Kernel & Userland Are Out of Sync
Three parts make up a computer system: the hardware, the system kernel and the userland. Userland applications need the kernel to make the hardware perform the tasks needed to play a music file, access the Internet or type a report. When a userland application and the kernel use a different version of a key compiler or system library, your system is said to be "out of sync." In BSD Unix systems, you may get a "Userland and kernel are out of sync" error message.
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Kernel and Userland Interactions
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A userland application you need to install and use may require a service or library file that is only found in the latest update to a particular kernel library or hardware driver. Some userland application programmers statically link libraries to their applications. A statically linked application does not compile if a library it needs is not present. If an application is dynamically linked to a library version you don't have, however, the application compiles, and you may not know your system is out of sync until you actually run the application.
BSD Approach
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BSD systems are distributed as a complete package that includes the kernel and a large number of userland applications. The advantage to this approach is that all applications, utilities, libraries and drivers are matched to a specific kernel version. In addition, all kernel and userland source files are compiled using the same compiler version and compiler libraries. Any version discrepancies between a userland application and the kernel services it needs are dealt with before a BSD distribution becomes available to the public.
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Linux Approach
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Unlike BSD, the source code for the Linux kernel is released separate from any userland applications. Linux distributions like Fedora or Debian may use the same version of the kernel, but structure their userland environment in different and often incompatible ways. It is possible for a Linux distribution to contain some userland applications that were compiled for one kernel version and others that were compiled against another kernel version.
Rebuilding World
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One way BSD users can avoid "userland and kernel are out of sync" errors is to rebuild the entire system whenever the need arises to upgrade the kernel or use the latest version of a hardware driver or userland application. This process is called "rebuilding world." The exact sequence of steps varies between BSD distributions. According to the FreeBSD handbook, however, the steps include backing up the current system, acquiring the source code to the desired BSD version, building a new source code compiler, compiling and installing the new kernel, then creating a new, fully synced userland.
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