fseek.c (6896B)
1 /*- 2 * Copyright (c) 1990, 1993 3 * The Regents of the University of California. All rights reserved. 4 * 5 * This code is derived from software contributed to Berkeley by 6 * Chris Torek. 7 * 8 * Redistribution and use in source and binary forms, with or without 9 * modification, are permitted provided that the following conditions 10 * are met: 11 * 1. Redistributions of source code must retain the above copyright 12 * notice, this list of conditions and the following disclaimer. 13 * 2. Redistributions in binary form must reproduce the above copyright 14 * notice, this list of conditions and the following disclaimer in the 15 * documentation and/or other materials provided with the distribution. 16 * 3. All advertising materials mentioning features or use of this software 17 * must display the following acknowledgement: 18 * This product includes software developed by the University of 19 * California, Berkeley and its contributors. 20 * 4. Neither the name of the University nor the names of its contributors 21 * may be used to endorse or promote products derived from this software 22 * without specific prior written permission. 23 * 24 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 25 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 26 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 27 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 28 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 29 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 30 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 31 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 32 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 33 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 34 * SUCH DAMAGE. 35 */ 36 37 #if defined(LIBC_SCCS) && !defined(lint) 38 static char rcsid[] = "$OpenBSD: fseek.c,v 1.2 1996/08/19 08:32:46 tholo Exp $"; 39 #endif /* LIBC_SCCS and not lint */ 40 41 #include <sys/types.h> 42 #include <sys/stat.h> 43 #include <fcntl.h> 44 #include <stdio.h> 45 #include <stdlib.h> 46 #include <errno.h> 47 #include "local.h" 48 49 #define POS_ERR (-(fpos_t)1) 50 51 /* 52 * Seek the given file to the given offset. 53 * `Whence' must be one of the three SEEK_* macros. 54 */ 55 int 56 fseek(fp, offset, whence) 57 register FILE *fp; 58 long offset; 59 int whence; 60 { 61 register fpos_t (*seekfn) __P((void *, fpos_t, int)); 62 fpos_t target, curoff; 63 size_t n; 64 struct stat st; 65 int havepos; 66 67 /* make sure stdio is set up */ 68 if (!__sdidinit) 69 __sinit(); 70 71 /* 72 * Have to be able to seek. 73 */ 74 if ((seekfn = fp->_seek) == NULL) { 75 errno = ESPIPE; /* historic practice */ 76 return (EOF); 77 } 78 79 /* 80 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument. 81 * After this, whence is either SEEK_SET or SEEK_END. 82 */ 83 switch (whence) { 84 85 case SEEK_CUR: 86 /* 87 * In order to seek relative to the current stream offset, 88 * we have to first find the current stream offset a la 89 * ftell (see ftell for details). 90 */ 91 __sflush(fp); /* may adjust seek offset on append stream */ 92 if (fp->_flags & __SOFF) 93 curoff = fp->_offset; 94 else { 95 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 96 if (curoff == -1L) 97 return (EOF); 98 } 99 if (fp->_flags & __SRD) { 100 curoff -= fp->_r; 101 if (HASUB(fp)) 102 curoff -= fp->_ur; 103 } else if (fp->_flags & __SWR && fp->_p != NULL) 104 curoff += fp->_p - fp->_bf._base; 105 106 offset += curoff; 107 whence = SEEK_SET; 108 havepos = 1; 109 break; 110 111 case SEEK_SET: 112 case SEEK_END: 113 curoff = 0; /* XXX just to keep gcc quiet */ 114 havepos = 0; 115 break; 116 117 default: 118 errno = EINVAL; 119 return (EOF); 120 } 121 122 /* 123 * Can only optimise if: 124 * reading (and not reading-and-writing); 125 * not unbuffered; and 126 * this is a `regular' Unix file (and hence seekfn==__sseek). 127 * We must check __NBF first, because it is possible to have __NBF 128 * and __SOPT both set. 129 */ 130 if (fp->_bf._base == NULL) 131 __smakebuf(fp); 132 if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT)) 133 goto dumb; 134 if ((fp->_flags & __SOPT) == 0) { 135 if (seekfn != __sseek || 136 fp->_file < 0 || fstat(fp->_file, &st) || 137 (st.st_mode & S_IFMT) != S_IFREG) { 138 fp->_flags |= __SNPT; 139 goto dumb; 140 } 141 fp->_blksize = st.st_blksize; 142 fp->_flags |= __SOPT; 143 } 144 145 /* 146 * We are reading; we can try to optimise. 147 * Figure out where we are going and where we are now. 148 */ 149 if (whence == SEEK_SET) 150 target = offset; 151 else { 152 if (fstat(fp->_file, &st)) 153 goto dumb; 154 target = st.st_size + offset; 155 } 156 157 if (!havepos) { 158 if (fp->_flags & __SOFF) 159 curoff = fp->_offset; 160 else { 161 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 162 if (curoff == POS_ERR) 163 goto dumb; 164 } 165 curoff -= fp->_r; 166 if (HASUB(fp)) 167 curoff -= fp->_ur; 168 } 169 170 /* 171 * Compute the number of bytes in the input buffer (pretending 172 * that any ungetc() input has been discarded). Adjust current 173 * offset backwards by this count so that it represents the 174 * file offset for the first byte in the current input buffer. 175 */ 176 if (HASUB(fp)) { 177 curoff += fp->_r; /* kill off ungetc */ 178 n = fp->_up - fp->_bf._base; 179 curoff -= n; 180 n += fp->_ur; 181 } else { 182 n = fp->_p - fp->_bf._base; 183 curoff -= n; 184 n += fp->_r; 185 } 186 187 /* 188 * If the target offset is within the current buffer, 189 * simply adjust the pointers, clear EOF, undo ungetc(), 190 * and return. (If the buffer was modified, we have to 191 * skip this; see fgetln.c.) 192 */ 193 if ((fp->_flags & __SMOD) == 0 && 194 target >= curoff && target < curoff + n) { 195 register int o = target - curoff; 196 197 fp->_p = fp->_bf._base + o; 198 fp->_r = n - o; 199 if (HASUB(fp)) 200 FREEUB(fp); 201 fp->_flags &= ~__SEOF; 202 return (0); 203 } 204 205 /* 206 * The place we want to get to is not within the current buffer, 207 * but we can still be kind to the kernel copyout mechanism. 208 * By aligning the file offset to a block boundary, we can let 209 * the kernel use the VM hardware to map pages instead of 210 * copying bytes laboriously. Using a block boundary also 211 * ensures that we only read one block, rather than two. 212 */ 213 curoff = target & ~(fp->_blksize - 1); 214 if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR) 215 goto dumb; 216 fp->_r = 0; 217 fp->_p = fp->_bf._base; 218 if (HASUB(fp)) 219 FREEUB(fp); 220 fp->_flags &= ~__SEOF; 221 n = target - curoff; 222 if (n) { 223 if (__srefill(fp) || fp->_r < n) 224 goto dumb; 225 fp->_p += n; 226 fp->_r -= n; 227 } 228 return (0); 229 230 /* 231 * We get here if we cannot optimise the seek ... just 232 * do it. Allow the seek function to change fp->_bf._base. 233 */ 234 dumb: 235 if (__sflush(fp) || 236 (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) { 237 return (EOF); 238 } 239 /* success: clear EOF indicator and discard ungetc() data */ 240 if (HASUB(fp)) 241 FREEUB(fp); 242 fp->_p = fp->_bf._base; 243 fp->_r = 0; 244 /* fp->_w = 0; */ /* unnecessary (I think...) */ 245 fp->_flags &= ~__SEOF; 246 return (0); 247 }