fardinanam / xv6-riscv

Xv6 for RISC-V

Working with xv6

Creating a new system call program

1. Create a new file <new_system_call_name> in user/ directory.

   The new file should contain the following:

       #include "kernel/param.h"
       #include "kernel/types.h"
       #include "kernel/stat.h"
       #include "user/user.h"
       
       int main(int argc, char *argv[]) {
           // code
           exit(exit_status);
       }

   For example, see the user/trace.c file

   Note: The file name should be as same as the command name that you want to use to execute the system call.

2. Add the new system call command name to user/Makefile as follows:

       UPROGS = \
           ...
           $U/_<new_command_name>\
           ...

   eg: $U/_trace \

   This will allow the make qemu to enable the system call.

   Note: The <new_command_name> part should be the same as the file name created in step 1.

3. Add an entry to user/usys.pl.

       entry("<new_system_call_name>");

   eg: entry("trace");

   This will route the system call into the kernel.

   Note: The <new_system_call_name> part does not have to be the same as the file name created in step 1. But all the names stated in the following steps should be the same as this one.

4. Add the prototype of the new system call in user/user.h.

   eg: int trace(int);

5. Define a system call number in kernel/syscall.h as follows:

       #define SYS_<new_system_call_name> <new_system_call_number>

   eg: #define SYS_trace 22

6. Define the system call function in kernel/proc.c

       <return type>
       <new_system_call_name>(<args>) {
           // code
       }

   eg: see the trace() function in kernel/proc.c

7. Add the prototype of the new system call in kernel/defs.h. Preferably under the proc.c comment.

   eg: int trace(int);

8. Open kernel/sysproc.c and add the handler function that will execute the system call which has just been defined in step 6.

       <return type>
       sys_<new_system_call_name>(void) {
           // code
       }

   eg: see the sys_trace() function in kernel/sysproc.c

9. In kernel/syscall.c,

   • extern the new system call handler function.

         extern int sys_<new_system_call_name>(void);

     eg: extern int sys_trace(void);

   • Add the new system call name to the syscalls array.

         [SYS_<new_system_call_name>] sys_<new_system_call_name>,

     eg: [SYS_trace] sys_trace,

10. Make required changes in other functions and files as required.