How/where is the working directory of a program stored?

Question

When a program accesses files, uses system(), etc., how and where is the current working directory of that program physically known/stored? Since logically the working directory of a program is similar to a global variable, it should ideally be thread-local, especially in languages like D where "global" variables are thread-local by default. Would it be possible to make the current working directory of a program thread-local?

Note: If you are not familiar with D specifically, even a language-agnostic answer would be useful.

Answer 1

On Linux, each process is represented by a process descriptor - a task_struct. This structure is defined in include/linux/sched.h in the kernel source.

One of the fields of task_struct is a pointer to an fs_struct, which stores filesystem-related information. fs_struct is defined in include/linux/fs_struct.h.

fs_struct has a field called pwd, which stores information about the current working directory (the filesystem it is on, and the details of the directory itself).