Why the environment variable is unset after using setenv( )

Question

I wrote a C program in Linux to set the values of environment variables using setenv, but after execution, when I run set or export, t

Answer 1

The C library treats environment variables as global settings you can read with getenv and adjust with setenv/putenv, and that inherit across calls to the exec family, but that is a convenient fiction. As far as the kernel is concerned, the environment variables are a second set of arguments to main. This becomes clear if you look at the actual system call, execve, underlying the exec family. This is its C prototype:

int execve(const char *filename, char *const argv[], char *const envp[]);
                                                     ^^^^^^^^^^^^^^^^^^

See that third argument? That's the only way for process A to set process B's environment variables.¹ Therefore, the only time process A can set process B's environment variables is when process A starts process B, via fork and execve.

Your program is started by the shell, so the shell got to set its copy of the environment variables, but there is no way for it to push a change back to the shell -- just as there is no way for a C function to change the values of its arguments in its caller.

¹ Do not bring up ptrace.