System.map
In Linux, the file is a symbol table used by the kernel.
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 panics and [Linux kernel (operating system)|kernel oops|kernel oops]es. 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 aSystem.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 is the type of a symbol. Thenm 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 absoluteB or b for uninitialized data section D or d for initialized data sectionG or g for initialized data section for small objects i for sections specific to DLLsN for debugging symbolp for stack unwind sectionR or r for read only data sectionS or s for uninitialized data section for small objectsT or t for text sectionU for undefinedV or v for weak objectW 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,System.map is located in the root of the source directory. However, some further software installation steps expect to locate the file elsewhere: