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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 1.8 2005/07/23 20:23:06 joerg Exp $
41 #include "namespace.h"
42 #include <sys/types.h>
48 #include "un-namespace.h"
51 #include "libc_private.h"
52 #include "priv_stdio.h"
54 #define POS_ERR (-(fpos_t)1)
57 fseek(FILE *fp, long offset, int whence)
59 return (fseeko(fp, offset, whence));
63 fseeko(fp, offset, whence)
70 /* make sure stdio is set up */
75 ret = _fseeko(fp, offset, whence);
81 * Seek the given file to the given offset.
82 * `Whence' must be one of the three SEEK_* macros.
85 _fseeko(FILE *fp, off_t offset, int whence)
87 fpos_t (*seekfn) (void *, fpos_t, int);
88 fpos_t target, curoff;
94 * Have to be able to seek.
96 if ((seekfn = fp->_seek) == NULL) {
97 errno = ESPIPE; /* historic practice */
102 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument.
103 * After this, whence is either SEEK_SET or SEEK_END.
109 * In order to seek relative to the current stream offset,
110 * we have to first find the current stream offset a la
111 * ftell (see ftell for details).
113 if (fp->pub._flags & __SOFF)
114 curoff = fp->_offset;
116 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
121 if (fp->pub._flags & __SRD) {
122 curoff -= fp->pub._r;
125 } else if (fp->pub._flags & __SWR && fp->pub._p != NULL)
126 curoff += fp->pub._p - fp->_bf._base;
135 curoff = 0; /* XXX just to keep gcc quiet */
145 * Can only optimise if:
146 * reading (and not reading-and-writing);
147 * not unbuffered; and
148 * this is a `regular' Unix file (and hence seekfn==__sseek).
149 * We must check __NBF first, because it is possible to have __NBF
150 * and __SOPT both set.
152 if (fp->_bf._base == NULL)
154 if (fp->pub._flags & (__SWR | __SRW | __SNBF | __SNPT))
156 if ((fp->pub._flags & __SOPT) == 0) {
157 if (seekfn != __sseek ||
158 fp->pub._fileno < 0 || _fstat(fp->pub._fileno, &st) ||
159 (st.st_mode & S_IFMT) != S_IFREG) {
160 fp->pub._flags |= __SNPT;
163 fp->_blksize = st.st_blksize;
164 fp->pub._flags |= __SOPT;
168 * We are reading; we can try to optimise.
169 * Figure out where we are going and where we are now.
171 if (whence == SEEK_SET)
174 if (_fstat(fp->pub._fileno, &st))
176 target = st.st_size + offset;
180 if (fp->pub._flags & __SOFF)
181 curoff = fp->_offset;
183 curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
184 if (curoff == POS_ERR)
187 curoff -= fp->pub._r;
193 * Compute the number of bytes in the input buffer (pretending
194 * that any ungetc() input has been discarded). Adjust current
195 * offset backwards by this count so that it represents the
196 * file offset for the first byte in the current input buffer.
199 curoff += fp->pub._r; /* kill off ungetc */
200 n = fp->_extra->_up - fp->_bf._base;
204 n = fp->pub._p - fp->_bf._base;
210 * If the target offset is within the current buffer,
211 * simply adjust the pointers, clear EOF, undo ungetc(),
212 * and return. (If the buffer was modified, we have to
213 * skip this; see fgetln.c.)
215 if ((fp->pub._flags & __SMOD) == 0 &&
216 target >= curoff && target < curoff + n) {
217 int o = target - curoff;
219 fp->pub._p = fp->_bf._base + o;
223 fp->pub._flags &= ~__SEOF;
228 * The place we want to get to is not within the current buffer,
229 * but we can still be kind to the kernel copyout mechanism.
230 * By aligning the file offset to a block boundary, we can let
231 * the kernel use the VM hardware to map pages instead of
232 * copying bytes laboriously. Using a block boundary also
233 * ensures that we only read one block, rather than two.
235 curoff = target & ~(fp->_blksize - 1);
236 if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR)
239 fp->pub._p = fp->_bf._base;
242 fp->pub._flags &= ~__SEOF;
245 if (__srefill(fp) || fp->pub._r < n)
253 * We get here if we cannot optimise the seek ... just
254 * do it. Allow the seek function to change fp->_bf._base.
258 (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) {
261 /* success: clear EOF indicator and discard ungetc() data */
264 fp->pub._p = fp->_bf._base;
266 /* fp->pub._w = 0; */ /* unnecessary (I think...) */
267 fp->pub._flags &= ~__SEOF;