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Segmentation faults can also occur independently of page faults: illegal access to a valid page is a segmentation fault, but not an invalid page fault, and segmentation faults can occur in the middle of a page (hence no page fault), for example in a buffer overflow that stays within a page but illegally overwrites memory.
On x86 there exists an older memory management mechanism known as segmentation. If the application loads a segment register with the selector of a non-present segment (which under POSIX-compliant OSes can only be done with assembly language), the exception is generated. Some OSes used that for swapping, but under Linux this generates SIGBUS.
$ ./test Segmentation fault Since the example code, when executed, generates a segmentation fault , lldb can be used to inspect the problem: $ lldb test (lldb) target create "test" Current executable set to 'test' (x86_64).
The operating system delays loading parts of the program from disk until the program attempts to use it and the page fault is generated. If the page is not loaded in memory at the time of the fault, then it is called a major or hard page fault. The page fault handler in the OS needs to find a free location: either a free page in memory, or a ...
The following is an excerpt from the Segfault story "Man clicks Internet banner ad": [3]. For the first time in recent memory, someone actually clicked an Internet banner ad.
Protection may encompass all accesses to a specified area of memory, write accesses, or attempts to execute the contents of the area. An attempt to access unauthorized [a] memory results in a hardware fault, e.g., a segmentation fault, storage violation exception, generally causing abnormal termination of the offending process.
Generally, if the fault originated in a user-mode program, the user-mode program is terminated. If, however, the fault originated in a core system driver or the operating system itself, the operating system usually saves diagnostic information either to a file or to the screen and stops operating.
A “Segmentation fault” is nothing more than an invalid page fault and the OS has chosen, via exception or other means, to close the process that generated the fault. See the Wiki page on “Page fault”, “General protection fault”, and “Bus Error” for a better idea of what is going on.