != 0)
                goto failed;

        if (getsockname(listener, (struct sockaddr *)&sock, &socklen) != 0)
                goto failed;

        if ((fd[1] = socket(PF_INET, SOCK_STREAM, 0)) == -1)
                goto failed;

        set_nonblocking(fd[1]);

        sock.sin_addr.s_addr = htonl(INADDR_LOOPBACK);

        if (connect(fd[1], (struct sockaddr *)&sock, sizeof sock) == -1) {
                if (errno != EINPROGRESS)
                        goto failed;
        } else
                connect_done = 1;

        if ((fd[0] = accept(listener, (struct sockaddr *)&sock, &socklen)) == -1)
                goto failed;

        close(listener);
        listener = -1;

        set_blocking(fd[1]);

        if (connect_done == 0) {
                if (connect(fd[1], (struct sockaddr *)&sock, sizeof sock) != 0
                    && errno != EISCONN)
                        goto failed;
        }

        /* all OK! */
        return 0;

 failed:
        if (fd[0] != -1)
                close(fd[0]);
        if (fd[1] != -1)
                close(fd[1]);
        if (listener != -1)
                close(listener);
        return -1;
}



/**
 * Run a program on a local tcp socket, so that we can talk to it's
 * stdin and stdout.  This is used to fake a connection to a daemon
 * for testing -- not for the normal case of running SSH.
 *
 * @return a socket which is attached to a subprocess running
 * "prog". stdin and stdout are attached. stderr is left attached to
 * the original stderr
 **/
int sock_exec(const char *prog)
{
        int fd[2];

        if (socketpair_tcp(fd) != 0) {
                rsyserr(FERROR, errno, "socketpair_tcp failed");
                return -1;
        }
        if (verbose >= 2)
                rprintf(FINFO, "Running socket program: \"%s\"\n", prog);
        if (fork() == 0) {
                close(fd[0]);
                close(0);
                close(1);
                dup(fd[1]);
                dup(fd[1]);
                exit(system(prog));
        }
        close(fd[1]);
        return fd[0];
}

---