ar_io.c

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/*-
 * Copyright (c) 1992 Keith Muller.
 * Copyright (c) 1992, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Keith Muller of the University of California, San Diego.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#ifndef lint
#if 0
static char sccsid[] = "@(#)ar_io.c     8.2 (Berkeley) 4/18/94";
#endif
#endif /* not lint */

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/mtio.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include "pax.h"
#include "options.h"
#include "extern.h"

/*
 * Routines which deal directly with the archive I/O device/file.
 */

#define DMOD            0666            /* default mode of created archives */
#define EXT_MODE        O_RDONLY        /* open mode for list/extract */
#define AR_MODE         (O_WRONLY | O_CREAT | O_TRUNC)  /* mode for archive */
#define APP_MODE        O_RDWR          /* mode for append */

static char none[] = "<NONE>";          /* pseudo name for no file */
static char stdo[] = "<STDOUT>";        /* pseudo name for stdout */
static char stdn[] = "<STDIN>";         /* pseudo name for stdin */
static int arfd = -1;                   /* archive file descriptor */
static int artyp = ISREG;               /* archive type: file/FIFO/tape */
static int arvol = 1;                   /* archive volume number */
static int lstrval = -1;                /* return value from last i/o */
static int io_ok;                       /* i/o worked on volume after resync */
static int did_io;                      /* did i/o ever occur on volume? */
static int done;                        /* set via tty termination */
static struct stat arsb;                /* stat of archive device at open */
static int invld_rec;                   /* tape has out of spec record size */
static int wr_trail = 1;                /* trailer was rewritten in append */
static int can_unlnk = 0;               /* do we unlink null archives?  */
const char *arcname;                    /* printable name of archive */
const char *gzip_program;               /* name of gzip program */
static pid_t zpid = -1;                 /* pid of child process */

static int get_phys(void);
extern sigset_t s_mask;
static void ar_start_gzip(int, const char *, int);

/*
 * ar_open()
 *      Opens the next archive volume. Determines the type of the device and
 *      sets up block sizes as required by the archive device and the format.
 *      Note: we may be called with name == NULL on the first open only.
 * Return:
 *      -1 on failure, 0 otherwise
 */

int
ar_open(const char *name)
{
        struct mtget mb;

        if (arfd != -1)
                (void)close(arfd);
        arfd = -1;
        can_unlnk = did_io = io_ok = invld_rec = 0;
        artyp = ISREG;
        flcnt = 0;

        /*
         * open based on overall operation mode
         */
        switch (act) {
        case LIST:
        case EXTRACT:
                if (name == NULL) {
                        arfd = STDIN_FILENO;
                        arcname = stdn;
                } else if ((arfd = open(name, EXT_MODE, DMOD)) < 0)
                        syswarn(0, errno, "Failed open to read on %s", name);
                if (arfd != -1 && gzip_program != NULL)
                        ar_start_gzip(arfd, gzip_program, 0);
                break;
        case ARCHIVE:
                if (name == NULL) {
                        arfd = STDOUT_FILENO;
                        arcname = stdo;
                } else if ((arfd = open(name, AR_MODE, DMOD)) < 0)
                        syswarn(0, errno, "Failed open to write on %s", name);
                else
                        can_unlnk = 1;
                if (arfd != -1 && gzip_program != NULL)
                        ar_start_gzip(arfd, gzip_program, 1);
                break;
        case APPND:
                if (name == NULL) {
                        arfd = STDOUT_FILENO;
                        arcname = stdo;
                } else if ((arfd = open(name, APP_MODE, DMOD)) < 0)
                        syswarn(0, errno, "Failed open to read/write on %s",
                                name);
                break;
        case COPY:
                /*
                 * arfd not used in COPY mode
                 */
                arcname = none;
                lstrval = 1;
                return(0);
        }
        if (arfd < 0)
                return(-1);

        if (chdname != NULL)
                if (chdir(chdname) != 0) {
                        syswarn(1, errno, "Failed chdir to %s", chdname);
                        return(-1);
                }
        /*
         * set up is based on device type
         */
        if (fstat(arfd, &arsb) < 0) {
                syswarn(0, errno, "Failed stat on %s", arcname);
                (void)close(arfd);
                arfd = -1;
                can_unlnk = 0;
                return(-1);
        }
        if (S_ISDIR(arsb.st_mode)) {
                paxwarn(0, "Cannot write an archive on top of a directory %s",
                    arcname);
                (void)close(arfd);
                arfd = -1;
                can_unlnk = 0;
                return(-1);
        }

        if (S_ISCHR(arsb.st_mode))
                artyp = ioctl(arfd, MTIOCGET, &mb) ? ISCHR : ISTAPE;
        else if (S_ISBLK(arsb.st_mode))
                artyp = ISBLK;
        else if ((lseek(arfd, (off_t)0L, SEEK_CUR) == -1) && (errno == ESPIPE))
                artyp = ISPIPE;
        else
                artyp = ISREG;

        /*
         * make sure we beyond any doubt that we only can unlink regular files
         * we created
         */
        if (artyp != ISREG)
                can_unlnk = 0;
        /*
         * if we are writing, we are done
         */
        if (act == ARCHIVE) {
                blksz = rdblksz = wrblksz;
                lstrval = 1;
                return(0);
        }

        /*
         * set default blksz on read. APPNDs writes rdblksz on the last volume
         * On all new archive volumes, we shift to wrblksz (if the user
         * specified one, otherwize we will continue to use rdblksz). We
         * must to set blocksize based on what kind of device the archive is
         * stored.
         */
        switch(artyp) {
        case ISTAPE:
                /*
                 * Tape drives come in at least two flavors. Those that support
                 * variable sized records and those that have fixed sized
                 * records. They must be treated differently. For tape drives
                 * that support variable sized records, we must make large
                 * reads to make sure we get the entire record, otherwise we
                 * will just get the first part of the record (up to size we
                 * asked). Tapes with fixed sized records may or may not return
                 * multiple records in a single read. We really do not care
                 * what the physical record size is UNLESS we are going to
                 * append. (We will need the physical block size to rewrite
                 * the trailer). Only when we are appending do we go to the
                 * effort to figure out the true PHYSICAL record size.
                 */
                blksz = rdblksz = MAXBLK;
                break;
        case ISPIPE:
        case ISBLK:
        case ISCHR:
                /*
                 * Blocksize is not a major issue with these devices (but must
                 * be kept a multiple of 512). If the user specified a write
                 * block size, we use that to read. Under append, we must
                 * always keep blksz == rdblksz. Otherwise we go ahead and use
                 * the device optimal blocksize as (and if) returned by stat
                 * and if it is within pax specs.
                 */
                if ((act == APPND) && wrblksz) {
                        blksz = rdblksz = wrblksz;
                        break;
                }

#if 0   /* Not on minix */
                if ((arsb.st_blksize > 0) && (arsb.st_blksize < MAXBLK) &&
                    ((arsb.st_blksize % BLKMULT) == 0))
                        rdblksz = arsb.st_blksize;
                else
#endif
                        rdblksz = DEVBLK;
                /*
                 * For performance go for large reads when we can without harm
                 */
                if ((act == APPND) || (artyp == ISCHR))
                        blksz = rdblksz;
                else
                        blksz = MAXBLK;
                break;
        case ISREG:
                /*
                 * if the user specified wrblksz works, use it. Under appends
                 * we must always keep blksz == rdblksz
                 */
                if ((act == APPND) && wrblksz && ((arsb.st_size%wrblksz)==0)){
                        blksz = rdblksz = wrblksz;
                        break;
                }
                /*
                 * See if we can find the blocking factor from the file size
                 */
                for (rdblksz = MAXBLK; rdblksz > 0; rdblksz -= BLKMULT)
                        if ((arsb.st_size % rdblksz) == 0)
                                break;
                /*
                 * When we cannot find a match, we may have a flawed archive.
                 */
                if (rdblksz <= 0)
                        rdblksz = FILEBLK;
                /*
                 * for performance go for large reads when we can
                 */
                if (act == APPND)
                        blksz = rdblksz;
                else
                        blksz = MAXBLK;
                break;
        default:
                /*
                 * should never happen, worse case, slow...
                 */
                blksz = rdblksz = BLKMULT;
                break;
        }
        lstrval = 1;
        return(0);
}

/*
 * ar_close()
 *      closes archive device, increments volume number, and prints i/o summary
 */
void
ar_close(void)
{
        int status;

        if (arfd < 0) {
                did_io = io_ok = flcnt = 0;
                return;
        }

        /*
         * Close archive file. This may take a LONG while on tapes (we may be
         * forced to wait for the rewind to complete) so tell the user what is
         * going on (this avoids the user hitting control-c thinking pax is
         * broken).
         */
        if (vflag && (artyp == ISTAPE)) {
                if (vfpart)
                        (void)putc('\n', listf);
                (void)fprintf(listf,
                        "%s: Waiting for tape drive close to complete...",
                        argv0);
                (void)fflush(listf);
        }

        /*
         * if nothing was written to the archive (and we created it), we remove
         * it
         */
        if (can_unlnk && (fstat(arfd, &arsb) == 0) && (S_ISREG(arsb.st_mode)) &&
            (arsb.st_size == 0)) {
                (void)unlink(arcname);
                can_unlnk = 0;
        }

        /*
         * for a quick extract/list, pax frequently exits before the child
         * process is done
         */
        if ((act == LIST || act == EXTRACT) && nflag && zpid > 0)
                kill(zpid, SIGINT);

        (void)close(arfd);

        /* Do not exit before child to ensure data integrity */
        if (zpid > 0)
                waitpid(zpid, &status, 0);

        if (vflag && (artyp == ISTAPE)) {
                (void)fputs("done.\n", listf);
                vfpart = 0;
                (void)fflush(listf);
        }
        arfd = -1;

        if (!io_ok && !did_io) {
                flcnt = 0;
                return;
        }
        did_io = io_ok = 0;

        /*
         * The volume number is only increased when the last device has data
         * and we have already determined the archive format.
         */
        if (frmt != NULL)
                ++arvol;

        if (!vflag) {
                flcnt = 0;
                return;
        }

        /*
         * Print out a summary of I/O for this archive volume.
         */
        if (vfpart) {
                (void)putc('\n', listf);
                vfpart = 0;
        }

        /*
         * If we have not determined the format yet, we just say how many bytes
         * we have skipped over looking for a header to id. there is no way we
         * could have written anything yet.
         */
        if (frmt == NULL) {
#       ifdef NET2_STAT
                (void)fprintf(listf, "%s: unknown format, %lu bytes skipped.\n",
                    argv0, rdcnt);
#       else
                (void)fprintf(listf, "%s: unknown format, %ju bytes skipped.\n",
                    argv0, (uintmax_t)rdcnt);
#       endif
                (void)fflush(listf);
                flcnt = 0;
                return;
        }

        if (strcmp(NM_CPIO, argv0) == 0)
                (void)fprintf(listf, "%llu blocks\n",
                    (unsigned long)((rdcnt ? rdcnt : wrcnt) / 5120));
        else if (strcmp(NM_TAR, argv0) != 0)
                (void)fprintf(listf,
#       ifdef NET2_STAT
                    "%s: %s vol %d, %lu files, %lu bytes read, %lu bytes written.\n",
                    argv0, frmt->name, arvol-1, flcnt, rdcnt, wrcnt);
#       else
                    "%s: %s vol %d, %ju files, %ju bytes read, %ju bytes written.\n",
                    argv0, frmt->name, arvol-1, (uintmax_t)flcnt,
                    (uintmax_t)rdcnt, (uintmax_t)wrcnt);
#       endif
        (void)fflush(listf);
        flcnt = 0;
}

/*
 * ar_drain()
 *      drain any archive format independent padding from an archive read
 *      from a socket or a pipe. This is to prevent the process on the
 *      other side of the pipe from getting a SIGPIPE (pax will stop
 *      reading an archive once a format dependent trailer is detected).
 */
void
ar_drain(void)
{
        int res;
        char drbuf[MAXBLK];

        /*
         * we only drain from a pipe/socket. Other devices can be closed
         * without reading up to end of file. We sure hope that pipe is closed
         * on the other side so we will get an EOF.
         */
        if ((artyp != ISPIPE) || (lstrval <= 0))
                return;

        /*
         * keep reading until pipe is drained
         */
        while ((res = read(arfd, drbuf, sizeof(drbuf))) > 0)
                ;
        lstrval = res;
}

/*
 * ar_set_wr()
 *      Set up device right before switching from read to write in an append.
 *      device dependent code (if required) to do this should be added here.
 *      For all archive devices we are already positioned at the place we want
 *      to start writing when this routine is called.
 * Return:
 *      0 if all ready to write, -1 otherwise
 */

int
ar_set_wr(void)
{
        off_t cpos;

        /*
         * we must make sure the trailer is rewritten on append, ar_next()
         * will stop us if the archive containing the trailer was not written
         */
        wr_trail = 0;

        /*
         * Add any device dependent code as required here
         */
        if (artyp != ISREG)
                return(0);
        /*
         * Ok we have an archive in a regular file. If we were rewriting a
         * file, we must get rid of all the stuff after the current offset
         * (it was not written by pax).
         */
        if (((cpos = lseek(arfd, (off_t)0L, SEEK_CUR)) < 0) ||
            (ftruncate(arfd, cpos) < 0)) {
                syswarn(1, errno, "Unable to truncate archive file");
                return(-1);
        }
        return(0);
}

/*
 * ar_app_ok()
 *      check if the last volume in the archive allows appends. We cannot check
 *      this until we are ready to write since there is no spec that says all
 *      volumes in a single archive have to be of the same type...
 * Return:
 *      0 if we can append, -1 otherwise.
 */

int
ar_app_ok(void)
{
        if (artyp == ISPIPE) {
                paxwarn(1, "Cannot append to an archive obtained from a pipe.");
                return(-1);
        }

        if (!invld_rec)
                return(0);
        paxwarn(1,"Cannot append, device record size %d does not support %s spec",
                rdblksz, argv0);
        return(-1);
}

/*
 * ar_read()
 *      read up to a specified number of bytes from the archive into the
 *      supplied buffer. When dealing with tapes we may not always be able to
 *      read what we want.
 * Return:
 *      Number of bytes in buffer. 0 for end of file, -1 for a read error.
 */

int
ar_read(char *buf, int cnt)
{
        int res = 0;

        /*
         * if last i/o was in error, no more reads until reset or new volume
         */
        if (lstrval <= 0)
                return(lstrval);

        /*
         * how we read must be based on device type
         */
        switch (artyp) {
        case ISTAPE:
                if ((res = read(arfd, buf, cnt)) > 0) {
                        /*
                         * CAUTION: tape systems may not always return the same
                         * sized records so we leave blksz == MAXBLK. The
                         * physical record size that a tape drive supports is
                         * very hard to determine in a uniform and portable
                         * manner.
                         */
                        io_ok = 1;
                        if (res != rdblksz) {
                                /*
                                 * Record size changed. If this is happens on
                                 * any record after the first, we probably have
                                 * a tape drive which has a fixed record size
                                 * we are getting multiple records in a single
                                 * read). Watch out for record blocking that
                                 * violates pax spec (must be a multiple of
                                 * BLKMULT).
                                 */
                                rdblksz = res;
                                if (rdblksz % BLKMULT)
                                        invld_rec = 1;
                        }
                        return(res);
                }
                break;
        case ISREG:
        case ISBLK:
        case ISCHR:
        case ISPIPE:
        default:
                /*
                 * Files are so easy to deal with. These other things cannot
                 * be trusted at all. So when we are dealing with character
                 * devices and pipes we just take what they have ready for us
                 * and return. Trying to do anything else with them runs the
                 * risk of failure.
                 */
                if ((res = read(arfd, buf, cnt)) > 0) {
                        io_ok = 1;
                        return(res);
                }
                break;
        }

        /*
         * We are in trouble at this point, something is broken...
         */
        lstrval = res;
        if (res < 0)
                syswarn(1, errno, "Failed read on archive volume %d", arvol);
        else
                paxwarn(0, "End of archive volume %d reached", arvol);
        return(res);
}

/*
 * ar_write()
 *      Write a specified number of bytes in supplied buffer to the archive
 *      device so it appears as a single "block". Deals with errors and tries
 *      to recover when faced with short writes.
 * Return:
 *      Number of bytes written. 0 indicates end of volume reached and with no
 *      flaws (as best that can be detected). A -1 indicates an unrecoverable
 *      error in the archive occured.
 */

int
ar_write(char *buf, int bsz)
{
        int res;
        off_t cpos;

        /*
         * do not allow pax to create a "bad" archive. Once a write fails on
         * an archive volume prevent further writes to it.
         */
        if (lstrval <= 0)
                return(lstrval);

        if ((res = write(arfd, buf, bsz)) == bsz) {
                wr_trail = 1;
                io_ok = 1;
                return(bsz);
        }
        /*
         * write broke, see what we can do with it. We try to send any partial
         * writes that may violate pax spec to the next archive volume.
         */
        if (res < 0)
                lstrval = res;
        else
                lstrval = 0;

        switch (artyp) {
        case ISREG:
                if ((res > 0) && (res % BLKMULT)) {
                        /*
                         * try to fix up partial writes which are not BLKMULT
                         * in size by forcing the runt record to next archive
                         * volume
                         */
                        if ((cpos = lseek(arfd, (off_t)0L, SEEK_CUR)) < 0)
                                break;
                        cpos -= (off_t)res;
                        if (ftruncate(arfd, cpos) < 0)
                                break;
                        res = lstrval = 0;
                        break;
                }
                if (res >= 0)
                        break;
                /*
                 * if file is out of space, handle it like a return of 0
                 */
                if ((errno == ENOSPC) || (errno == EFBIG)
#ifdef EDQUOT
                        || (errno == EDQUOT)
#endif
                        )
                        res = lstrval = 0;
                break;
        case ISTAPE:
        case ISCHR:
        case ISBLK:
                if (res >= 0)
                        break;
                if (errno == EACCES) {
                        paxwarn(0, "Write failed, archive is write protected.");
                        res = lstrval = 0;
                        return(0);
                }
                /*
                 * see if we reached the end of media, if so force a change to
                 * the next volume
                 */
                if ((errno == ENOSPC) || (errno == EIO) || (errno == ENXIO))
                        res = lstrval = 0;
                break;
        case ISPIPE:
        default:
                /*
                 * we cannot fix errors to these devices
                 */
                break;
        }

        /*
         * Better tell the user the bad news...
         * if this is a block aligned archive format, we may have a bad archive
         * if the format wants the header to start at a BLKMULT boundary. While
         * we can deal with the mis-aligned data, it violates spec and other
         * archive readers will likely fail. if the format is not block
         * aligned, the user may be lucky (and the archive is ok).
         */
        if (res >= 0) {
                if (res > 0)
                        wr_trail = 1;
                io_ok = 1;
        }

        /*
         * If we were trying to rewrite the trailer and it didn't work, we
         * must quit right away.
         */
        if (!wr_trail && (res <= 0)) {
                paxwarn(1,"Unable to append, trailer re-write failed. Quitting.");
                return(res);
        }

        if (res == 0)
                paxwarn(0, "End of archive volume %d reached", arvol);
        else if (res < 0)
                syswarn(1, errno, "Failed write to archive volume: %d", arvol);
        else if (!frmt->blkalgn || ((res % frmt->blkalgn) == 0))
                paxwarn(0,"WARNING: partial archive write. Archive MAY BE FLAWED");
        else
                paxwarn(1,"WARNING: partial archive write. Archive IS FLAWED");
        return(res);
}

/*
 * ar_rdsync()
 *      Try to move past a bad spot on a flawed archive as needed to continue
 *      I/O. Clears error flags to allow I/O to continue.
 * Return:
 *      0 when ok to try i/o again, -1 otherwise.
 */

int
ar_rdsync(void)
{
        long fsbz;
        off_t cpos;
        off_t mpos;
        struct mtop mb;

        /*
         * Fail resync attempts at user request (done) or this is going to be
         * an update/append to an existing archive. If last i/o hit media end,
         * we need to go to the next volume not try a resync.
         */
        if ((done > 0) || (lstrval == 0))
                return(-1);

        if ((act == APPND) || (act == ARCHIVE)) {
                paxwarn(1, "Cannot allow updates to an archive with flaws.");
                return(-1);
        }
        if (io_ok)
                did_io = 1;

        switch(artyp) {
        case ISTAPE:
                /*
                 * if the last i/o was a successful data transfer, we assume
                 * the fault is just a bad record on the tape that we are now
                 * past. If we did not get any data since the last resync try
                 * to move the tape forward one PHYSICAL record past any
                 * damaged tape section. Some tape drives are stubborn and need
                 * to be pushed.
                 */
                if (io_ok) {
                        io_ok = 0;
                        lstrval = 1;
                        break;
                }
                mb.mt_op = MTFSR;
                mb.mt_count = 1;
                if (ioctl(arfd, MTIOCTOP, &mb) < 0)
                        break;
                lstrval = 1;
                break;
        case ISREG:
        case ISCHR:
        case ISBLK:
                /*
                 * try to step over the bad part of the device.
                 */
                io_ok = 0;
#if 0
        /* no blksize on minix */
                if (((fsbz = arsb.st_blksize) <= 0) || (artyp != ISREG))
#endif
                        fsbz = BLKMULT;
                if ((cpos = lseek(arfd, (off_t)0L, SEEK_CUR)) < 0)
                        break;
                mpos = fsbz - (cpos % (off_t)fsbz);
                if (lseek(arfd, mpos, SEEK_CUR) < 0)
                        break;
                lstrval = 1;
                break;
        case ISPIPE:
        default:
                /*
                 * cannot recover on these archive device types
                 */
                io_ok = 0;
                break;
        }
        if (lstrval <= 0) {
                paxwarn(1, "Unable to recover from an archive read failure.");
                return(-1);
        }
        paxwarn(0, "Attempting to recover from an archive read failure.");
        return(0);
}

/*
 * ar_fow()
 *      Move the I/O position within the archive foward the specified number of
 *      bytes as supported by the device. If we cannot move the requested
 *      number of bytes, return the actual number of bytes moved in skipped.
 * Return:
 *      0 if moved the requested distance, -1 on complete failure, 1 on
 *      partial move (the amount moved is in skipped)
 */

int
ar_fow(off_t sksz, off_t *skipped)
{
        off_t cpos;
        off_t mpos;

        *skipped = 0;
        if (sksz <= 0)
                return(0);

        /*
         * we cannot move foward at EOF or error
         */
        if (lstrval <= 0)
                return(lstrval);

        /*
         * Safer to read forward on devices where it is hard to find the end of
         * the media without reading to it. With tapes we cannot be sure of the
         * number of physical blocks to skip (we do not know physical block
         * size at this point), so we must only read foward on tapes!
         */
        if (artyp != ISREG)
                return(0);

        /*
         * figure out where we are in the archive
         */
        if ((cpos = lseek(arfd, (off_t)0L, SEEK_CUR)) >= 0) {
                /*
                 * we can be asked to move farther than there are bytes in this
                 * volume, if so, just go to file end and let normal buf_fill()
                 * deal with the end of file (it will go to next volume by
                 * itself)
                 */
                if ((mpos = cpos + sksz) > arsb.st_size) {
                        *skipped = arsb.st_size - cpos;
                        mpos = arsb.st_size;
                } else
                        *skipped = sksz;
                if (lseek(arfd, mpos, SEEK_SET) >= 0)
                        return(0);
        }
        syswarn(1, errno, "Forward positioning operation on archive failed");
        lstrval = -1;
        return(-1);
}

/*
 * ar_rev()
 *      move the i/o position within the archive backwards the specified byte
 *      count as supported by the device. With tapes drives we RESET rdblksz to
 *      the PHYSICAL blocksize.
 *      NOTE: We should only be called to move backwards so we can rewrite the
 *      last records (the trailer) of an archive (APPEND).
 * Return:
 *      0 if moved the requested distance, -1 on complete failure
 */

int
ar_rev(off_t sksz)
{
        off_t cpos;
        struct mtop mb;
        int phyblk;

        /*
         * make sure we do not have try to reverse on a flawed archive
         */
        if (lstrval < 0)
                return(lstrval);

        switch(artyp) {
        case ISPIPE:
                if (sksz <= 0)
                        break;
                /*
                 * cannot go backwards on these critters
                 */
                paxwarn(1, "Reverse positioning on pipes is not supported.");
                lstrval = -1;
                return(-1);
        case ISREG:
        case ISBLK:
        case ISCHR:
        default:
                if (sksz <= 0)
                        break;

                /*
                 * For things other than files, backwards movement has a very
                 * high probability of failure as we really do not know the
                 * true attributes of the device we are talking to (the device
                 * may not even have the ability to lseek() in any direction).
                 * First we figure out where we are in the archive.
                 */
                if ((cpos = lseek(arfd, (off_t)0L, SEEK_CUR)) < 0) {
                        syswarn(1, errno,
                           "Unable to obtain current archive byte offset");
                        lstrval = -1;
                        return(-1);
                }

                /*
                 * we may try to go backwards past the start when the archive
                 * is only a single record. If this hapens and we are on a
                 * multi volume archive, we need to go to the end of the
                 * previous volume and continue our movement backwards from
                 * there.
                 */
                if ((cpos -= sksz) < (off_t)0L) {
                        if (arvol > 1) {
                                /*
                                 * this should never happen
                                 */
                                paxwarn(1,"Reverse position on previous volume.");
                                lstrval = -1;
                                return(-1);
                        }
                        cpos = (off_t)0L;
                }
                if (lseek(arfd, cpos, SEEK_SET) < 0) {
                        syswarn(1, errno, "Unable to seek archive backwards");
                        lstrval = -1;
                        return(-1);
                }
                break;
        case ISTAPE:
                /*
                 * Calculate and move the proper number of PHYSICAL tape
                 * blocks. If the sksz is not an even multiple of the physical
                 * tape size, we cannot do the move (this should never happen).
                 * (We also cannot handler trailers spread over two vols).
                 * get_phys() also makes sure we are in front of the filemark.
                 */
                if ((phyblk = get_phys()) <= 0) {
                        lstrval = -1;
                        return(-1);
                }

                /*
                 * make sure future tape reads only go by physical tape block
                 * size (set rdblksz to the real size).
                 */
                rdblksz = phyblk;

                /*
                 * if no movement is required, just return (we must be after
                 * get_phys() so the physical blocksize is properly set)
                 */
                if (sksz <= 0)
                        break;

                /*
                 * ok we have to move. Make sure the tape drive can do it.
                 */
                if (sksz % phyblk) {
                        paxwarn(1,
                            "Tape drive unable to backspace requested amount");
                        lstrval = -1;
                        return(-1);
                }

                /*
                 * move backwards the requested number of bytes
                 */
                mb.mt_op = MTBSR;
                mb.mt_count = sksz/phyblk;
                if (ioctl(arfd, MTIOCTOP, &mb) < 0) {
                        syswarn(1,errno, "Unable to backspace tape %d blocks.",
                            mb.mt_count);
                        lstrval = -1;
                        return(-1);
                }
                break;
        }
        lstrval = 1;
        return(0);
}

/*
 * get_phys()
 *      Determine the physical block size on a tape drive. We need the physical
 *      block size so we know how many bytes we skip over when we move with
 *      mtio commands. We also make sure we are BEFORE THE TAPE FILEMARK when
 *      return.
 *      This is one really SLOW routine...
 * Return:
 *      physical block size if ok (ok > 0), -1 otherwise
 */

static int
get_phys(void)
{
        int padsz = 0;
        int res;
        int phyblk;
        struct mtop mb;
        char scbuf[MAXBLK];

        /*
         * move to the file mark, and then back up one record and read it.
         * this should tell us the physical record size the tape is using.
         */
        if (lstrval == 1) {
                /*
                 * we know we are at file mark when we get back a 0 from
                 * read()
                 */
                while ((res = read(arfd, scbuf, sizeof(scbuf))) > 0)
                        padsz += res;
                if (res < 0) {
                        syswarn(1, errno, "Unable to locate tape filemark.");
                        return(-1);
                }
        }

        /*
         * move backwards over the file mark so we are at the end of the
         * last record.
         */
        mb.mt_op = MTBSF;
        mb.mt_count = 1;
        if (ioctl(arfd, MTIOCTOP, &mb) < 0) {
                syswarn(1, errno, "Unable to backspace over tape filemark.");
                return(-1);
        }

        /*
         * move backwards so we are in front of the last record and read it to
         * get physical tape blocksize.
         */
        mb.mt_op = MTBSR;
        mb.mt_count = 1;
        if (ioctl(arfd, MTIOCTOP, &mb) < 0) {
                syswarn(1, errno, "Unable to backspace over last tape block.");
                return(-1);
        }
        if ((phyblk = read(arfd, scbuf, sizeof(scbuf))) <= 0) {
                syswarn(1, errno, "Cannot determine archive tape blocksize.");
                return(-1);
        }

        /*
         * read foward to the file mark, then back up in front of the filemark
         * (this is a bit paranoid, but should be safe to do).
         */
        while ((res = read(arfd, scbuf, sizeof(scbuf))) > 0)
                ;
        if (res < 0) {
                syswarn(1, errno, "Unable to locate tape filemark.");
                return(-1);
        }
        mb.mt_op = MTBSF;
        mb.mt_count = 1;
        if (ioctl(arfd, MTIOCTOP, &mb) < 0) {
                syswarn(1, errno, "Unable to backspace over tape filemark.");
                return(-1);
        }

        /*
         * set lstrval so we know that the filemark has not been seen
         */
        lstrval = 1;

        /*
         * return if there was no padding
         */
        if (padsz == 0)
                return(phyblk);

        /*
         * make sure we can move backwards over the padding. (this should
         * never fail).
         */
        if (padsz % phyblk) {
                paxwarn(1, "Tape drive unable to backspace requested amount");
                return(-1);
        }

        /*
         * move backwards over the padding so the head is where it was when
         * we were first called (if required).
         */
        mb.mt_op = MTBSR;
        mb.mt_count = padsz/phyblk;
        if (ioctl(arfd, MTIOCTOP, &mb) < 0) {
                syswarn(1,errno,"Unable to backspace tape over %d pad blocks",
                    mb.mt_count);
                return(-1);
        }
        return(phyblk);
}

/*
 * ar_next()
 *      prompts the user for the next volume in this archive. For some devices
 *      we may allow the media to be changed. Otherwise a new archive is
 *      prompted for. By pax spec, if there is no controlling tty or an eof is
 *      read on tty input, we must quit pax.
 * Return:
 *      0 when ready to continue, -1 when all done
 */

int
ar_next(void)
{
        char buf[PAXPATHLEN+2];
        static int freeit = 0;
        sigset_t o_mask;

        /*
         * WE MUST CLOSE THE DEVICE. A lot of devices must see last close, (so
         * things like writing EOF etc will be done) (Watch out ar_close() can
         * also be called via a signal handler, so we must prevent a race.
         */
        if (sigprocmask(SIG_BLOCK, &s_mask, &o_mask) < 0)
                syswarn(0, errno, "Unable to set signal mask");
        ar_close();
        if (sigprocmask(SIG_SETMASK, &o_mask, NULL) < 0)
                syswarn(0, errno, "Unable to restore signal mask");

        if (done || !wr_trail || strcmp(NM_TAR, argv0) == 0)
                return(-1);

        tty_prnt("\nATTENTION! %s archive volume change required.\n", argv0);

        /*
         * if i/o is on stdin or stdout, we cannot reopen it (we do not know
         * the name), the user will be forced to type it in.
         */
        if (strcmp(arcname, stdo) && strcmp(arcname, stdn) && (artyp != ISREG)
            && (artyp != ISPIPE)) {
                if (artyp == ISTAPE) {
                        tty_prnt("%s ready for archive tape volume: %d\n",
                                arcname, arvol);
                        tty_prnt("Load the NEXT TAPE on the tape drive");
                } else {
                        tty_prnt("%s ready for archive volume: %d\n",
                                arcname, arvol);
                        tty_prnt("Load the NEXT STORAGE MEDIA (if required)");
                }

                if ((act == ARCHIVE) || (act == APPND))
                        tty_prnt(" and make sure it is WRITE ENABLED.\n");
                else
                        tty_prnt("\n");

                for(;;) {
                        tty_prnt("Type \"y\" to continue, \".\" to quit %s,",
                                argv0);
                        tty_prnt(" or \"s\" to switch to new device.\nIf you");
                        tty_prnt(" cannot change storage media, type \"s\"\n");
                        tty_prnt("Is the device ready and online? > ");

                        if ((tty_read(buf,sizeof(buf))<0) || !strcmp(buf,".")){
                                done = 1;
                                lstrval = -1;
                                tty_prnt("Quitting %s!\n", argv0);
                                vfpart = 0;
                                return(-1);
                        }

                        if ((buf[0] == '\0') || (buf[1] != '\0')) {
                                tty_prnt("%s unknown command, try again\n",buf);
                                continue;
                        }

                        switch (buf[0]) {
                        case 'y':
                        case 'Y':
                                /*
                                 * we are to continue with the same device
                                 */
                                if (ar_open(arcname) >= 0)
                                        return(0);
                                tty_prnt("Cannot re-open %s, try again\n",
                                        arcname);
                                continue;
                        case 's':
                        case 'S':
                                /*
                                 * user wants to open a different device
                                 */
                                tty_prnt("Switching to a different archive\n");
                                break;
                        default:
                                tty_prnt("%s unknown command, try again\n",buf);
                                continue;
                        }
                        break;
                }
        } else
                tty_prnt("Ready for archive volume: %d\n", arvol);

        /*
         * have to go to a different archive
         */
        for (;;) {
                tty_prnt("Input archive name or \".\" to quit %s.\n", argv0);
                tty_prnt("Archive name > ");

                if ((tty_read(buf, sizeof(buf)) < 0) || !strcmp(buf, ".")) {
                        done = 1;
                        lstrval = -1;
                        tty_prnt("Quitting %s!\n", argv0);
                        vfpart = 0;
                        return(-1);
                }
                if (buf[0] == '\0') {
                        tty_prnt("Empty file name, try again\n");
                        continue;
                }
                if (!strcmp(buf, "..")) {
                        tty_prnt("Illegal file name: .. try again\n");
                        continue;
                }
                if (strlen(buf) > PAXPATHLEN) {
                        tty_prnt("File name too long, try again\n");
                        continue;
                }

                /*
                 * try to open new archive
                 */
                if (ar_open(buf) >= 0) {
                        if (freeit) {
                                (void)free((char *)(uintptr_t)arcname);
                                freeit = 0;
                        }
                        if ((arcname = strdup(buf)) == NULL) {
                                done = 1;
                                lstrval = -1;
                                paxwarn(0, "Cannot save archive name.");
                                return(-1);
                        }
                        freeit = 1;
                        break;
                }
                tty_prnt("Cannot open %s, try again\n", buf);
                continue;
        }
        return(0);
}

/*
 * ar_start_gzip()
 * starts the gzip compression/decompression process as a child, using magic
 * to keep the fd the same in the calling function (parent).
 */
static void
ar_start_gzip(int fd, const char *gzip_prog, int wr)
{
        int fds[2];
        const char *gzip_flags;

        if (pipe(fds) < 0)
                err(1, "could not pipe");
        zpid = fork();
        if (zpid < 0)
                err(1, "could not fork");

        /* parent */
        if (zpid) {
                if (wr)
                        dup2(fds[1], fd);
                else
                        dup2(fds[0], fd);
                close(fds[0]);
                close(fds[1]);
        } else {
                if (wr) {
                        dup2(fds[0], STDIN_FILENO);
                        dup2(fd, STDOUT_FILENO);
                        gzip_flags = "-c";
                } else {
                        dup2(fds[1], STDOUT_FILENO);
                        dup2(fd, STDIN_FILENO);
                        gzip_flags = "-dc";
                }
                close(fds[0]);
                close(fds[1]);
                if (execlp(gzip_prog, gzip_prog, gzip_flags,
                    (char *)NULL) < 0)
                        err(1, "could not exec");
                /* NOTREACHED */
        }
}

---