source: buchla-68k/libcio/lseek.c

/*
   =============================================================================
        lseek.c -- position a file to a specified byte location
        Version 7 -- 1987-10-28 -- D.N. Lynx Crowe
   =============================================================================
*/

#define DEBUGIT         0

#include "ram.h"

int32_t lseek(int16_t fd, int32_t pos, int16_t how)
{
        register struct fcb *fp;

        if ((fd < 0) OR (fd > MAXCHAN)) {

#if     DEBUGIT
        if (fsdebug)
                printf("lseek(%d):  bad file index\n", fd);
#endif

                errno = EBADF;          /* file number bad */
                return(-1L);
        }

        if (chantab[fd].c_seek EQ 0) {

#if     DEBUGIT
        if (fsdebug)
                printf("lseek(%d):  device not seekable\n", fd);
#endif

                errno = EINVAL;         /* device can't seek */
                return(-1L);
        }

        fp = chantab[fd].c_arg;         /* get fcb pointer from channel table */

        switch (how) {                  /* dispatch off of seek type */

        case 2:                         /* relative to end of file */
                pos += fp->curlen;
                break;

        case 1:                         /* relative to current position */
                pos += fp->offset + ((int32_t)fp->curlsn << FILESHFT);

        case 0:                         /* relative to start of file */
                break;

        default:
                errno = EINVAL;         /* invalid seek type */
                return(-1L);
        }

        if (pos < 0) {                          /* trap seeks before BOF */

                fp->offset = 0;
                fp->curlsn = 0;
                fp->modefl |= FC_ERR;

#if     DEBUGIT
        if (fsdebug)
                printf("lseek(%d):  seek (%ld) is before BOF\n", fd, pos);
#endif

                errno = EINVAL;
                return(-1L);

        } else if (pos > fp->curlen) {          /* trap seeks past EOF */

                fp->offset = fp->curlen & (BPSEC -1);
                fp->curlsn = (int16_t)(fp->curlen >> FILESHFT);
                fp->modefl |= FC_ERR;

#if     DEBUGIT
        if (fsdebug)
                printf("lseek(%d):  seek (%ld) is after EOF (%ld)\n",
                        fd, pos, fp->curlen);
#endif

                errno = EINVAL;
                return(-1L);
        }

        fp->offset = pos & ((int32_t)BPSEC - 1);        /* calculate sector offset */
        fp->curlsn = (int16_t)(pos >> FILESHFT);        /* calculate logical sector */

        if (_seek(fp) < 0) {                    /* position to the physical sector */

                fp->modefl |= FC_ERR;           /* couldn't seek */
                errno = EIO;
                return(-1L);
        }

        if (pos EQ fp->curlen)
                fp->modefl |= FC_EOF;

#if     DEBUGIT
        if (fsdebug)
                printf("lseek():  pos=%d, curlsn=%ld, curdsn=%ld, offset=%u\n",
                        pos, fp->curlsn, fp->curdsn, fp->offset);
#endif

        return(pos);                            /* return current position */
}