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Full Discussion: System calls
Top Forums Programming System calls Post 302176832 by Perderabo on Wednesday 19th of March 2008 08:57:41 AM
Old 03-19-2008
A system call usually involves a single machine language instruction that behaves very much like an interrupt. Anyone can invoke it. But it jumps to a specific routine in the kernel called syscall. syscall can only do system calls. The jump to syscall also switch into kernel mode. There is not a way to to switch to kernel mode and jump to arbitrary point in the kernel. You must jump to syscall. (At least this is what happens on a reasonably designed computer.) Also, there is no way to switch kernel mode and continue executing user code even if you are the kernel. In kernel mode you must be running code in kernel address space.
 

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SYSCALL_MODULE(9)					   BSD Kernel Developer's Manual					 SYSCALL_MODULE(9)

NAME
SYSCALL_MODULE -- syscall kernel module declaration macro SYNOPSIS
#include <sys/param.h> #include <sys/kernel.h> #include <sys/proc.h> #include <sys/module.h> #include <sys/sysent.h> SYSCALL_MODULE(name, int *offset, struct sysent *new_sysent, modeventhand_t evh, void *arg); DESCRIPTION
The SYSCALL_MODULE() macro declares a new syscall. SYSCALL_MODULE() expands into a kernel module declaration with name 'sys/name'. The rest of the arguments expected by this macro are: offset A pointer to an int which saves the offset in struct sysent where the syscall is allocated. new_sysent is a pointer to a structure that specifies the function implementing the syscall and the number of arguments this function needs (see <sys/sysent.h>). evh A pointer to the kernel module event handler function with the argument arg. Please refer to module(9) for more information. arg The argument passed to the callback functions of the evh event handler when it is called. EXAMPLES
A minimal example for a syscall module can be found in /usr/share/examples/kld/syscall/module/syscall.c. SEE ALSO
module(9) /usr/share/examples/kld/syscall/module/syscall.c AUTHORS
This manual page was written by Alexander Langer <alex@FreeBSD.org>. BSD
January 24, 2015 BSD
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