Some clarity on the arguments of `fexecve`

The Linux man pages say that the third argument to fexecve is envp[] which my research has led me to believe is the environment to execute the file descriptor e.g. “/use/python3” for a Python file.

1. Is this true? Please correct me if I’m wrong

2. What if I’m using an ELF binary? What would the argument be then?

Note: I’ve tried this with the second and third arguments empty *char[] (if that’s what the types were) and I got:

> fexecve: invalid argument

Thanks in advance. I’m doing this on my phone so excuse any weird text. Also, the file descriptor is a memfd, if that means anything.

The envp argument is just like the same argument to execve() or execle(). It's an array of strings, where each element is an environment variable in the format variable=value, which will be used as the initial set of environment variables in the new process. The end of the array is indicated by a NULL value.

fexecve() is the same as execve(), except that it gets the program to run from an open file descriptor rather than a pathname; this can be useful to avoid some race conditions resulting from the executable or a directory in its pathname being replaced after an existence check.

The details of the envp parameter can be found in the execve() man page. If you don't need to provide a custom environment, you can use the global variable environ to have the new process inherit the current process's environment (this is how execl() and execv() work, since they don't take an explicit envp argument).