KSYMS(4) FreeBSD Kernel Interfaces Manual KSYMS(4)


ksymskernel symbol table interface


device ksyms


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 formatted 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.


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.




An open(2) of /dev/ksyms will fail if:
The device is already open. A process must close /dev/ksyms before it can be opened again.
There is a resource shortage in the kernel.
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.


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).


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


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.

April 5, 2009 FreeBSD