Spawn a new process reading the binary `executable_path`, passing `argv` as arguments to `main()` and setting `envp` as argument.
Places the process ID of the new process in `pid`.
If `executable_path` passed to `posix_spawn` is a relative path, it is resolved relative to the current working directory.
If `executable_path` passed to `posix_spawnp` is a relative path, it is resolved by searching through directories specified in the `PATH` environment variable.
The `posix_spawn_file_actions_t` and `posix_spawnattr_t` arguments may be `nullptr`. If they aren't, see [`posix_spawn_file_actions`(2)](help://man/3/posix_spawn_file_actions_init) and [`posix_spawnattr`(2)](help://man/3/posix_spawnattr_init) for what they do.
4.`executable_path` is loaded and starts running, as if `execve` or `execvpe` was called.
## Return value
If the process is successfully forked, returns 0.
Otherwise, returns an error number. This function does *not* return -1 on error and does *not* set `errno` like most other functions, it instead returns what other functions set `errno` to as result.
If the process forks successfully but spawnattr or file action processing or exec fail, `posix_spawn` returns 0 and the child exits with exit code `127`.
## Example
This simple example launches `/bin/Calculator`.
To make the child process use the parent's environment, it passes `environ` from `unistd.h`.
```**c++
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <spawn.h>
int main()
{
const char* argv[] = { "Calculator", nullptr };
pid_t child_pid;
if ((errno = posix_spawn(&child_pid, "/bin/Calculator", nullptr, nullptr, const_cast<char**>(argv), environ)))
