函数os_file_pread

/*******************************************************************//**
Does a synchronous read operation in Posix.
@return    number of bytes read, -1 if error */
static
ssize_t
os_file_pread(
/*==========*/
    os_file_t    file,    /*!< in: handle to a file */
    void*        buf,    /*!< in: buffer where to read */
    ulint        n,    /*!< in: number of bytes to read */
    ulint        offset,    /*!< in: least significant 32 bits of file
                offset from where to read */
    ulint        offset_high) /*!< in: most significant 32 bits of
                offset */
{
    off_t    offs;
#if defined(HAVE_PREAD) && !defined(HAVE_BROKEN_PREAD)
    ssize_t    n_bytes;
#endif /* HAVE_PREAD && !HAVE_BROKEN_PREAD */

    ut_a((offset & 0xFFFFFFFFUL) == offset);

    /* If off_t is > 4 bytes in size, then we assume we can pass a
    64-bit address */

    if (sizeof(off_t) > 4) {
        offs = (off_t)offset + (((off_t)offset_high) << 32);

    } else {
        offs = (off_t)offset;

        if (offset_high > 0) {
            fprintf(stderr,
                "InnoDB: Error: file read at offset > 4 GB\n");
        }
    }

    os_n_file_reads++;

#if defined(HAVE_PREAD) && !defined(HAVE_BROKEN_PREAD)
    os_mutex_enter(os_file_count_mutex);
    os_file_n_pending_preads++;
    os_n_pending_reads++;
    os_mutex_exit(os_file_count_mutex);

    n_bytes = pread(file, buf, (ssize_t)n, offs);

    os_mutex_enter(os_file_count_mutex);
    os_file_n_pending_preads--;
    os_n_pending_reads--;
    os_mutex_exit(os_file_count_mutex);

    return(n_bytes);
#else
    {
        off_t    ret_offset;
        ssize_t    ret;
#ifndef UNIV_HOTBACKUP
        ulint    i;
#endif /* !UNIV_HOTBACKUP */

        os_mutex_enter(os_file_count_mutex);
        os_n_pending_reads++;
        os_mutex_exit(os_file_count_mutex);

#ifndef UNIV_HOTBACKUP
        /* Protect the seek / read operation with a mutex */
        i = ((ulint) file) % OS_FILE_N_SEEK_MUTEXES;

        os_mutex_enter(os_file_seek_mutexes[i]);
#endif /* !UNIV_HOTBACKUP */

        ret_offset = lseek(file, offs, SEEK_SET);

        if (ret_offset < 0) {
            ret = -1;
        } else {
            ret = read(file, buf, (ssize_t)n);
        }

#ifndef UNIV_HOTBACKUP
        os_mutex_exit(os_file_seek_mutexes[i]);
#endif /* !UNIV_HOTBACKUP */

        os_mutex_enter(os_file_count_mutex);
        os_n_pending_reads--;
        os_mutex_exit(os_file_count_mutex);

        return(ret);
    }
#endif
}