How to capture environment after fork and exec in c? - c

Question

I have a C function to do a fork and exec that will be called twice.
The first call executes a shell script (call it setenv.sh) which can be any kind of shell (bash/korn/c/perl etc) that will set environment variables. The envp array will be NULL for this call but the intent was that it will return a populated array based on environ from the child process after setenv.sh has run.
The second call will be a C or java program that needs a certain environment to run so for this call, the envp array will be the populated one returned from the first call.
int execute(char **args, int argc, char **envp)
{
char *function = "execute";
int status, i;
pid_t p, pid;
extern int errno;
sigset_t mask, savemask;
struct sigaction ignore, saveint, savequit;
int fd[2];
pipe(fd);
sigemptyset(&ignore.sa_mask);
ignore.sa_handler = SIG_IGN;
ignore.sa_flags=0;
sigaction(SIGINT, &ignore, &saveint);
sigaction(SIGQUIT, &ignore, &savequit);
sigemptyset(&mask);
sigaddset(&mask, SIGCHLD);
sigprocmask(SIG_BLOCK, &mask, &savemask);
if ((pid=fork()) < 0) status = -1;
if (pid ==0) {
/* Child */
close(fd[0]);
sigaction(SIGINT, &saveint, (struct sigaction *) 0);
sigaction(SIGQUIT, &savequit, (struct sigaction *) 0);
sigprocmask(SIG_SETMASK, &savemask, (sigset_t *) 0);
printf("Command Line Parameters\n");
printf("-----------------------\n");
for (i = 0; i < argc; i++) {
printf("[%d]: %s\n", (i+1), args[i]);
}
if (execve(*args, args, envp) < 0)
{
sprintf(err_data,"Failed to execute %s", args[0]);
perror(err_data);
return(FAILED);
}
write(fd[1], &environ, sizeof(environ));
close(fd[1]);
}
while (waitpid(pid, &status, 0) < 0) {
if (errno != EINTR) {
status = -1;
break;
}
}
if (status==0) {
read(fd[0], &envp, sizeof(envp));
}
close(fd[0]);
sigaction(SIGINT, &saveint, (struct sigaction *) 0);
sigaction(SIGQUIT, &savequit, (struct sigaction *) 0);
sigprocmask(SIG_SETMASK, &savemask, (sigset_t *) 0);
return(status);
}
This function is working fine without the pipe code to execute a real program passed in and i can also pass it a set of environment variables in an envp array and it runs in that environment fine.
However, in testing with the pipe included, i find after the exec of setenv.sh, the child process never executes the writing of environ to the pipe and the parent then just blocks on the read from pipe.
I understand why it doesnt work - because the exec of the shell script overwrites the original C code in the child. The question is, is there a way to achieve the aim of running a shell script with exec and capturing the resulting ENVIRONMENT back in the parent (not the same as capturing stdin/stdout/stderr). Assume you cannot change the contents of setenv.sh because it may be provided by a third party.
No need to nitpick over error handling etc.. , this is a work in progress so just after some inputs in how to achieve the aim.
An alternative i considered was parsing the setenv.sh script in the parent to obtain the variables into an array which can then be passed to the real program. Problem with this is the setenv.sh script might contain if statement blocks and includes of other shell scripts so i really wanted to capture the environment at the end of the run of setenv.sh (by exec'ing it) and passing this back to the parent.
Any suggestions appreciated ?

Answer 1

You basically can't solve this generally without using debugging facilities of your operating system and digging into the memory of your child process. Which basically requires you to do write half of a debugger.
The closest you can get with a third-party script is something like this. Let's say that the script is for /bin/bash. You write your own wrapper script like this:
#!/bin/bash
. setenv.sh
env >&3
Where 3 is the file descriptor number of your pipe. You can write equivalent scripts for other shells. The only reason this works though is because the "setenv.sh" script is executed inside your wrapper script without creating a child process. Environment variables can only be communicated to children of a process.
In a system I use at work we have environment variables that need to be unified between many different programs that come from various scripts, many of which we don't have any control over. The way we resolved that mess is that instead of environment variables we require those scripts to output "KEY=VALUE\n" lines and then import them into scripts, makefiles, etc. through simple scripts (if required). That's probably the best you can do.

Answer 2

You can use :
extern char **environ;
environ is defined as a global variable in the Glibc source file posix/environ.c.