CODEWITHSUNDEEP
unixfreebsdsystem-calls
Jothikumar Rathinamoorthy
Jothikumar Rathinamoorthy

Reputation: 73

Passing return values from a system call to user process in freebsd

I have implemented a system call which returns values like 0, 1 and 2, according to some conditions in the process priority. I am performing a syscall() in a file. When the system call returns 0, I am getting the return value of syscall() as 0. But, when it return 1 or 2, I am getting -1 for 1 and the system call number for any other values. Please let me know, is it possible to return values apart from 0 and -1.

Upvotes: 1

Views: 693

Answers (1)

Garrett Wollman
Garrett Wollman

Reputation: 46

The kernel implementation of a system call does not return a value, it returns an error code. The first argument to your function is a "struct thread", and you pass the return value back to userspace by setting td->td_retval[0] to the desired return value. See, for example, the implementation of getpid() in sys/kern/kern_prot.c.

Upvotes: 3

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