11.4 Return Values

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.

11.4.1 Man Pages

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:

If successful, 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.

11.4.2 Where Are the Return Values?

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 yet.

Tip: If you cannot find the answer here or anywhere else, study libc source code and see how it interfaces with the kernel.

11.4.3 Where Is errno?

Actually, nowhere...

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.

11.4.4 Determining an Error Occurred

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.