setenv, unsetenv, putenv - c++

Question

I am working on a custom shell for a systems programming class. We were instructed to implement the built-in setenv() and unsetenv() commands with a hint of check man pages for putenv().
My issue is that setenv(char*, char*, int) and putenv(char*) do not seem to be working at all. My code for executing a command entered is as follows:
//... skipping past stuff for IO redirection
pid = fork();
if(pid == 0){
//child
if(!strcmp(_simpleCommands[0]->_arguments[0],"printenv")){
//check if command is "printenv"
extern char **environ;
int i;
for(i = 0; environ[i] != NULL; i++){
printf("%s\n",environ[i]);
}
exit(0);
}
if(!strcmp(_simpleCommands[0]->_arguments[0],"setenv")){
//if command is "setenv" get parameters char* A, char* B
char * p = _simpleCommands[0]->_arguments[1];
char * s = _simpleCommands[0]->_arguments[2];
//putenv(char* s) needs to be formatted A=B; A is variable B is value
char param[strlen(p) + strlen(s) + 1];
strcat(param,p);
strcat(param,"=");
strcat(param,s);
putenv(param);
//setenv(p,s,1);
exit(0);
}
if(!strcmp(_simpleCommands[0]->_arguments[0],"unsetenv")){
//remove environment variable
unsetenv(_simpleCommands[0]->_arguments[0]);
exit(0);
}
//execute command
execvp(_simpleCommands[0]->_arguments[0],_simpleCommands->_arguments);
perror("-myshell");
_exit(1);
}
//omitting restore IO defaults...
If I run printenv it works properly, but if I try to set a new variable using either putenv() or setenv() my printenv() command returns the exact same thing, so it does not appear to be working.
As a side note, the problem may not be with the functions or how I called them, because my shell is executing the commands as though it had to format a wildcard (* or ?) which I am not sure should happen.

Answer 1

You appear to be calling fork unconditionally before examining the command line. But some shell built-in commands need to run in the parent process, so that their effect persists. All the built-ins that manipulate the environment fall in this category.
As an aside, I wouldn't try to use the C library's environment manipulation functions if I were writing a shell. I'd use three-argument main, copy envp into a data structure under my full control, and then feed that back into execve. This is partially because I'm a control freak, and partially because it's nigh-impossible to do anything complicated with setenv and/or putenv and not have a memory leak. See this older SO question for gory details.

Answer 2

What make you think it is not working? I wrote a simple test case below...and it worked as expected.
Making sure you setevn and prientevn are called in the same process.
#include <stdlib.h>
#include <assert.h>
int main()
{
char * s= "stack=overflow";
int ret = putenv(s);
assert(ret == 0);
//printout all the env
extern char **environ;
int i;
for(i = 0; environ[i] != NULL; i++){
printf("%s\n",environ[i]);
}
return 0;
}
pierr#ubuntu:~/workspace/so/c/env$ ./test | grep stack
stack=overflow