port/stdio/flush.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */

/*
 * Copyright (c) 1988, 2010, Oracle and/or its affiliates. All rights reserved.
 */

/*  Copyright (c) 1988 AT&T */
/*    All Rights Reserved   */

#include "lint.h"
#include "mtlib.h"
#include "file64.h"
#include <sys/types.h>
#include <stdlib.h>
#include <stdio.h>
#include <thread.h>
#include <synch.h>
#include <unistd.h>
#include <string.h>
#include "stdiom.h"
#include <wchar.h>
#include <sys/stat.h>
#include <stddef.h>
#include <errno.h>
#include <fcntl.h>

#define _iob    __iob

#undef end

#define FILE_ARY_SZ 8 /* a nice size for FILE array & end_buffer_ptrs */

#ifdef  _LP64

/*
 * Macros to declare and loop over a fp or fp/xfp combo to
 * avoid some of the _LP64 ifdef hell.
 */

#define FPDECL(fp)      FILE *fp
#define FIRSTFP(lp, fp)     fp = lp->iobp
#define NEXTFP(fp)      fp++
#define FPLOCK(fp)      &fp->_lock
#define FPSTATE(fp)     &fp->_state

#define xFILE           FILE

#else

#define FPDECL(fp)      FILE *fp; xFILE *x##fp
#define FIRSTFP(lp, fp)     x##fp = lp->iobp; \
                fp = x##fp ? &x##fp->_iob : &_iob[0]
#define NEXTFP(fp)      (x##fp ? fp = &(++x##fp)->_iob : ++fp)
#define FPLOCK(fp)      x##fp ? \
                    &x##fp->xlock : &_xftab[IOPIND(fp)]._lock
#define FPSTATE(fp)     x##fp ? \
                    &x##fp->xstate : &_xftab[IOPIND(fp)]._state

/* The extended 32-bit file structure for use in link buffers */
typedef struct xFILE {
    FILE            _iob;       /* must be first! */
    struct xFILEdata    _xdat;
} xFILE;

#define xmagic          _xdat._magic
#define xend            _xdat._end
#define xlock           _xdat._lock
#define xstate          _xdat._state

#define FILEx(fp)       ((struct xFILE *)(uintptr_t)fp)

/*
 * The magic number stored is actually the pointer scrambled with
 * a magic number.  Pointers to data items live everywhere in memory
 * so we scramble the pointer in order to avoid accidental collisions.
 */
#define XFILEMAGIC      0x63687367
#define XMAGIC(xfp)     ((uintptr_t)(xfp) ^ XFILEMAGIC)

#endif /* _LP64 */

struct _link_   /* manages a list of streams */
{
    xFILE *iobp;        /* the array of (x)FILE's */
                /* NULL for the __first_link in ILP32 */
    int niob;       /* length of the arrays */
    struct _link_   *next;  /* next in the list */
};

/*
 * With dynamic linking, iob may be in either the library or in the user's
 * a.out, so the run time linker fixes up the first entry in __first_link at
 * process startup time.
 *
 * In 32 bit processes, we don't have xFILE[FILE_ARY_SZ] but FILE[],
 * and _xftab[] instead; this is denoted by having iobp set to NULL in
 * 32 bit mode for the first link entry.
 */
struct _link_ __first_link =    /* first in linked list */
{
#if !defined(_LP64)
    NULL,
#else
    &_iob[0],
#endif
    _NFILE,
    NULL
};

/*
 * Information cached to speed up searches.  We remember where we
 * last found a free FILE* and we remember whether we saw any fcloses
 * in between.  We also count the number of chunks we allocated, see
 * _findiop() for an explanation.
 * These variables are all protected by _first_link_lock.
 */
static struct _link_ *lastlink = NULL;
static int fcloses;
static int nchunks;

static mutex_t _first_link_lock = DEFAULTMUTEX;

static int _fflush_l_iops(void);
static FILE *getiop(FILE *, rmutex_t *, mbstate_t *);

/*
 * All functions that understand the linked list of iob's follow.
 */
#pragma weak _cleanup = __cleanup
void
__cleanup(void)     /* called at process end to flush ouput streams */
{
    (void) fflush(NULL);
}

/*
 * For fork1-safety (see libc_prepare_atfork(), etc).
 */
void
stdio_locks()
{
    (void) mutex_lock(&_first_link_lock);
    /*
     * XXX: We should acquire all of the iob locks here.
     */
}

void
stdio_unlocks()
{
    /*
     * XXX: We should release all of the iob locks here.
     */
    (void) mutex_unlock(&_first_link_lock);
}

void
_flushlbf(void)     /* fflush() all line-buffered streams */
{
    FPDECL(fp);
    int i;
    struct _link_ *lp;
    /* Allow compiler to optimize the loop */
    int threaded = __libc_threaded;

    if (threaded)
        cancel_safe_mutex_lock(&_first_link_lock);

    lp = &__first_link;
    do {
        FIRSTFP(lp, fp);
        for (i = lp->niob; --i >= 0; NEXTFP(fp)) {
            /*
             * The additional _IONBF check guards againsts
             * allocated but uninitialized iops (see _findiop).
             * We also automatically skip non allocated iop's.
             * Don't block on locks.
             */
            if ((fp->_flag & (_IOLBF | _IOWRT | _IONBF)) ==
                (_IOLBF | _IOWRT)) {
                if (threaded) {
                    rmutex_t *lk = FPLOCK(fp);
                    if (cancel_safe_mutex_trylock(lk) != 0)
                        continue;
                    /* Recheck after locking */
                    if ((fp->_flag & (_IOLBF | _IOWRT)) ==
                        (_IOLBF | _IOWRT)) {
                        (void) _fflush_u(fp);
                    }
                    cancel_safe_mutex_unlock(lk);
                } else {
                    (void) _fflush_u(fp);
                }
            }
        }
    } while ((lp = lp->next) != NULL);

    if (threaded)
        cancel_safe_mutex_unlock(&_first_link_lock);
}

/* allocate an unused stream; NULL if cannot */
FILE *
_findiop(void)
{
    struct _link_ *lp, **prev;

    /* used so there only needs to be one malloc() */
#ifdef _LP64
    typedef struct  {
        struct _link_   hdr;
        FILE    iob[FILE_ARY_SZ];
    } Pkg;
#else
    typedef union {
        struct {                /* Normal */
            struct _link_   hdr;
            xFILE   iob[FILE_ARY_SZ];
        } Pkgn;
        struct {                /* Reversed */
            xFILE   iob[FILE_ARY_SZ];
            struct _link_   hdr;
        } Pkgr;
    } Pkg;
    uintptr_t delta;
#endif
    Pkg *pkgp;
    struct _link_ *hdr;
    FPDECL(fp);
    int i;
    int threaded = __libc_threaded;

    if (threaded)
        cancel_safe_mutex_lock(&_first_link_lock);

    if (lastlink == NULL) {
rescan:
        fcloses = 0;
        lastlink = &__first_link;
    }

    lp = lastlink;

    /*
     * lock to make testing of fp->_flag == 0 and acquiring the fp atomic
     * and for allocation of new links
     * low contention expected on _findiop(), hence coarse locking.
     * for finer granularity, use fp->_lock for allocating an iop
     * and make the testing of lp->next and allocation of new link atomic
     * using lp->_lock
     */

    do {
        prev = &lp->next;
        FIRSTFP(lp, fp);

        for (i = lp->niob; --i >= 0; NEXTFP(fp)) {
            FILE *ret;
            if (threaded) {
                ret = getiop(fp, FPLOCK(fp), FPSTATE(fp));
                if (ret != NULL) {
                    cancel_safe_mutex_unlock(
                        &_first_link_lock);
                    return (ret);
                }
            } else {
                ret = getiop(fp, NULL, FPSTATE(fp));
                if (ret != NULL)
                    return (ret);
            }
        }
    } while ((lastlink = lp = lp->next) != NULL);

    /*
     * If there was a sufficient number of  fcloses since we last started
     * at __first_link, we rescan all fp's again.  We do not rescan for
     * all fcloses; that would simplify the algorithm but would make
     * search times near O(n) again.
     * Worst case behaviour would still be pretty bad (open a full set,
     * then continously opening and closing one FILE * gets you a full
     * scan each time).  That's why we over allocate 1 FILE for each
     * 32 chunks.  More over allocation is better; this is a nice
     * empirical value which doesn't cost a lot of memory, doesn't
     * overallocate until we reach 256 FILE *s and keeps the performance
     * pretty close to the optimum.
     */
    if (fcloses > nchunks/32)
        goto rescan;

    /*
     * Need to allocate another and put it in the linked list.
     */
    if ((pkgp = malloc(sizeof (Pkg))) == NULL) {
        if (threaded)
            cancel_safe_mutex_unlock(&_first_link_lock);
        return (NULL);
    }

    (void) memset(pkgp, 0, sizeof (Pkg));

#ifdef _LP64
    hdr = &pkgp->hdr;
    hdr->iobp = &pkgp->iob[0];
#else
    /*
     * The problem with referencing a word after a FILE* is the possibility
     * of a SIGSEGV if a non-stdio issue FILE structure ends on a page
     * boundary.  We run this check so we never need to run an expensive
     * check like mincore() in order to know whether it is
     * safe to dereference ((xFILE*)fp)->xmagic.
     * We allocate the block with two alternative layouts; if one
     * layout is not properly aligned for our purposes, the other layout
     * will be because the size of _link_ is small compared to
     * sizeof (xFILE).
     * The check performed is this:
     *  If the distance from pkgp to the end of the page is
     *  less than the the offset of the last xmagic field in the
     *  xFILE structure, (the 0x1000 boundary is inside our just
     *  allocated structure) and the distance modulo the size of xFILE
     *  is identical to the offset of the first xmagic in the
     *  structure (i.e., XXXXXX000 points to an xmagic field),
     *  we need to use the reverse structure.
     */
    if ((delta = 0x1000 - ((uintptr_t)pkgp & 0xfff)) <=
        offsetof(Pkg, Pkgn.iob[FILE_ARY_SZ-1].xmagic) &&
        delta % sizeof (struct xFILE) ==
        offsetof(Pkg, Pkgn.iob[0].xmagic)) {
        /* Use reversed structure */
        hdr = &pkgp->Pkgr.hdr;
        hdr->iobp = &pkgp->Pkgr.iob[0];
    } else {
        /* Use normal structure */
        hdr = &pkgp->Pkgn.hdr;
        hdr->iobp = &pkgp->Pkgn.iob[0];
    }
#endif /* _LP64 */

    hdr->niob = FILE_ARY_SZ;
    nchunks++;

#ifdef  _LP64
    fp = hdr->iobp;
    for (i = 0; i < FILE_ARY_SZ; i++)
        (void) mutex_init(&fp[i]._lock,
            USYNC_THREAD | LOCK_RECURSIVE, NULL);
#else
    xfp = hdr->iobp;
    fp = &xfp->_iob;

    for (i = 0; i < FILE_ARY_SZ; i++) {
        xfp[i].xmagic = XMAGIC(&xfp[i]);
        (void) mutex_init(&xfp[i].xlock,
            USYNC_THREAD | LOCK_RECURSIVE, NULL);
    }
#endif  /*  _LP64   */

    lastlink = *prev = hdr;
    fp->_ptr = 0;
    fp->_base = 0;
    fp->_flag = 0377; /* claim the fp by setting low 8 bits */
    if (threaded)
        cancel_safe_mutex_unlock(&_first_link_lock);

    return (fp);
}

static void
isseekable(FILE *iop)
{
    struct stat64 fstatbuf;
    int save_errno;

    save_errno = errno;

    if (fstat64(GET_FD(iop), &fstatbuf) != 0) {
        /*
         * when we don't know what it is we'll
         * do the old behaviour and flush
         * the stream
         */
        SET_SEEKABLE(iop);
        errno = save_errno;
        return;
    }

    /*
     * check for what is non-SEEKABLE
     * otherwise assume it's SEEKABLE so we get the old
     * behaviour and flush the stream
     */

    if (S_ISFIFO(fstatbuf.st_mode) || S_ISCHR(fstatbuf.st_mode) ||
        S_ISSOCK(fstatbuf.st_mode) || S_ISDOOR(fstatbuf.st_mode)) {
        CLEAR_SEEKABLE(iop);
    } else {
        SET_SEEKABLE(iop);
    }

    errno = save_errno;
}

#ifdef  _LP64
void
_setbufend(FILE *iop, Uchar *end)   /* set the end pointer for this iop */
{
    iop->_end = end;

    isseekable(iop);
}

#undef _realbufend

Uchar *
_realbufend(FILE *iop)      /* get the end pointer for this iop */
{
    return (iop->_end);
}

#else /* _LP64 */

/*
 * Awkward functions not needed for the sane 64 bit environment.
 */
/*
 * xmagic must not be aligned on a 4K boundary. We guarantee this in
 * _findiop().
 */
#define VALIDXFILE(xfp) \
    (((uintptr_t)&(xfp)->xmagic & 0xfff) && \
        (xfp)->xmagic == XMAGIC(FILEx(xfp)))

static struct xFILEdata *
getxfdat(FILE *iop)
{
    if (STDIOP(iop))
        return (&_xftab[IOPIND(iop)]);
    else if (VALIDXFILE(FILEx(iop)))
        return (&FILEx(iop)->_xdat);
    else
        return (NULL);
}

void
_setbufend(FILE *iop, Uchar *end)   /* set the end pointer for this iop */
{
    struct xFILEdata *dat = getxfdat(iop);

    if (dat != NULL)
        dat->_end = end;

    isseekable(iop);

    /*
     * For binary compatibility with user programs using the
     * old _bufend macro.  This is *so* broken, fileno()
     * is not the proper index.
     */
    if (iop->_magic < _NFILE)
        _bufendtab[iop->_magic] = end;

}

Uchar *
_realbufend(FILE *iop)      /* get the end pointer for this iop */
{
    struct xFILEdata *dat = getxfdat(iop);

    if (dat != NULL)
        return (dat->_end);

    return (NULL);
}

/*
 * _reallock() is invoked in each stdio call through the IOB_LCK() macro,
 * it is therefor extremely performance sensitive.  We get better performance
 * by inlining the STDIOP check in IOB_LCK and inlining a custom version
 * of getfxdat() here.
 */
rmutex_t *
_reallock(FILE *iop)
{
    if (VALIDXFILE(FILEx(iop)))
        return (&FILEx(iop)->xlock);

    return (NULL);
}

#endif  /*  _LP64   */

/* make sure _cnt, _ptr are correct */
void
_bufsync(FILE *iop, Uchar *bufend)
{
    ssize_t spaceleft;

    spaceleft = bufend - iop->_ptr;
    if (bufend < iop->_ptr) {
        iop->_ptr = bufend;
        iop->_cnt = 0;
    } else if (spaceleft < iop->_cnt)
        iop->_cnt = spaceleft;
}

/* really write out current buffer contents */
int
_xflsbuf(FILE *iop)
{
    ssize_t n;
    Uchar *base = iop->_base;
    Uchar *bufend;
    ssize_t num_wrote;

    /*
     * Hopefully, be stable with respect to interrupts...
     */
    n = iop->_ptr - base;
    iop->_ptr = base;
    bufend = _bufend(iop);
    if (iop->_flag & (_IOLBF | _IONBF))
        iop->_cnt = 0;      /* always go to a flush */
    else
        iop->_cnt = bufend - base;

    if (_needsync(iop, bufend)) /* recover from interrupts */
        _bufsync(iop, bufend);

    if (n > 0) {
        int fd = GET_FD(iop);
        while ((num_wrote = write(fd, base, (size_t)n)) != n) {
            if (num_wrote <= 0) {
                if (!cancel_active())
                    iop->_flag |= _IOERR;
                return (EOF);
            }
            n -= num_wrote;
            base += num_wrote;
        }
    }
    return (0);
}

/* flush (write) buffer */
int
fflush(FILE *iop)
{
    int res;
    rmutex_t *lk;

    if (iop) {
        FLOCKFILE(lk, iop);
        res = _fflush_u(iop);
        FUNLOCKFILE(lk);
    } else {
        res = _fflush_l_iops();     /* flush all iops */
    }
    return (res);
}

static int
_fflush_l_iops(void)        /* flush all buffers */
{
    FPDECL(iop);

    int i;
    struct _link_ *lp;
    int res = 0;
    rmutex_t *lk;
    /* Allow the compiler to optimize the load out of the loop */
    int threaded = __libc_threaded;

    if (threaded)
        cancel_safe_mutex_lock(&_first_link_lock);

    lp = &__first_link;

    do {
        /*
         * We need to grab the file locks or file corruption
         * will happen.  But we first check the flags field
         * knowing that when it is 0, it isn't allocated and
         * cannot be allocated while we're holding the
         * _first_link_lock.  And when _IONBF is set (also the
         * case when _flag is 0377, or alloc in progress), we
         * also ignore it.
         *
         * Ignore locked streams; it will appear as if
         * concurrent updates happened after fflush(NULL).  Note
         * that we even attempt to lock if the locking is set to
         * "by caller".  We don't want to penalize callers of
         * __fsetlocking() by not flushing their files.  Note: if
         * __fsetlocking() callers don't employ any locking, they
         * may still face corruption in fflush(NULL); but that's
         * no change from earlier releases.
         */
        FIRSTFP(lp, iop);
        for (i = lp->niob; --i >= 0; NEXTFP(iop)) {
            unsigned int flag = iop->_flag;

            /* flag 0, flag 0377, or _IONBF set */
            if (flag == 0 || (flag & _IONBF) != 0)
                continue;

            if (threaded) {
                lk = FPLOCK(iop);
                if (cancel_safe_mutex_trylock(lk) != 0)
                    continue;
            }

            if (!(iop->_flag & _IONBF)) {
                /*
                 * don't need to worry about the _IORW case
                 * since the iop will also marked with _IOREAD
                 * or _IOWRT whichever we are really doing
                 */
                if (iop->_flag & _IOWRT) {
                    /* Flush write buffers */
                    res |= _fflush_u(iop);
                } else if (iop->_flag & _IOREAD) {
                    /*
                     * flush seekable read buffers
                     * don't flush non-seekable read buffers
                     */
                    if (GET_SEEKABLE(iop)) {
                        res |= _fflush_u(iop);
                    }
                }
            }
            if (threaded)
                cancel_safe_mutex_unlock(lk);
        }
    } while ((lp = lp->next) != NULL);
    if (threaded)
        cancel_safe_mutex_unlock(&_first_link_lock);
    return (res);
}

/* flush buffer */
int
_fflush_u(FILE *iop)
{
    int res = 0;

    /* this portion is always assumed locked */
    if (!(iop->_flag & _IOWRT)) {
        (void) lseek64(GET_FD(iop), -iop->_cnt, SEEK_CUR);
        iop->_cnt = 0;
        /* needed for ungetc & multibyte pushbacks */
        iop->_ptr = iop->_base;
        if (iop->_flag & _IORW) {
            iop->_flag &= ~_IOREAD;
        }
        return (0);
    }
    if (iop->_base != NULL && iop->_ptr > iop->_base) {
        res = _xflsbuf(iop);
    }
    if (iop->_flag & _IORW) {
        iop->_flag &= ~_IOWRT;
        iop->_cnt = 0;
    }
    return (res);
}

/* flush buffer and close stream */
int
fclose(FILE *iop)
{
    int res = 0;
    rmutex_t *lk;

    if (iop == NULL) {
        return (EOF);       /* avoid passing zero to FLOCKFILE */
    }

    FLOCKFILE(lk, iop);
    if (iop->_flag == 0) {
        FUNLOCKFILE(lk);
        return (EOF);
    }
    /* Is not unbuffered and opened for read and/or write ? */
    if (!(iop->_flag & _IONBF) && (iop->_flag & (_IOWRT | _IOREAD | _IORW)))
        res = _fflush_u(iop);
    if (close(GET_FD(iop)) < 0)
        res = EOF;
    if (iop->_flag & _IOMYBUF) {
        (void) free((char *)iop->_base - PUSHBACK);
    }
    iop->_base = NULL;
    iop->_ptr = NULL;
    iop->_cnt = 0;
    iop->_flag = 0;         /* marks it as available */
    FUNLOCKFILE(lk);

    if (__libc_threaded)
        cancel_safe_mutex_lock(&_first_link_lock);
    fcloses++;
    if (__libc_threaded)
        cancel_safe_mutex_unlock(&_first_link_lock);

    return (res);
}

/* close all open streams */
int
fcloseall(void)
{
    FPDECL(iop);

    struct _link_ *lp;
    rmutex_t *lk;

    if (__libc_threaded)
        cancel_safe_mutex_lock(&_first_link_lock);

    lp = &__first_link;

    do {
        int i;

        FIRSTFP(lp, iop);
        for (i = lp->niob; --i >= 0; NEXTFP(iop)) {
            /* code stolen from fclose(), above */

            FLOCKFILE(lk, iop);
            if (iop->_flag == 0) {
                FUNLOCKFILE(lk);
                continue;
            }

            /* Not unbuffered and opened for read and/or write? */
            if (!(iop->_flag & _IONBF) &&
                (iop->_flag & (_IOWRT | _IOREAD | _IORW)))
                (void) _fflush_u(iop);
            (void) close(GET_FD(iop));
            if (iop->_flag & _IOMYBUF)
                free((char *)iop->_base - PUSHBACK);
            iop->_base = NULL;
            iop->_ptr = NULL;
            iop->_cnt = 0;
            iop->_flag = 0;     /* marks it as available */
            FUNLOCKFILE(lk);
            fcloses++;
        }
    } while ((lp = lp->next) != NULL);

    if (__libc_threaded)
        cancel_safe_mutex_unlock(&_first_link_lock);

    return (0);
}

/* flush buffer, close fd but keep the stream used by freopen() */
int
close_fd(FILE *iop)
{
    int res = 0;
    mbstate_t *mb;

    if (iop == NULL || iop->_flag == 0)
        return (EOF);
    /* Is not unbuffered and opened for read and/or write ? */
    if (!(iop->_flag & _IONBF) && (iop->_flag & (_IOWRT | _IOREAD | _IORW)))
        res = _fflush_u(iop);
    if (close(GET_FD(iop)) < 0)
        res = EOF;
    if (iop->_flag & _IOMYBUF) {
        (void) free((char *)iop->_base - PUSHBACK);
    }
    iop->_base = NULL;
    iop->_ptr = NULL;
    mb = _getmbstate(iop);
    if (mb != NULL)
        (void) memset(mb, 0, sizeof (mbstate_t));
    iop->_cnt = 0;
    _setorientation(iop, _NO_MODE);
    return (res);
}

static FILE *
getiop(FILE *fp, rmutex_t *lk, mbstate_t *mb)
{
    if (lk != NULL && cancel_safe_mutex_trylock(lk) != 0)
        return (NULL);  /* locked: fp in use */

    if (fp->_flag == 0) {   /* unused */
#ifndef _LP64
        fp->__orientation = 0;
#endif /* _LP64 */
        fp->_cnt = 0;
        fp->_ptr = NULL;
        fp->_base = NULL;
        fp->_flag = 0377;   /* claim the fp by setting low 8 bits */
        (void) memset(mb, 0, sizeof (mbstate_t));
        FUNLOCKFILE(lk);
        return (fp);
    }
    FUNLOCKFILE(lk);
    return (NULL);
}

#ifndef _LP64
/*
 * DESCRIPTION:
 * This function gets the pointer to the mbstate_t structure associated
 * with the specified iop.
 *
 * RETURNS:
 * If the associated mbstate_t found, the pointer to the mbstate_t is
 * returned.  Otherwise, NULL is returned.
 */
mbstate_t *
_getmbstate(FILE *iop)
{
    struct xFILEdata *dat = getxfdat(iop);

    if (dat != NULL)
        return (&dat->_state);

    return (NULL);
}

/*
 * More 32-bit only functions.
 * They lookup/set large fd's for extended FILE support.
 */

/*
 * The negative value indicates that Extended fd FILE's has not
 * been enabled by the user.
 */
static int bad_fd = -1;

int
_file_get(FILE *iop)
{
    int altfd;

    /*
     * Failure indicates a FILE * not allocated through stdio;
     * it means the flag values are probably bogus and that if
     * a file descriptor is set, it's in _magic.
     * Inline getxfdat() for performance reasons.
     */
    if (STDIOP(iop))
        altfd = _xftab[IOPIND(iop)]._altfd;
    else if (VALIDXFILE(FILEx(iop)))
        altfd = FILEx(iop)->_xdat._altfd;
    else
        return (iop->_magic);
    /*
     * if this is not an internal extended FILE then check
     * if _file is being changed from underneath us.
     * It should not be because if
     * it is then then we lose our ability to guard against
     * silent data corruption.
     */
    if (!iop->__xf_nocheck && bad_fd > -1 && iop->_magic != bad_fd) {
        (void) fprintf(stderr,
            "Application violated extended FILE safety mechanism.\n"
            "Please read the man page for extendedFILE.\nAborting\n");
        abort();
    }
    return (altfd);
}

int
_file_set(FILE *iop, int fd, const char *type)
{
    struct xFILEdata *dat;
    int Fflag;

    /* Already known to contain at least one byte */
    while (*++type != '\0')
        ;

    Fflag = type[-1] == 'F';
    if (!Fflag && bad_fd < 0) {
        errno = EMFILE;
        return (-1);
    }

    dat = getxfdat(iop);
    iop->__extendedfd = 1;
    iop->__xf_nocheck = Fflag;
    dat->_altfd = fd;
    iop->_magic = (unsigned char)bad_fd;
    return (0);
}

/*
 * Activates extended fd's in FILE's
 */

static const int tries[] = {196, 120, 60, 3};
#define NTRIES  (sizeof (tries)/sizeof (int))

int
enable_extended_FILE_stdio(int fd, int action)
{
    int i;

    if (action < 0)
        action = SIGABRT;   /* default signal */

    if (fd < 0) {
        /*
         * search for an available fd and make it the badfd
         */
        for (i = 0; i < NTRIES; i++) {
            fd = fcntl(tries[i], F_BADFD, action);
            if (fd >= 0)
                break;
        }
        if (fd < 0) /* failed to find an available fd */
            return (-1);
    } else {
        /* caller requests that fd be the chosen badfd */
        int nfd = fcntl(fd, F_BADFD, action);
        if (nfd < 0 || nfd != fd)
            return (-1);
    }
    bad_fd = fd;
    return (0);
}
#endif