In
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
, the file is a
symbol table
In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier (or symbols), constants, procedures and functions in a program's source code is associated with inf ...
used by the
kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel (image processing), a matrix used for image convolution
* Compute kernel, in GPGPU programming
* Kernel method, in machine lea ...
.
A symbol table is a look-up between symbol names and their addresses in memory. A symbol name may be the name of a variable or the name of a function. The System.map is required when the address of a symbol name, or the symbol name of an address, is needed. It is especially useful for debugging
kernel panic
A kernel panic (sometimes abbreviated as KP) is a safety measure taken by an operating system's kernel upon detecting an internal fatal error in which either it is unable to safely recover or continuing to run the system would have a highe ...
s and
kernel oopses. The kernel does the address-to-name translation itself when
CONFIG_KALLSYMS is enabled so that tools like ksymoops are not required.
Internals
The following is part of a
System.map file:
c041bc90 b packet_sklist
c041bc94 b packet_sklist_lock
c041bc94 b packet_socks_nr
c041bc98 A __bss_stop
c041bc98 A _end
c041c000 A pg0
ffffe400 A __kernel_vsyscall
ffffe410 A SYSENTER_RETURN
ffffe420 A __kernel_sigreturn
ffffe440 A __kernel_rt_sigreturn
Because addresses may change from one build to the next, a new
System.map is generated for each build of the kernel.
Symbol types
The character between the address and the symbol (separated by spaces) is the ''type'' of a symbol. The
nm utility program on
Unix systems lists the symbols from object files. The
System.map is directly related to it, in that this file is produced by
nm on the whole kernel program just like
nm lists the symbols and their types for any small object programs.
Some of these types are:
*
A for absolute
*
B or
b for uninitialized data section (called BSS)
*
D or
d for initialized data section
*
G or
g for initialized data section for small objects (global)
*
i for sections specific to DLLs
*
N for debugging symbol
*
p for stack unwind section
*
R or
r for read only data section
*
S or
s for uninitialized data section for small objects
*
T or
t for text (code) section
*
U for undefined
*
V or
v for weak object
*
W or
w for weak objects which have not been tagged so
*
- for stabs symbol in an a.out object file
*
? for "symbol type unknown"
Filesystem location
After building the
Linux kernel
The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...
,
System.map is located in the root of the source directory. However, some further software installation steps expect to locate the file elsewhere:
* as
/boot/System.map-$(uname
uname (short for ''unix name'') is a computer program in Unix and Unix-like computer operating systems that prints the name, version and other details about the current machine and the operating system running on it.
History
The uname system cal ...
-r)
* building
SVGAlib
SVGAlib was an open-source low-level graphics library which ran on Linux and FreeBSD and allowed programs to change video mode and display full-screen graphics, without the use of a windowing system
In computing, a windowing system (or window ...
expects to find
/lib/modules/$(uname -r)/build/System.map
See also
*
Linux kernel oops
In computing, an oops is a serious but non-fatal error in the Linux kernel. An oops often precedes a kernel panic, but may in some cases allow continued operation with compromised reliability. The term does not stand for anything, other than that ...
*
Kernel panic
A kernel panic (sometimes abbreviated as KP) is a safety measure taken by an operating system's kernel upon detecting an internal fatal error in which either it is unable to safely recover or continuing to run the system would have a highe ...
References
Executable file formats
Linux kernel
{{linux-stub