KSYMS(4) BSD Kernel Interfaces Manual KSYMS(4)NAMEksyms — kernel symbol table interface
The /dev/ksyms character device provides a read-only interface to a snap‐
shot 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.
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.
The STRTAB section contains the symbol name strings from
the kernel and any loaded modules that the symbol table
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 mod‐
ules into the kernel while the /dev/ksyms file is open but may contain
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
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.
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 ALSOioctl(2), nlist(3), elf(5), kldload(8)HISTORY
A ksyms device exists in many different operating systems. This imple‐
mentation is similar in function to the Solaris and NetBSD ksyms driver.
The ksyms driver first appeared in FreeBSD 8.0 to support lockstat(1).
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
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.
The ksyms driver was written by Stacey Son ⟨firstname.lastname@example.org⟩.
BSD April 5, 2009 BSD