CONFIG(5) File Formats Manual CONFIG(5) NAME config -- kernel configuration file format DESCRIPTION A kernel configuration file specifies the configuration of a FreeBSD ker- nel. It is processed by config(8) to create a build environment where a kernel may be built using make(1). Lexical Structure A kernel configuration file comprises a sequence of specification direc- tives. A specification directive starts with a keyword at the beginning of the line and is followed by additional parameters. A specification directive may be terminated by a semicolon `;' or by a new- line. Long input lines may be broken into shorter lines by starting the second and subsequent lines with a white space character. Case is significant, "machine" and "MACHINE" are different tokens. A double quote character `"' starts a quoted string. All characters up to the next quote character form the value of the quoted string. A `"' char- acter may be inserted into a quoted string by using the sequence `\"'. Numbers are specified using C-style syntax. A `#' character starts a comment; all characters from the `#' character till the end of the current line are ignored. Whitespace between tokens is ignored, except inside quoted strings. White- space following a comment line is ignored. Configuration Directives Kernel configuration directives may appear in any order in a kernel config- uration file. Directives are processed in order of appearance with subse- quent directive lines overriding the effect of prior ones. The list of keywords and their meanings are as follows: cpu cputype Specify the CPU this kernel will run on. There can be more than one cpu directive in a configuration file. The allowed list of CPU names is architecture specific and is defined in the file sys/conf/options.<arch>. device name [, name [...]] devices name [, name [...]] Configures the specified devices for inclusion into the kernel im- age. Devices that are common to all architectures are defined in the file sys/conf/files. Devices that are specific to architecture arch are defined in the file sys/conf/files.<arch>. env filename Specifies a filename containing a kernel environment definition. The kernel will augment this compiled-in environment with the envi- ronment prepared for it at boot time by loader(8). Environment variables specified in the loader(8) environment will take prece- dence over environment variables specified in filename, and envi- ronment variables specified in the dynamic environment take prece- dence over both of these. loader_env.disabled=1 may be specified in the static environment to disable the loader(8) environment. Disabling the loader(8) should be done with caution and due consideration for whether or not it supplies environment variables needed for properly booting the sys- tem. static_env.disabled=1 may be specified in the loader(8) environment to disable use of the static environment. This option has no ef- fect if specified in any environment after the loader(8) environ- ment is processed. This option is not usable in conjunction with loader_env.disabled. This directive is useful for setting kernel tunables in embedded environments that do not start from loader(8). All env and envvar directives will be processed and added to the static environment in reversed order of appearance so that later specified variables properly override earlier specified variables. Note that within filename, the first appearance of a given variable will be the first one seen by the kernel, effectively shadowing any later appearances of the same variable within filename. envvar setting Specifies an individual environment setting to be added to the ker- nel's compiled-in environment. setting must be of the form "name=value". Optional quotes are supported in both name and value. All env and envvar directives will be processed and added to the static environment in reversed order of appearance so that later specified variables properly override earlier specified variables. files filename Specifies a file containing a list of files specific to that kernel configuration file (a la files.<arch>). hints filename Specifies a file to load a static device configuration specifica- tion from. From FreeBSD 5.0 onwards, the kernel reads the system's device configuration at boot time (see device.hints(5)). This di- rective configures the kernel to use the static device configura- tion listed in filename. Hints provided in this static device configuration will be over- written in the order in which they're encountered. Hints in the compiled-in environment takes precedence over compiled-in hints, and hints in the environment prepared for the kernel by loader(8) takes precedence over hints in the compiled-in environment. Once the dynamic environment becomes available, all compiled-in hints will be added to the dynamic environment if they do not al- ready have an override in the dynamic environment. The dynamic en- vironment will then be used for all searches of hints. static_hints.disabled=1 may be specified in either a compiled-in environment or the loader(8) environment to disable use of these hints files. This option has no effect if specified in any envi- ronment after the loader(8) environment is processed. The file filename must conform to the syntax specified by de- vice.hints(5). Multiple hints lines are allowed. The resulting hints will be the files concatenated in reverse order of appearance so that hints in later files properly override hints in earlier files. ident name Set the kernel name to name. At least one ident directive is re- quired. include filename Read subsequent text from file filename and return to the current file after filename is successfully processed. includeoptions filename Specifies a file containing a list of additional options specific to that kernel configuration file. This is useful for build envi- ronments that need to add custom options and is often used in con- junction with the makeoption directive. machine arch [cpuarch] Specifies the architecture of the machine the kernel is being com- piled for. Legal values for arch include: arm64 The 64-bit ARM application architecture. arm The ARM architecture amd64 The AMD x86-64 architecture. i386 The Intel x86 based PC architecture. powerpc The IBM PowerPC architecture. riscv The RISC-V architecture. If argument cpuarch is specified, it points config(8) to the cpu architecture of the machine. When cpuarch is not specified, it is assumed to be the same as arch. arch corresponds to MACHINE. cpuarch corresponds to MACHINE_ARCH. A kernel configuration file may have only one machine directive, unless the second one matches the machine argument in the first one exactly. makeoption options makeoptions options Add options to the generated makefile. The options argument is a comma separated list of one or more op- tion specifications. Each option specification has the form MakeVariableName[=Value] MakeVariableName+=Value and results in the appropriate make(1) variable definition being inserted into the generated makefile. If only the name of the make(1) variable is specified, value is assumed to be the empty string. Note that, as the common makefiles overwrite the CFLAGS variable after having processed the configuration file, customizing CFLAGS directly via makeoptions is not possible. Nonetheless, custom com- piler flags can be specified using the CONF_CFLAGS variable in- stead. Its content is appended to CFLAGS after the common make- files have set the latter, allowing to override their compilation flags. Example: makeoptions MYMAKEOPTION="foo" makeoptions MYMAKEOPTION+="bar" makeoptions MYNULLMAKEOPTION makeoptions CONF_CFLAGS+="-DSOME_CONTROLLING_MACRO" maxusers number This optional directive is used to configure the size of some ker- nel data structures. The parameter number can be 0 (the default) or an integer greater than or equal to 2. A value of 0 indicates that the kernel should configure its data structures according to the size of available physical memory. If auto configuration is requested, the kernel will set this tunable to a value between 32 and 384 for 32-bit systems, or scale the value higher based on available memory for 64-bit systems. As explained in tuning(7), this tunable can also be set at boot time using loader(8). nocpu cputype Remove the specified CPU from the list of previously selected CPUs. This directive can be used to cancel the effect of cpu directives in files included using include. nodevice name [, name [...]] nodevices name [, name [...]] Remove the specified devices from the list of previously selected devices. This directive can be used to cancel the effects of device or devices directives in files included using include. nomakeoption name nomakeoptions name Removes previously defined make(1) option name from the kernel build. This directive can be used to cancel the effects of makeoption directives in files included using include. nooption name [, name [...]] nooptions name [, name [...]] Remove the specified kernel options from the list of previously de- fined options. This directive can be used to cancel the effects of option or options directives in files included using include. option optionspec [, optionspec [...]] options optionspec [, optionspec [...]] Add compile time kernel options to the kernel build. Each option specification has the form name[=value] If value is not specified, it is assumed to be NULL. Options com- mon to all architectures are specified in the file sys/conf/options. Options specific to architecture arch are speci- fied in the file sys/conf/options.<arch>. FILES sys/compile/NAME Compile directory created from a kernel configura- tion. sys/conf/Makefile.arch Makefile fragments for architecture arch. sys/conf/files Devices common to all architectures. sys/conf/files.arch Devices for architecture arch. sys/conf/options Options common to all architectures. sys/conf/options.arch Options for architecture arch. SEE ALSO kenv(1), make(1), device.hints(5), loader.conf(5), config(8), kldload(8), loader(8) Samuel J. Leffler and Michael J. Karels, Building 4.4BSD Kernels with Con- fig. HISTORY The config(8) utility first appeared in 4.1BSD, and was subsequently re- vised in 4.4BSD. The kernel configuration mechanism changed further in FreeBSD 4.0 and FreeBSD 5.0, moving toward an architecture supporting dynamic kernel con- figuration. FreeBSD 15.1 June 13, 2025 CONFIG(5)
NAME | DESCRIPTION | FILES | SEE ALSO | HISTORY
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