System.map

In Linux, the System.map 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 kernel oopses. The kernel does the address-to-name translation itself when CONFIG_KALLSYMS is enabled so that tools like ksymoops are not required.^[1]

Internals

The following is part of a System.map file:^[2]

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.^[3]

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:^[5]

• as /boot/System.map-$(uname -r)
• building SVGAlib expects to find /lib/modules/$(uname -r)/build/System.map

See also

• Linux kernel oops
• Kernel panic

References