/proc filesystem is inherently not portable between operating systems. There are Unices where it does not exist at all.
/proc filesystem on AIX is documented here: https://www.ibm.com/support/knowledgecenter/en/ssw_aix_61/com.ibm.aix.files/proc.htm
a.out file in the
object subdirectory, it says:
[...] The name
a.out also appears in the directory as a synonym for the executable file associated with the text of the running process.
object directory makes it possible for a controlling process to get access to the object file and any shared libraries (and consequently the symbol tables), without the process first obtaining the specific path names of those files.
This means that accessing the
a.out file in that structure is more or less the opposite of what you want to do. It allows a user program to access the executable of a process without knowing its full path.
There is also structures in
/proc on AIX that contains the basename of the executable, but again, this is not what you're interested in.
Since a process can set its own zeroth command line argument (the name of the process that shows up in
ps would also not be a surefire solution to find the absolute path to the running executable, even if it was executed with a full path.
What you could do is to hunt down the executable by its inode.
An example of how to do this is described in the currently accepted answer to the question "How to identify executable path with its PID on AIX 5 or more".
It may also be good to ask yourself why you need this information from querying a running process, as it's usually clear from studying a script or program exactly what external utilities it is invoking.