Question

I am reading about device drivers and I have a question related to the UNIX philosophy regarding everything as file.

When a user issues a command say for eg, opening a file then what comes into action – System call or File Operation?
sys_open is a system call and open is a file operation. Can you please elaborate on the topic.

Thanks in advance.

Answer 1

Quick answer, I hope it’ll help:

All system calls work the same way. The system call number is stored somewhere (e.g. in a register) together with the system call parameters. In case of open system calls parameters are: pointer to the filename and permissions string. Then the open function raises a software interruption using the adequate intruction (syscall, int …, it depends on the HW).

As for any interruption, the kernel is invoked (in kernel mode) to handle the interruption. The system detects that the interruption was caused by a system call, then read the system call number in the register sees it is a open system call, create the file descriptor in the kernel memory and proceed to actually open the file by calling the driver open function. The file descriptor id is then stored back into a register and returns to user mode.

The file descriptor is then retrieved from the register and returned by open().