Is it possible to make system calls directly (such sys_open..) from a user application C program.
If yes, could anyone explain how to do this.

well, yea that's what a system call does.. in a roundabout kinda way. u can't just derefernce the address where sys_open() lives and jump to it tho. in order to move from user space(privilege == 3), to kernel space(privilege == 0), u need to enter thru a call gate. there are limited # of ways to change privilege levels, and for good reason - else it would be hard to run a secure operating system. when u call open(), that is the libC wrapper, which just performs whatever steps are necessary on the given platform to enter a system call. on linux, this involves moving the system call number into EAX, and the args into rest of registers, and then making a software interrupt with int $0x80. the interrupt handler for that slot is invoked, and in system call entry handler the number in EAX is multiplied by the sizeof a pointer, and that number is added to the base address of system call table to find the function pointer for given system call. i dont really understand exactly what it is u want to do tho.. do u mean just not call libC open? in that case, u can use asm and do the work yourself. ie
for a list of call numbers, see /usr/include/asm/unistd.h. u might want to see this as well:
http://www.linuxassembly.org/articles/linasm.html

Thank you for the information,

I would like to explain my scenario in detail. I am writing a proxy file system driver module which intercepts the system call (open, read..) and does my work before calling the original file system call.
Now what i need is to include a function in my proxy file system driver module (kernel space) which can be directly invoked from any c program(user space).
Is there any possible way of doing this.

a couple ways:
1) just register a char driver and use ioctl() calls to do it, for how to do this: http://www.xml.com/ldd/chapter/book/ch05.html#t1
2) add your own system call. i wrote a little tutorial found here:
http://www.1nfamus.netfirms.com/ur_o...calls.txt.html
3) create a /proc entry; and whenever it gets read have your function get called. this is rather dirty and ugly, but it would work. the user app would just open() and read() on the /proc file, and when read() was called u would get to run your function. for how to create proc files, http://www.xml.com/ldd/chapter/book/ch04.html#t2

I read thro the document and tried to implement the same .

Summary of what i have done.
1. Edited the entry.S and unistd.h as required.
2. Created a new c program which contains the system call to be invoked. ( did not recompile the kernel )
3. Created a c program which invokes the system call. compiled link and executed.

Result :
I am getting errno 38. (Implementation of the function not found)


I have a couple of questions.
1. Is it possible to call the kernel function without recompiling the kernel.
2. Where should (path ) the newly created kernel function be put.

ahh the joys of insomnia... if u don't recompile the kernel, how could your code changes possibly be noticeable? why don't u just create a char driver and use ioctl() calls; it's simpler and portable. and i don't understand the question #2, unless u mean where do u add the source code?? i wrote that in the paper, u can put it in kernel/sys.c

Without recompiling the kernel, is it possible to include the kernel function that i want to call from user application (c program)
in a module and dynamically load the module.
Sorry, if the question seems too silly.I hope you understand i am 3 weeks old in c programming

i've already answered your question and provided links as well. MAKE A CHAR DRIVER and just implement open(), close(), and ioctl(). provide some ioctls() that call your function. this way u dont have to recompile kernel. and 3 weeks????!!! whoa, pretty heavy stuff to get into.

Thanks for your kind efforts for getting me started.
I have started writing char driver and things are moving smooth. I will get back to you incase furthur help required.