fseek.c revision 4169
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) 38static char sccsid[] = "@(#)fseek.c 8.3 (Berkeley) 1/2/94"; 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 */ 55int 56fseek(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 if (fp->_flags & __SOFF) 92 curoff = fp->_offset; 93 else { 94 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 95 if (curoff == -1L) 96 return (EOF); 97 } 98 if (fp->_flags & __SRD) { 99 curoff -= fp->_r; 100 if (HASUB(fp)) 101 curoff -= fp->_ur; 102 } else if (fp->_flags & __SWR && fp->_p != NULL) 103 curoff += fp->_p - fp->_bf._base; 104 105 offset += curoff; 106 whence = SEEK_SET; 107 havepos = 1; 108 break; 109 110 case SEEK_SET: 111 case SEEK_END: 112 curoff = 0; /* XXX just to keep gcc quiet */ 113 havepos = 0; 114 break; 115 116 default: 117 errno = EINVAL; 118 return (EOF); 119 } 120 121 /* 122 * Can only optimise if: 123 * reading (and not reading-and-writing); 124 * not unbuffered; and 125 * this is a `regular' Unix file (and hence seekfn==__sseek). 126 * We must check __NBF first, because it is possible to have __NBF 127 * and __SOPT both set. 128 */ 129 if (fp->_bf._base == NULL) 130 __smakebuf(fp); 131 if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT)) 132 goto dumb; 133 if ((fp->_flags & __SOPT) == 0) { 134 if (seekfn != __sseek || 135 fp->_file < 0 || fstat(fp->_file, &st) || 136 (st.st_mode & S_IFMT) != S_IFREG) { 137 fp->_flags |= __SNPT; 138 goto dumb; 139 } 140 fp->_blksize = st.st_blksize; 141 fp->_flags |= __SOPT; 142 } 143 144 /* 145 * We are reading; we can try to optimise. 146 * Figure out where we are going and where we are now. 147 */ 148 if (whence == SEEK_SET) 149 target = offset; 150 else { 151 if (fstat(fp->_file, &st)) 152 goto dumb; 153 target = st.st_size + offset; 154 } 155 156 if (!havepos) { 157 if (fp->_flags & __SOFF) 158 curoff = fp->_offset; 159 else { 160 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 161 if (curoff == POS_ERR) 162 goto dumb; 163 } 164 curoff -= fp->_r; 165 if (HASUB(fp)) 166 curoff -= fp->_ur; 167 } 168 169 /* 170 * Compute the number of bytes in the input buffer (pretending 171 * that any ungetc() input has been discarded). Adjust current 172 * offset backwards by this count so that it represents the 173 * file offset for the first byte in the current input buffer. 174 */ 175 if (HASUB(fp)) { 176 curoff += fp->_r; /* kill off ungetc */ 177 n = fp->_up - fp->_bf._base; 178 curoff -= n; 179 n += fp->_ur; 180 } else { 181 n = fp->_p - fp->_bf._base; 182 curoff -= n; 183 n += fp->_r; 184 } 185 186 /* 187 * If the target offset is within the current buffer, 188 * simply adjust the pointers, clear EOF, undo ungetc(), 189 * and return. (If the buffer was modified, we have to 190 * skip this; see fgetln.c.) 191 */ 192 if ((fp->_flags & __SMOD) == 0 && 193 target >= curoff && target < curoff + n) { 194 register int o = target - curoff; 195 196 fp->_p = fp->_bf._base + o; 197 fp->_r = n - o; 198 if (HASUB(fp)) 199 FREEUB(fp); 200 fp->_flags &= ~__SEOF; 201 return (0); 202 } 203 204 /* 205 * The place we want to get to is not within the current buffer, 206 * but we can still be kind to the kernel copyout mechanism. 207 * By aligning the file offset to a block boundary, we can let 208 * the kernel use the VM hardware to map pages instead of 209 * copying bytes laboriously. Using a block boundary also 210 * ensures that we only read one block, rather than two. 211 */ 212 curoff = target & ~(fp->_blksize - 1); 213 if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR) 214 goto dumb; 215 fp->_r = 0; 216 fp->_p = fp->_bf._base; 217 if (HASUB(fp)) 218 FREEUB(fp); 219 fp->_flags &= ~__SEOF; 220 n = target - curoff; 221 if (n) { 222 if (__srefill(fp) || fp->_r < n) 223 goto dumb; 224 fp->_p += n; 225 fp->_r -= n; 226 } 227 return (0); 228 229 /* 230 * We get here if we cannot optimise the seek ... just 231 * do it. Allow the seek function to change fp->_bf._base. 232 */ 233dumb: 234 if (__sflush(fp) || 235 (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) { 236 return (EOF); 237 } 238 /* success: clear EOF indicator and discard ungetc() data */ 239 if (HASUB(fp)) 240 FREEUB(fp); 241 fp->_p = fp->_bf._base; 242 fp->_r = 0; 243 /* fp->_w = 0; */ /* unnecessary (I think...) */ 244 fp->_flags &= ~__SEOF; 245 return (0); 246} 247