gdb undocumented errno 2

`errno 2` corresponds to the error `No such file or directory`.

In GDB, the `errno` command can be used to print the error message associated with the most recent system call error. However, `errno` is not a documented command in GDB and may not be available in all versions or configurations of GDB.

To use `errno` in GDB, you can follow these steps:

1. Set a breakpoint in your program where you expect the error to occur.
2. Run the program until the breakpoint is hit.
3. Use the `errno` command to print the error message associated with the most recent system call error.
4. Continue debugging the program as needed.

Here's an example:

(gdb) break main
(gdb) run
...
Breakpoint 1, main () at main.c:10
10      int fd = open("nonexistent_file.txt", O_RDONLY);
(gdb) errno
$1 = 2
(gdb) continue
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x00005555555546f3 in main () at main.c:16
16      return 0;

In this example, we set a breakpoint at the beginning of `main`, and then ran the program. When the breakpoint was hit, we used the `open` system call to attempt to open a nonexistent file, resulting in `errno` being set to 2. We then used the `errno` command to print the error message, and continued running the program until it segfaulted.