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 * %sccs.include.redist.c% 9 */ 10 11 #if defined(LIBC_SCCS) && !defined(lint) 12 static char sccsid[] = "@(#)fseek.c 8.2 (Berkeley) 11/04/93"; 13 #endif /* LIBC_SCCS and not lint */ 14 15 #include <sys/types.h> 16 #include <sys/stat.h> 17 #include <fcntl.h> 18 #include <stdio.h> 19 #include <stdlib.h> 20 #include <errno.h> 21 #include "local.h" 22 23 #define POS_ERR (-(fpos_t)1) 24 25 /* 26 * Seek the given file to the given offset. 27 * `Whence' must be one of the three SEEK_* macros. 28 */ 29 int 30 fseek(fp, offset, whence) 31 register FILE *fp; 32 long offset; 33 int whence; 34 { 35 register fpos_t (*seekfn) __P((void *, fpos_t, int)); 36 fpos_t target, curoff; 37 size_t n; 38 struct stat st; 39 int havepos; 40 41 /* make sure stdio is set up */ 42 if (!__sdidinit) 43 __sinit(); 44 45 /* 46 * Have to be able to seek. 47 */ 48 if ((seekfn = fp->_seek) == NULL) { 49 errno = ESPIPE; /* historic practice */ 50 return (EOF); 51 } 52 53 /* 54 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument. 55 * After this, whence is either SEEK_SET or SEEK_END. 56 */ 57 switch (whence) { 58 59 case SEEK_CUR: 60 /* 61 * In order to seek relative to the current stream offset, 62 * we have to first find the current stream offset a la 63 * ftell (see ftell for details). 64 */ 65 if (fp->_flags & __SOFF) 66 curoff = fp->_offset; 67 else { 68 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 69 if (curoff == -1L) 70 return (EOF); 71 } 72 if (fp->_flags & __SRD) { 73 curoff -= fp->_r; 74 if (HASUB(fp)) 75 curoff -= fp->_ur; 76 } else if (fp->_flags & __SWR && fp->_p != NULL) 77 curoff += fp->_p - fp->_bf._base; 78 79 offset += curoff; 80 whence = SEEK_SET; 81 havepos = 1; 82 break; 83 84 case SEEK_SET: 85 case SEEK_END: 86 curoff = 0; /* XXX just to keep gcc quiet */ 87 havepos = 0; 88 break; 89 90 default: 91 errno = EINVAL; 92 return (EOF); 93 } 94 95 /* 96 * Can only optimise if: 97 * reading (and not reading-and-writing); 98 * not unbuffered; and 99 * this is a `regular' Unix file (and hence seekfn==__sseek). 100 * We must check __NBF first, because it is possible to have __NBF 101 * and __SOPT both set. 102 */ 103 if (fp->_bf._base == NULL) 104 __smakebuf(fp); 105 if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT)) 106 goto dumb; 107 if ((fp->_flags & __SOPT) == 0) { 108 if (seekfn != __sseek || 109 fp->_file < 0 || fstat(fp->_file, &st) || 110 (st.st_mode & S_IFMT) != S_IFREG) { 111 fp->_flags |= __SNPT; 112 goto dumb; 113 } 114 fp->_blksize = st.st_blksize; 115 fp->_flags |= __SOPT; 116 } 117 118 /* 119 * We are reading; we can try to optimise. 120 * Figure out where we are going and where we are now. 121 */ 122 if (whence == SEEK_SET) 123 target = offset; 124 else { 125 if (fstat(fp->_file, &st)) 126 goto dumb; 127 target = st.st_size + offset; 128 } 129 130 if (!havepos) { 131 if (fp->_flags & __SOFF) 132 curoff = fp->_offset; 133 else { 134 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR); 135 if (curoff == POS_ERR) 136 goto dumb; 137 } 138 curoff -= fp->_r; 139 if (HASUB(fp)) 140 curoff -= fp->_ur; 141 } 142 143 /* 144 * Compute the number of bytes in the input buffer (pretending 145 * that any ungetc() input has been discarded). Adjust current 146 * offset backwards by this count so that it represents the 147 * file offset for the first byte in the current input buffer. 148 */ 149 if (HASUB(fp)) { 150 curoff += fp->_r; /* kill off ungetc */ 151 n = fp->_up - fp->_bf._base; 152 curoff -= n; 153 n += fp->_ur; 154 } else { 155 n = fp->_p - fp->_bf._base; 156 curoff -= n; 157 n += fp->_r; 158 } 159 160 /* 161 * If the target offset is within the current buffer, 162 * simply adjust the pointers, clear EOF, undo ungetc(), 163 * and return. (If the buffer was modified, we have to 164 * skip this; see fgetline.c.) 165 */ 166 if ((fp->_flags & __SMOD) == 0 && 167 target >= curoff && target < curoff + n) { 168 register int o = target - curoff; 169 170 fp->_p = fp->_bf._base + o; 171 fp->_r = n - o; 172 if (HASUB(fp)) 173 FREEUB(fp); 174 fp->_flags &= ~__SEOF; 175 return (0); 176 } 177 178 /* 179 * The place we want to get to is not within the current buffer, 180 * but we can still be kind to the kernel copyout mechanism. 181 * By aligning the file offset to a block boundary, we can let 182 * the kernel use the VM hardware to map pages instead of 183 * copying bytes laboriously. Using a block boundary also 184 * ensures that we only read one block, rather than two. 185 */ 186 curoff = target & ~(fp->_blksize - 1); 187 if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR) 188 goto dumb; 189 fp->_r = 0; 190 if (HASUB(fp)) 191 FREEUB(fp); 192 fp->_flags &= ~__SEOF; 193 n = target - curoff; 194 if (n) { 195 if (__srefill(fp) || fp->_r < n) 196 goto dumb; 197 fp->_p += n; 198 fp->_r -= n; 199 } 200 return (0); 201 202 /* 203 * We get here if we cannot optimise the seek ... just 204 * do it. Allow the seek function to change fp->_bf._base. 205 */ 206 dumb: 207 if (__sflush(fp) || 208 (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) { 209 return (EOF); 210 } 211 /* success: clear EOF indicator and discard ungetc() data */ 212 if (HASUB(fp)) 213 FREEUB(fp); 214 fp->_p = fp->_bf._base; 215 fp->_r = 0; 216 /* fp->_w = 0; */ /* unnecessary (I think...) */ 217 fp->_flags &= ~__SEOF; 218 return (0); 219 } 220