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NAME

     ksyms -- kernel symbol table interface

SYNOPSIS

     device ksyms

DESCRIPTION

     The /dev/ksyms character device provides a read-only interface to a
     snapshot of the kernel symbol table.  The in-kernel symbol manager is
     designed to be able to handle many types of symbols tables, however, only
     elf(5) symbol tables are supported by this device.  The ELF format image
     contains two sections: a symbol table and a corresponding string table.

           Symbol Table
                   The SYMTAB section contains the symbol table entries
                   present in the current running kernel, including the symbol
                   table entries of any loaded modules. The symbols are
                   ordered by the kernel module load time starting with kernel
                   file symbols first, followed by the first loaded module's
                   symbols and so on.

           String Table
                   The STRTAB section contains the symbol name strings from
                   the kernel and any loaded modules that the symbol table
                   entries reference.

     Elf formated symbol table data read from the /dev/ksyms file represents
     the state of the kernel at the time when the device is opened.  Since
     /dev/ksyms has no text or data, most of the fields are initialized to
     NULL.  The ksyms driver does not block the loading or unloading of
     modules into the kernel while the /dev/ksyms file is open but may contain
     stale data.

IOCTLS

     The ioctl(2) command codes below are defined in <sys/ksyms.h>.

     The (third) argument to the ioctl(2) should be a pointer to the type
     indicated.

           KIOCGSIZE (size_t)
                   Returns the total size of the current symbol table.  This
                   can be used when allocating a buffer to make a copy of the
                   kernel symbol table.

           KIOCGADDR (void *)
                   Returns the address of the kernel symbol table mapped in
                   the process memory.

FILES

     /dev/ksyms

ERRORS

     An open(2) of /dev/ksyms will fail if:

     [EBUSY]            The device is already open.  A process must close
                        /dev/ksyms before it can be opened again.

     [ENOMEM]           There is a resource shortage in the kernel.

     [ENXIO]            The driver was unsuccessful in creating a snapshot of
                        the kernel symbol table.  This may occur if the kernel
                        was in the process of loading or unloading a module.

SEE ALSO

     ioctl(2), nlist(3), elf(5), kldload(8)

HISTORY

     A ksyms device exists in many different operating systems.  This
     implementation is similar in function to the Solaris and NetBSD ksyms
     driver.

     The ksyms driver first appeared in FreeBSD 8.0 to support lockstat(1).

BUGS

     Because files can be dynamically linked into the kernel at any time the
     symbol information can vary.  When you open the /dev/ksyms file, you have
     access to an ELF image which represents a snapshot of the state of the
     kernel symbol information at that instant in time. Keeping the device
     open does not block the loading or unloading of kernel modules.  To get a
     new snapshot you must close and re-open the device.

     A process is only allowed to open the /dev/ksyms file once at a time.
     The process must close the /dev/ksyms before it is allowed to open it
     again.

     The ksyms driver uses the calling process' memory address space to store
     the snapshot.  ioctl(2) can be used to get the memory address where the
     symbol table is stored to save kernel memory.  mmap(2) may also be used
     but it will map it to another address.

AUTHORS

     The ksyms driver was written by Stacey Son <sson@freebsd.org>.