fseek.c revision 1.7
1 /* $NetBSD: fseek.c,v 1.7 1995/02/02 02:09:39 jtc Exp $ */ 2 3 /*- 4 * Copyright (c) 1990, 1993 5 * The Regents of the University of California. All rights reserved. 6 * 7 * This code is derived from software contributed to Berkeley by 8 * Chris Torek. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 3. All advertising materials mentioning features or use of this software 19 * must display the following acknowledgement: 20 * This product includes software developed by the University of 21 * California, Berkeley and its contributors. 22 * 4. Neither the name of the University nor the names of its contributors 23 * may be used to endorse or promote products derived from this software 24 * without specific prior written permission. 25 * 26 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 27 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 28 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 29 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 30 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 31 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 32 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 33 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 34 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 35 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 36 * SUCH DAMAGE. 37 */ 38 39 #if defined(LIBC_SCCS) && !defined(lint) 40 #if 0 41 static char sccsid[] = "@(#)fseek.c 8.3 (Berkeley) 1/2/94"; 42 #endif 43 static char rcsid[] = "$NetBSD: fseek.c,v 1.7 1995/02/02 02:09:39 jtc Exp $"; 44 #endif /* LIBC_SCCS and not lint */ 45 46 #include <sys/types.h> 47 #include <sys/stat.h> 48 #include <fcntl.h> 49 #include <stdio.h> 50 #include <stdlib.h> 51 #include <errno.h> 52 #include "local.h" 53 54 #define POS_ERR (-(fpos_t)1) 55 56 /* 57 * Seek the given file to the given offset. 58 * `Whence' must be one of the three SEEK_* macros. 59 */ 60 int 61 fseek(fp, offset, whence) 62 register FILE *fp; 63 long offset; 64 int whence; 65 { 66 register fpos_t (*seekfn) __P((void *, fpos_t, int)); 67 fpos_t target, curoff; 68 size_t n; 69 struct stat st; 70 int havepos; 71 72 /* make sure stdio is set up */ 73 if (!__sdidinit) 74 __sinit(); 75 76 /* 77 * Have to be able to seek. 78 */ 79 if ((seekfn = fp->_seek) == NULL) { 80 errno = ESPIPE; /* historic practice */ 81 return (EOF); 82 } 83 84 /* 85 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument. 86 * After this, whence is either SEEK_SET or SEEK_END. 87 */ 88 switch (whence) { 89 90 case SEEK_CUR: 91 /* 92 * In order to seek relative to the current stream offset, 93 * we have to first find the current stream offset a la 94 * ftell (see ftell for details). 95 */ 96 if (fp->_flags & __SOFF) 97 curoff = fp->_offset; 98 else { 99 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 100 if (curoff == -1L) 101 return (EOF); 102 } 103 if (fp->_flags & __SRD) { 104 curoff -= fp->_r; 105 if (HASUB(fp)) 106 curoff -= fp->_ur; 107 } else if (fp->_flags & __SWR && fp->_p != NULL) 108 curoff += fp->_p - fp->_bf._base; 109 110 offset += curoff; 111 whence = SEEK_SET; 112 havepos = 1; 113 break; 114 115 case SEEK_SET: 116 case SEEK_END: 117 curoff = 0; /* XXX just to keep gcc quiet */ 118 havepos = 0; 119 break; 120 121 default: 122 errno = EINVAL; 123 return (EOF); 124 } 125 126 /* 127 * Can only optimise if: 128 * reading (and not reading-and-writing); 129 * not unbuffered; and 130 * this is a `regular' Unix file (and hence seekfn==__sseek). 131 * We must check __NBF first, because it is possible to have __NBF 132 * and __SOPT both set. 133 */ 134 if (fp->_bf._base == NULL) 135 __smakebuf(fp); 136 if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT)) 137 goto dumb; 138 if ((fp->_flags & __SOPT) == 0) { 139 if (seekfn != __sseek || 140 fp->_file < 0 || fstat(fp->_file, &st) || 141 (st.st_mode & S_IFMT) != S_IFREG) { 142 fp->_flags |= __SNPT; 143 goto dumb; 144 } 145 fp->_blksize = st.st_blksize; 146 fp->_flags |= __SOPT; 147 } 148 149 /* 150 * We are reading; we can try to optimise. 151 * Figure out where we are going and where we are now. 152 */ 153 if (whence == SEEK_SET) 154 target = offset; 155 else { 156 if (fstat(fp->_file, &st)) 157 goto dumb; 158 target = st.st_size + offset; 159 } 160 161 if (!havepos) { 162 if (fp->_flags & __SOFF) 163 curoff = fp->_offset; 164 else { 165 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 166 if (curoff == POS_ERR) 167 goto dumb; 168 } 169 curoff -= fp->_r; 170 if (HASUB(fp)) 171 curoff -= fp->_ur; 172 } 173 174 /* 175 * Compute the number of bytes in the input buffer (pretending 176 * that any ungetc() input has been discarded). Adjust current 177 * offset backwards by this count so that it represents the 178 * file offset for the first byte in the current input buffer. 179 */ 180 if (HASUB(fp)) { 181 curoff += fp->_r; /* kill off ungetc */ 182 n = fp->_up - fp->_bf._base; 183 curoff -= n; 184 n += fp->_ur; 185 } else { 186 n = fp->_p - fp->_bf._base; 187 curoff -= n; 188 n += fp->_r; 189 } 190 191 /* 192 * If the target offset is within the current buffer, 193 * simply adjust the pointers, clear EOF, undo ungetc(), 194 * and return. (If the buffer was modified, we have to 195 * skip this; see fgetln.c.) 196 */ 197 if ((fp->_flags & __SMOD) == 0 && 198 target >= curoff && target < curoff + n) { 199 register int o = target - curoff; 200 201 fp->_p = fp->_bf._base + o; 202 fp->_r = n - o; 203 if (HASUB(fp)) 204 FREEUB(fp); 205 fp->_flags &= ~__SEOF; 206 return (0); 207 } 208 209 /* 210 * The place we want to get to is not within the current buffer, 211 * but we can still be kind to the kernel copyout mechanism. 212 * By aligning the file offset to a block boundary, we can let 213 * the kernel use the VM hardware to map pages instead of 214 * copying bytes laboriously. Using a block boundary also 215 * ensures that we only read one block, rather than two. 216 */ 217 curoff = target & ~(fp->_blksize - 1); 218 if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR) 219 goto dumb; 220 fp->_r = 0; 221 fp->_p = fp->_bf._base; 222 if (HASUB(fp)) 223 FREEUB(fp); 224 fp->_flags &= ~__SEOF; 225 n = target - curoff; 226 if (n) { 227 if (__srefill(fp) || fp->_r < n) 228 goto dumb; 229 fp->_p += n; 230 fp->_r -= n; 231 } 232 return (0); 233 234 /* 235 * We get here if we cannot optimise the seek ... just 236 * do it. Allow the seek function to change fp->_bf._base. 237 */ 238 dumb: 239 if (__sflush(fp) || 240 (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) { 241 return (EOF); 242 } 243 /* success: clear EOF indicator and discard ungetc() data */ 244 if (HASUB(fp)) 245 FREEUB(fp); 246 fp->_p = fp->_bf._base; 247 fp->_r = 0; 248 /* fp->_w = 0; */ /* unnecessary (I think...) */ 249 fp->_flags &= ~__SEOF; 250 return (0); 251 } 252
Indexes created Tue Sep 23 07:09:52 GMT 2025