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36 * @(#)fseek.c 8.3 (Berkeley) 1/2/94
37 * $FreeBSD: src/lib/libc/stdio/fseek.c,v 1.9.2.1 2001/03/05 10:56:58 obrien Exp $
38 * $DragonFly: src/lib/libc/stdio/fseek.c,v 1.2 2003/06/17 04:26:46 dillon Exp $
41 #include <sys/types.h>
48 #include "libc_private.h"
50 #define POS_ERR (-(fpos_t)1)
53 fseek(fp, offset, whence)
58 return (fseeko(fp, offset, whence));
62 * Seek the given file to the given offset.
63 * `Whence' must be one of the three SEEK_* macros.
66 fseeko(fp, offset, whence)
71 register fpos_t (*seekfn) __P((void *, fpos_t, int));
72 fpos_t target, curoff;
77 /* make sure stdio is set up */
83 * Have to be able to seek.
85 if ((seekfn = fp->_seek) == NULL) {
86 errno = ESPIPE; /* historic practice */
92 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument.
93 * After this, whence is either SEEK_SET or SEEK_END.
99 * In order to seek relative to the current stream offset,
100 * we have to first find the current stream offset a la
101 * ftell (see ftell for details).
103 if (fp->_flags & __SOFF)
104 curoff = fp->_offset;
106 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
112 if (fp->_flags & __SRD) {
116 } else if (fp->_flags & __SWR && fp->_p != NULL)
117 curoff += fp->_p - fp->_bf._base;
126 curoff = 0; /* XXX just to keep gcc quiet */
137 * Can only optimise if:
138 * reading (and not reading-and-writing);
139 * not unbuffered; and
140 * this is a `regular' Unix file (and hence seekfn==__sseek).
141 * We must check __NBF first, because it is possible to have __NBF
142 * and __SOPT both set.
144 if (fp->_bf._base == NULL)
146 if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT))
148 if ((fp->_flags & __SOPT) == 0) {
149 if (seekfn != __sseek ||
150 fp->_file < 0 || fstat(fp->_file, &st) ||
151 (st.st_mode & S_IFMT) != S_IFREG) {
152 fp->_flags |= __SNPT;
155 fp->_blksize = st.st_blksize;
156 fp->_flags |= __SOPT;
160 * We are reading; we can try to optimise.
161 * Figure out where we are going and where we are now.
163 if (whence == SEEK_SET)
166 if (fstat(fp->_file, &st))
168 target = st.st_size + offset;
172 if (fp->_flags & __SOFF)
173 curoff = fp->_offset;
175 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
176 if (curoff == POS_ERR)
185 * Compute the number of bytes in the input buffer (pretending
186 * that any ungetc() input has been discarded). Adjust current
187 * offset backwards by this count so that it represents the
188 * file offset for the first byte in the current input buffer.
191 curoff += fp->_r; /* kill off ungetc */
192 n = fp->_up - fp->_bf._base;
196 n = fp->_p - fp->_bf._base;
202 * If the target offset is within the current buffer,
203 * simply adjust the pointers, clear EOF, undo ungetc(),
204 * and return. (If the buffer was modified, we have to
205 * skip this; see fgetln.c.)
207 if ((fp->_flags & __SMOD) == 0 &&
208 target >= curoff && target < curoff + n) {
209 register int o = target - curoff;
211 fp->_p = fp->_bf._base + o;
215 fp->_flags &= ~__SEOF;
221 * The place we want to get to is not within the current buffer,
222 * but we can still be kind to the kernel copyout mechanism.
223 * By aligning the file offset to a block boundary, we can let
224 * the kernel use the VM hardware to map pages instead of
225 * copying bytes laboriously. Using a block boundary also
226 * ensures that we only read one block, rather than two.
228 curoff = target & ~(fp->_blksize - 1);
229 if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR)
232 fp->_p = fp->_bf._base;
235 fp->_flags &= ~__SEOF;
238 if (__srefill(fp) || fp->_r < n)
247 * We get here if we cannot optimise the seek ... just
248 * do it. Allow the seek function to change fp->_bf._base.
252 (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) {
256 /* success: clear EOF indicator and discard ungetc() data */
259 fp->_p = fp->_bf._base;
261 /* fp->_w = 0; */ /* unnecessary (I think...) */
262 fp->_flags &= ~__SEOF;