A system call would not be useful most of the time if it did not return some kind of a
value: The file descriptor of an open file, the number of bytes read to a buffer, the
system time, etc.
Additionally, the system needs to inform us if an error occurs: A file does not exist,
system resources are exhausted, we passed an invalid parameter, etc.
The traditional place to look for information about various system calls under UNIX® systems are the manual pages. FreeBSD describes its
system calls in section 2, sometimes in section 3.
For example, open(2) says:
open() returns a non-negative integer,
termed a file descriptor. It returns -1 on failure, and sets
errno to indicate the error.
The assembly language programmer new to UNIX and
FreeBSD will immediately ask the puzzling question: Where is errno and how do I get to it?
Note: The information presented in the manual pages applies to C programs. The
assembly language programmer needs additional information.
Unfortunately, it depends... For most system calls it is in EAX, but not for all. A good rule of thumb, when working with a
system call for the first time, is to look for the return value in EAX. If it is not there, you need further research.
Note: I am aware of one system call that returns the value in EDX:
SYS_fork. All others I have
worked with use EAX. But I have not worked with them all
Tip: If you cannot find the answer here or anywhere else, study libc source code and see how it interfaces with the kernel.
errno is part of the C language, not the UNIX kernel. When accessing kernel services directly, the error
code is returned in EAX, the same register the proper return
value generally ends up in.
This makes perfect sense. If there is no error, there is no error code. If there is an
error, there is no return value. One register can contain either.
When using the standard FreeBSD calling convention, the carry
flag is cleared upon success, set upon failure.
When using the Linux emulation mode, the signed value in EAX is non-negative upon success, and contains the return value. In
case of an error, the value is negative, i.e., -errno.