/* $OpenBSD: zopen.c,v 1.9 2002/12/08 16:07:54 mickey Exp $ */ /* $NetBSD: zopen.c,v 1.5 1995/03/26 09:44:53 glass Exp $ */ /*- * Copyright (c) 1985, 1986, 1992, 1993 * The Regents of the University of California. All rights reserved. * * This code is derived from software contributed to Berkeley by * Diomidis Spinellis and James A. Woods, derived from original * work by Spencer Thomas and Joseph Orost. * * 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. * 3. All advertising materials mentioning features or use of this software * must display the following acknowledgement: * This product includes software developed by the University of * California, Berkeley and its contributors. * 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. * * From: @(#)zopen.c 8.1 (Berkeley) 6/27/93 */ #if 0 static char sccsid[] = "@(#)zopen.c 8.1 (Berkeley) 6/27/93"; #else const char z_rcsid[] = "$OpenBSD: zopen.c,v 1.9 2002/12/08 16:07:54 mickey Exp $"; #endif /*- * fcompress.c - File compression ala IEEE Computer, June 1984. * * Compress authors: * Spencer W. Thomas (decvax!utah-cs!thomas) * Jim McKie (decvax!mcvax!jim) * Steve Davies (decvax!vax135!petsd!peora!srd) * Ken Turkowski (decvax!decwrl!turtlevax!ken) * James A. Woods (decvax!ihnp4!ames!jaw) * Joe Orost (decvax!vax135!petsd!joe) * * Cleaned up and converted to library returning I/O streams by * Diomidis Spinellis . * * zopen(filename, mode, bits) * Returns a FILE * that can be used for read or write. The modes * supported are only "r" and "w". Seeking is not allowed. On * reading the file is decompressed, on writing it is compressed. * The output is compatible with compress(1) with 16 bit tables. * Any file produced by compress(1) can be read. */ #include #include #include #include #include #include #include #include #include #include #include "compress.h" #define BITS 16 /* Default bits. */ #define HSIZE 69001 /* 95% occupancy */ #define ZBUFSIZ 8192 /* I/O buffer size */ /* A code_int must be able to hold 2**BITS values of type int, and also -1. */ typedef long code_int; typedef long count_int; static const u_char z_magic[] = {'\037', '\235'}; /* 1F 9D */ #define BIT_MASK 0x1f /* Defines for third byte of header. */ #define BLOCK_MASK 0x80 /* * Masks 0x40 and 0x20 are free. I think 0x20 should mean that there is * a fourth header byte (for expansion). */ #define INIT_BITS 9 /* Initial number of bits/code. */ #define MAXCODE(n_bits) ((1 << (n_bits)) - 1) struct s_zstate { int zs_fd; /* File stream for I/O */ char zs_mode; /* r or w */ enum { S_START, S_MIDDLE, S_EOF } zs_state; /* State of computation */ int zs_n_bits; /* Number of bits/code. */ int zs_maxbits; /* User settable max # bits/code. */ code_int zs_maxcode; /* Maximum code, given n_bits. */ code_int zs_maxmaxcode; /* Should NEVER generate this code. */ count_int zs_htab [HSIZE]; u_short zs_codetab [HSIZE]; code_int zs_hsize; /* For dynamic table sizing. */ code_int zs_free_ent; /* First unused entry. */ /* * Block compression parameters -- after all codes are used up, * and compression rate changes, start over. */ int zs_block_compress; int zs_clear_flg; long zs_ratio; count_int zs_checkpoint; long zs_in_count; /* Length of input. */ long zs_bytes_out; /* Length of compressed output. */ long zs_out_count; /* # of codes output (for debugging).*/ u_char zs_buf[ZBUFSIZ]; /* I/O buffer */ u_char *zs_bp; /* Current I/O window in the zs_buf */ int zs_offset; /* Number of bits in the zs_buf */ union { struct { long zs_fcode; code_int zs_ent; code_int zs_hsize_reg; int zs_hshift; } w; /* Write paramenters */ struct { u_char *zs_stackp, *zs_ebp; int zs_finchar; code_int zs_code, zs_oldcode, zs_incode; int zs_size; } r; /* Read parameters */ } u; }; /* Definitions to retain old variable names */ #define zs_fcode u.w.zs_fcode #define zs_ent u.w.zs_ent #define zs_hsize_reg u.w.zs_hsize_reg #define zs_hshift u.w.zs_hshift #define zs_stackp u.r.zs_stackp #define zs_finchar u.r.zs_finchar #define zs_code u.r.zs_code #define zs_oldcode u.r.zs_oldcode #define zs_incode u.r.zs_incode #define zs_size u.r.zs_size #define zs_ebp u.r.zs_ebp /* * To save much memory, we overlay the table used by compress() with those * used by decompress(). The tab_prefix table is the same size and type as * the codetab. The tab_suffix table needs 2**BITS characters. We get this * from the beginning of htab. The output stack uses the rest of htab, and * contains characters. There is plenty of room for any possible stack * (stack used to be 8000 characters). */ #define htabof(i) zs->zs_htab[i] #define codetabof(i) zs->zs_codetab[i] #define tab_prefixof(i) codetabof(i) #define tab_suffixof(i) ((u_char *)(zs->zs_htab))[i] #define de_stack ((u_char *)&tab_suffixof(1 << BITS)) #define CHECK_GAP 10000 /* Ratio check interval. */ /* * the next two codes should not be changed lightly, as they must not * lie within the contiguous general code space. */ #define FIRST 257 /* First free entry. */ #define CLEAR 256 /* Table clear output code. */ static int cl_block(struct s_zstate *); static void cl_hash(struct s_zstate *, count_int); static code_int getcode(struct s_zstate *); static int output(struct s_zstate *, code_int); /*- * Algorithm from "A Technique for High Performance Data Compression", * Terry A. Welch, IEEE Computer Vol 17, No 6 (June 1984), pp 8-19. * * Algorithm: * Modified Lempel-Ziv method (LZW). Basically finds common * substrings and replaces them with a variable size code. This is * deterministic, and can be done on the fly. Thus, the decompression * procedure needs no input table, but tracks the way the table was built. */ /*- * compress write * * Algorithm: use open addressing double hashing (no chaining) on the * prefix code / next character combination. We do a variant of Knuth's * algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime * secondary probe. Here, the modular division first probe is gives way * to a faster exclusive-or manipulation. Also do block compression with * an adaptive reset, whereby the code table is cleared when the compression * ratio decreases, but after the table fills. The variable-length output * codes are re-sized at this point, and a special CLEAR code is generated * for the decompressor. Late addition: construct the table according to * file size for noticeable speed improvement on small files. Please direct * questions about this implementation to ames!jaw. */ int zwrite(cookie, wbp, num) void *cookie; const char *wbp; int num; { code_int i; int c, disp; struct s_zstate *zs; const u_char *bp; u_char tmp; int count; zs = cookie; count = num; bp = (u_char *)wbp; switch (zs->zs_state) { case S_EOF: return 0; case S_START: zs->zs_state = S_MIDDLE; zs->zs_maxmaxcode = 1L << zs->zs_maxbits; if (write(zs->zs_fd, z_magic, sizeof(z_magic)) != sizeof(z_magic)) return (-1); tmp = (u_char)(zs->zs_maxbits | zs->zs_block_compress); if (write(zs->zs_fd, &tmp, sizeof(tmp)) != sizeof(tmp)) return (-1); zs->zs_bp = zs->zs_buf; zs->zs_offset = 0; zs->zs_bytes_out = 3; /* Includes 3-byte header mojo. */ zs->zs_out_count = 0; zs->zs_clear_flg = 0; zs->zs_ratio = 0; zs->zs_in_count = 1; zs->zs_checkpoint = CHECK_GAP; zs->zs_maxcode = MAXCODE(zs->zs_n_bits = INIT_BITS); zs->zs_free_ent = ((zs->zs_block_compress) ? FIRST : 256); zs->zs_ent = *bp++; --count; zs->zs_hshift = 0; for (zs->zs_fcode = (long)zs->zs_hsize; zs->zs_fcode < 65536L; zs->zs_fcode *= 2L) zs->zs_hshift++; /* Set hash code range bound. */ zs->zs_hshift = 8 - zs->zs_hshift; zs->zs_hsize_reg = zs->zs_hsize; /* Clear hash table. */ cl_hash(zs, (count_int)zs->zs_hsize_reg); case S_MIDDLE: for (i = 0; count-- > 0;) { c = *bp++; zs->zs_in_count++; zs->zs_fcode = (long)(((long)c << zs->zs_maxbits) + zs->zs_ent); /* Xor hashing. */ i = ((c << zs->zs_hshift) ^ zs->zs_ent); if (htabof(i) == zs->zs_fcode) { zs->zs_ent = codetabof(i); continue; } else if ((long)htabof(i) < 0) /* Empty slot. */ goto nomatch; /* Secondary hash (after G. Knott). */ disp = zs->zs_hsize_reg - i; if (i == 0) disp = 1; probe: if ((i -= disp) < 0) i += zs->zs_hsize_reg; if (htabof(i) == zs->zs_fcode) { zs->zs_ent = codetabof(i); continue; } if ((long)htabof(i) >= 0) goto probe; nomatch: if (output(zs, (code_int) zs->zs_ent) == -1) return (-1); zs->zs_out_count++; zs->zs_ent = c; if (zs->zs_free_ent < zs->zs_maxmaxcode) { /* code -> hashtable */ codetabof(i) = zs->zs_free_ent++; htabof(i) = zs->zs_fcode; } else if ((count_int)zs->zs_in_count >= zs->zs_checkpoint && zs->zs_block_compress) { if (cl_block(zs) == -1) return (-1); } } } return (num); } int zclose(cookie) void *cookie; { struct s_zstate *zs; int rval; zs = cookie; if (zs->zs_mode == 'w') { /* Put out the final code. */ if (output(zs, (code_int) zs->zs_ent) == -1) { (void)close(zs->zs_fd); free(zs); return (-1); } zs->zs_out_count++; if (output(zs, (code_int) - 1) == -1) { (void)close(zs->zs_fd); free(zs); return (-1); } } rval = close(zs->zs_fd); free(zs); return (rval); } /*- * Output the given code. * Inputs: * code: A n_bits-bit integer. If == -1, then EOF. This assumes * that n_bits =< (long)wordsize - 1. * Outputs: * Outputs code to the file. * Assumptions: * Chars are 8 bits long. * Algorithm: * Maintain a BITS character long buffer (so that 8 codes will * fit in it exactly). Use the VAX insv instruction to insert each * code in turn. When the buffer fills up empty it and start over. */ static const u_char lmask[9] = {0xff, 0xfe, 0xfc, 0xf8, 0xf0, 0xe0, 0xc0, 0x80, 0x00}; static const u_char rmask[9] = {0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff}; static int output(zs, ocode) struct s_zstate *zs; code_int ocode; { int bits; if (ocode >= 0) { int r_off; u_char *bp; /* Get to the first byte. */ bp = zs->zs_bp + (zs->zs_offset >> 3); r_off = zs->zs_offset & 7; bits = zs->zs_n_bits; /* * Since ocode is always >= 8 bits, only need to mask the first * hunk on the left. */ *bp = (*bp & rmask[r_off]) | ((ocode << r_off) & lmask[r_off]); bp++; bits -= (8 - r_off); ocode >>= 8 - r_off; /* Get any 8 bit parts in the middle (<=1 for up to 16 bits) */ if (bits >= 8) { *bp++ = ocode; ocode >>= 8; bits -= 8; } /* Last bits. */ if (bits) *bp = ocode; zs->zs_offset += zs->zs_n_bits; if (zs->zs_offset == (zs->zs_n_bits << 3)) { zs->zs_bp += zs->zs_n_bits; zs->zs_offset = 0; } /* * If the next entry is going to be too big for the ocode size, * then increase it, if possible. */ if (zs->zs_free_ent > zs->zs_maxcode || (zs->zs_clear_flg > 0)) { /* * Write the whole buffer, because the input side won't * discover the size increase until after it has read it */ if (zs->zs_offset > 0) { zs->zs_bp += zs->zs_n_bits; zs->zs_offset = 0; } if (zs->zs_clear_flg) { zs->zs_maxcode = MAXCODE(zs->zs_n_bits = INIT_BITS); zs->zs_clear_flg = 0; } else { zs->zs_n_bits++; if (zs->zs_n_bits == zs->zs_maxbits) zs->zs_maxcode = zs->zs_maxmaxcode; else zs->zs_maxcode = MAXCODE(zs->zs_n_bits); } } if (zs->zs_bp + zs->zs_n_bits > &zs->zs_buf[ZBUFSIZ]) { bits = zs->zs_bp - zs->zs_buf; if (write(zs->zs_fd, zs->zs_buf, bits) != bits) return (-1); zs->zs_bytes_out += bits; if (zs->zs_offset > 0) fprintf (stderr, "zs_offset != 0\n"); zs->zs_bp = zs->zs_buf; } } else { /* At EOF, write the rest of the buffer. */ if (zs->zs_offset > 0) zs->zs_bp += (zs->zs_offset + 7) / 8; if (zs->zs_bp > zs->zs_buf) { bits = zs->zs_bp - zs->zs_buf; if (write(zs->zs_fd, zs->zs_buf, bits) != bits) return (-1); zs->zs_bytes_out += bits; } zs->zs_offset = 0; zs->zs_bp = zs->zs_buf; } return (0); } /* * Decompress read. This routine adapts to the codes in the file building * the "string" table on-the-fly; requiring no table to be stored in the * compressed file. The tables used herein are shared with those of the * compress() routine. See the definitions above. */ int zread(cookie, rbp, num) void *cookie; char *rbp; int num; { u_int count; struct s_zstate *zs; u_char *bp, header[3]; if (num == 0) return (0); zs = cookie; count = num; bp = (u_char *)rbp; switch (zs->zs_state) { case S_START: zs->zs_state = S_MIDDLE; zs->zs_bp = zs->zs_buf; break; case S_MIDDLE: goto middle; case S_EOF: goto eof; } /* Check the magic number */ if (read(zs->zs_fd, header, sizeof(header)) != sizeof(header) || memcmp(header, z_magic, sizeof(z_magic)) != 0) { errno = EFTYPE; return (-1); } zs->zs_maxbits = header[2]; /* Set -b from file. */ zs->zs_block_compress = zs->zs_maxbits & BLOCK_MASK; zs->zs_maxbits &= BIT_MASK; zs->zs_maxmaxcode = 1L << zs->zs_maxbits; if (zs->zs_maxbits > BITS) { errno = EFTYPE; return (-1); } /* As above, initialize the first 256 entries in the table. */ zs->zs_maxcode = MAXCODE(zs->zs_n_bits = INIT_BITS); for (zs->zs_code = 255; zs->zs_code >= 0; zs->zs_code--) { tab_prefixof(zs->zs_code) = 0; tab_suffixof(zs->zs_code) = (u_char) zs->zs_code; } zs->zs_free_ent = zs->zs_block_compress ? FIRST : 256; zs->zs_finchar = zs->zs_oldcode = getcode(zs); if (zs->zs_oldcode == -1) /* EOF already? */ return (0); /* Get out of here */ /* First code must be 8 bits = char. */ *bp++ = (u_char)zs->zs_finchar; count--; zs->zs_stackp = de_stack; while ((zs->zs_code = getcode(zs)) > -1) { if ((zs->zs_code == CLEAR) && zs->zs_block_compress) { for (zs->zs_code = 255; zs->zs_code >= 0; zs->zs_code--) tab_prefixof(zs->zs_code) = 0; zs->zs_clear_flg = 1; zs->zs_free_ent = FIRST - 1; if ((zs->zs_code = getcode(zs)) == -1) /* O, untimely death! */ break; } zs->zs_incode = zs->zs_code; /* Special case for KwKwK string. */ if (zs->zs_code >= zs->zs_free_ent) { *zs->zs_stackp++ = zs->zs_finchar; zs->zs_code = zs->zs_oldcode; } /* Generate output characters in reverse order. */ while (zs->zs_code >= 256) { *zs->zs_stackp++ = tab_suffixof(zs->zs_code); zs->zs_code = tab_prefixof(zs->zs_code); } *zs->zs_stackp++ = zs->zs_finchar = tab_suffixof(zs->zs_code); /* And put them out in forward order. */ middle: do { if (count-- == 0) return (num); *bp++ = *--zs->zs_stackp; } while (zs->zs_stackp > de_stack); /* Generate the new entry. */ if ((zs->zs_code = zs->zs_free_ent) < zs->zs_maxmaxcode) { tab_prefixof(zs->zs_code) = (u_short) zs->zs_oldcode; tab_suffixof(zs->zs_code) = zs->zs_finchar; zs->zs_free_ent = zs->zs_code + 1; } /* Remember previous code. */ zs->zs_oldcode = zs->zs_incode; } zs->zs_state = S_EOF; eof: return (num - count); } /*- * Read one code from the standard input. If EOF, return -1. * Inputs: * stdin * Outputs: * code or -1 is returned. */ static code_int getcode(zs) struct s_zstate *zs; { code_int gcode; int r_off, bits; u_char *bp; if (zs->zs_clear_flg > 0 || zs->zs_offset >= zs->zs_size || zs->zs_free_ent > zs->zs_maxcode) { zs->zs_bp += zs->zs_n_bits; /* * If the next entry will be too big for the current gcode * size, then we must increase the size. This implies reading * a new buffer full, too. */ if (zs->zs_free_ent > zs->zs_maxcode) { zs->zs_n_bits++; if (zs->zs_n_bits == zs->zs_maxbits) /* Won't get any bigger now. */ zs->zs_maxcode = zs->zs_maxmaxcode; else zs->zs_maxcode = MAXCODE(zs->zs_n_bits); } if (zs->zs_clear_flg > 0) { zs->zs_maxcode = MAXCODE(zs->zs_n_bits = INIT_BITS); zs->zs_clear_flg = 0; } /* fill the buffer up to the neck */ if (zs->zs_bp + zs->zs_n_bits > zs->zs_ebp) { for (bp = zs->zs_buf; zs->zs_bp < zs->zs_ebp; *bp++ = *zs->zs_bp++); if ((bits = read(zs->zs_fd, bp, ZBUFSIZ - (bp - zs->zs_buf))) < 0) return -1; zs->zs_bp = zs->zs_buf; zs->zs_ebp = bp + bits; } zs->zs_offset = 0; zs->zs_size = MIN(zs->zs_n_bits, zs->zs_ebp - zs->zs_bp); if (zs->zs_size == 0) return -1; /* Round size down to integral number of codes. */ zs->zs_size = (zs->zs_size << 3) - (zs->zs_n_bits - 1); } bp = zs->zs_bp; r_off = zs->zs_offset; bits = zs->zs_n_bits; /* Get to the first byte. */ bp += (r_off >> 3); r_off &= 7; /* Get first part (low order bits). */ gcode = (*bp++ >> r_off); bits -= (8 - r_off); r_off = 8 - r_off; /* Now, roffset into gcode word. */ /* Get any 8 bit parts in the middle (<=1 for up to 16 bits). */ if (bits >= 8) { gcode |= *bp++ << r_off; r_off += 8; bits -= 8; } /* High order bits. */ gcode |= (*bp & rmask[bits]) << r_off; zs->zs_offset += zs->zs_n_bits; return (gcode); } static int cl_block(zs) /* Table clear for block compress. */ struct s_zstate *zs; { long rat; zs->zs_checkpoint = zs->zs_in_count + CHECK_GAP; if (zs->zs_in_count > 0x007fffff) { /* Shift will overflow. */ rat = zs->zs_bytes_out >> 8; if (rat == 0) /* Don't divide by zero. */ rat = 0x7fffffff; else rat = zs->zs_in_count / rat; } else rat = (zs->zs_in_count << 8) / zs->zs_bytes_out; /* 8 fractional bits. */ if (rat > zs->zs_ratio) zs->zs_ratio = rat; else { zs->zs_ratio = 0; cl_hash(zs, (count_int) zs->zs_hsize); zs->zs_free_ent = FIRST; zs->zs_clear_flg = 1; if (output(zs, (code_int) CLEAR) == -1) return (-1); } return (0); } static void cl_hash(zs, cl_hsize) /* Reset code table. */ struct s_zstate *zs; count_int cl_hsize; { count_int *htab_p; long i, m1; m1 = -1; htab_p = zs->zs_htab + cl_hsize; i = cl_hsize - 16; do { /* Might use Sys V memset(3) here. */ *(htab_p - 16) = m1; *(htab_p - 15) = m1; *(htab_p - 14) = m1; *(htab_p - 13) = m1; *(htab_p - 12) = m1; *(htab_p - 11) = m1; *(htab_p - 10) = m1; *(htab_p - 9) = m1; *(htab_p - 8) = m1; *(htab_p - 7) = m1; *(htab_p - 6) = m1; *(htab_p - 5) = m1; *(htab_p - 4) = m1; *(htab_p - 3) = m1; *(htab_p - 2) = m1; *(htab_p - 1) = m1; htab_p -= 16; } while ((i -= 16) >= 0); for (i += 16; i > 0; i--) *--htab_p = m1; } FILE * zopen(name, mode, bits) const char *name; const char *mode; int bits; { int fd; void *cookie; if ((fd = open(name, (*mode=='r'? O_RDONLY:O_WRONLY|O_CREAT), S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH)) == -1) return NULL; if ((cookie = z_open(fd, mode, bits)) == NULL) { close(fd); return NULL; } return funopen(cookie, (*mode == 'r'?zread:NULL), (*mode == 'w'?zwrite:NULL), NULL, zclose); } void * z_open(fd, mode, bits) int fd; const char *mode; int bits; { struct s_zstate *zs; if ((mode[0] != 'r' && mode[0] != 'w') || mode[1] != '\0' || bits < 0 || bits > BITS) { errno = EINVAL; return (NULL); } if ((zs = calloc(1, sizeof(struct s_zstate))) == NULL) return (NULL); /* User settable max # bits/code. */ zs->zs_maxbits = bits ? bits : BITS; /* Should NEVER generate this code. */ zs->zs_maxmaxcode = 1 << zs->zs_maxbits; zs->zs_hsize = HSIZE; /* For dynamic table sizing. */ zs->zs_free_ent = 0; /* First unused entry. */ zs->zs_block_compress = BLOCK_MASK; zs->zs_clear_flg = 0; zs->zs_ratio = 0; zs->zs_checkpoint = CHECK_GAP; zs->zs_in_count = 1; /* Length of input. */ zs->zs_out_count = 0; /* # of codes output (for debugging).*/ zs->zs_state = S_START; zs->zs_offset = 0; zs->zs_size = 0; zs->zs_mode = mode[0]; zs->zs_bp = zs->zs_ebp = zs->zs_buf; zs->zs_fd = fd; return zs; } int z_check_header(fd, sb, ofn) int fd; struct stat *sb; const char *ofn; { int f; u_char buf[sizeof(z_magic)]; off_t off = lseek(fd, 0, SEEK_CUR); f = (read(fd, buf, sizeof(buf)) == sizeof(buf) && !memcmp(buf, z_magic, sizeof(buf))); lseek (fd, off, SEEK_SET); return f; }