UNIX Filesystems: Evolution, Design, and Implementation

System Call Handling

Arguments passed to system calls are placed on the user stack prior to invoking a hardware instruction that then transfers the calling process from user mode to kernel mode. Once inside the kernel, any system call handler needs to be able to access the arguments, because the process may sleep awaiting some resource, resulting in a context switch, the kernel needs to copy these arguments into the kernel address space.

The sysent[] array specifies all of the system calls available, including the number of arguments.

By executing a hardware trap instruction, control is passed from user space to the kernel and the kernel trap() function runs to determine the system call to be processed. The C library function linked with the user program stores a unique value on the user stack corresponding to the system call. The kernel uses this value to locate the entry in sysent[] to understand how many arguments are being passed.

For a read() or write() system call, the arguments are accessible as follows:

fd = u.u_ar0[R0]
u_base = u.u_arg[0]
u_count = u.u_arg[1]

This is a little strange because the first and subsequent arguments are accessed in a different manner. This is partly due to the hardware on which 5th Edition UNIX was based and partly due to the method that the original authors chose to handle traps.

If any error is detected during system call handling, u_error is set to record the error found. For example, if an attempt is made to mount an already mounted filesystem, ...