Is there just one errno buffer. For example, if I run a function and the function returns a -1 and sets errno, then that errno will have a value. However, if say I run several processes using fork and they each run a function, each time a child runs that function and returns an error(assuming they all returned an error) errno would get overwritten each time?

Basically there's just one errno per memory space?

So say I run httpd and mysqld. And say both error out and set errno, first httpd then mysqld. Are there two errno that I can check say one for httpd and one for mysqld? Or is there just one and the value in errno will be that of mysqld since it overwrote it last?

I'm not a specialist when it comes to processes, threads etc. So I don't know for sure what happens when you start threading and/or forking.

But (in global terms) both httpd and mysqld have their own errno; so a 'bad file descriptor' in the one and a 'file not found' in the other will be correctly reported by the respective programs. And you can not access them using another program (they are internal).

No, each process and each thread has their own errno.

None of the other processes (or threads, if you used threads instead of processes) would see any change in their errno. The process or thread that made the call (with the error occurring) is the only one that sees the error.

If you use processes (fork() call), all variables and data structures work that way. At the fork() call, they get copied, that's all. There is nothing strange about errno, it is just a normal variable; the C library functions just happen to use it to describe the error when an error occurs.

If you use threads and a relatively recent compiler, you can create per-thread variables that work exactly like errno using the __thread keyword. For example,
Each of your threads will see a consistent value (that is, a value that only changes when the thread itself modifies it) for my_var, but each thread will see their own value. (Also, for example printf("my_var is at %p\n", &my_var); will print a different address for my_var for each thread.)

Yes, httpd has it's own errno, and mysqld has one for itself.

(Technically, both httpd and mysqld use either multiple threads or multiple processes, so they have more than one errno; one for every process and every thread.)

Note that errno is not Linux-specific. The Linux kernel does not even know it exists. It is purely a C standard thing, and is defined by the C library. (Although it is defined by the C library, and many programs can use the same C library at the same time, each process will still get their own copy: the C library just defines it. The value is stored separately for each process and thread in their data area.)

I hope this clears it up for you.

Thank you, yes that cleared it up.