Non-Standard Syscalls

Discussion in 'Linux Kernel' started by DevynCJohnson, May 18, 2014.

  1. DevynCJohnson

    DevynCJohnson Well-Known Member Staff Member Staff Writer

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    Series Index - http://www.linux.org/threads/linux-kernel-reading-guide.5384/

    There is another set of kernel calls called the non-standard syscalls or unimplemented system calls. These kernel calls are not part of the vanilla kernel, or at least not in recent versions.


    afs_syscall() - This is a seldomly used system call for OpenAFS. OpenAFS is an open-source alternative to the Andrew FileSystem, both of which are distributed filesystems. This syscall managed the Input/Output of OpenAFS. Most developers now use ioctl() instead.

    break() - No info available. I cannot find any information about this syscall. If anyone knows anything, please share with us.

    getpmsg() - This kernel call is the same as getmsg(). However, getpmsg() offered calling processes greater control than getmsg(). Both of these syscalls allowed a process to get a message from a stream.

    gtty(), stty() - This system call is mostly obsolete due to ioctl(). gtty() may still be used for backwards compatibility, but in general, developers may no longer see this syscall. It is (or was) used to control terminal devices through the device files (/dev/tty*). stty() is an equivalent of gtty().

    idle() - This kernel call makes the CPU idle.

    lock() - This system call locks or unlocks access to registers.

    madvise1() - This syscall is not used in the vanilla kernel, but is used in the Android-Linux kernels. madvice1() is an alias to madvice(), both of which advise the kernel on how to allocate and manage memory for the calling process.

    mpx() - This kernel call creates and manages multiplexed files.

    phys() - mmap() is now used instead of phys().

    prof() - This syscall is a profiler which is used to measure the performance of the kernel.

    putpmsg() - This is equivalent to putmsg(). Both of these syscalls are used to put messages on the message stream.

    security() - Some Linux Security Modules (LSMs) use (or had used) this kernel call to define system calls for security purposes. Most LSMs now use socket use socketcall().

    tuxcall() - This call comes from a TUX module and is sent to the kernel. The call asks the kernel to perform some task for the module. A TUX module is basically a server application/daemon in the form of a Linux module. Imagine an Apache server being a kernel module; that is essentially how TUX works.

    vserver() - This is a virtualization system call that is used by this specialized kernel (http://linux-vserver.org/Welcome_to_Linux-VServer.org).


    Last Note on System Calls

    This last "syscall" does not belong with the others, but I will mention it here. On some syscall tables (I will explain that in a moment), people may see a ni(), sys_ni(), ni_syscall(), or something of that manner. This is a syscall place holder. To better understand this, it is important to know that syscalls are assigned a syscall number. For instance, here is a syscall table for the v3.10 Android's Linux kernel (https://android.googlesource.com/ke...oid-3.10-adf/arch/m32r/kernel/syscall_table.S). Notice that each syscall has a number associated with it. For instance, write() is syscall "6". When syscalls are given new numbers or are removed, the ni() syscall is a null kernel call that reserves that syscall number for later or private use. These syscall tables may also be called "system call maps". The Linux kernel (when executing) stores the syscall map in the "%eax" register. These numbers are probably only important to syscall developers or assembly programmers.

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