Variable suddenly changing inside fork() child process C - very weird behaviour - c

Question

I'm hoping someone here could help me cause I'm witnessing some weird behavior from my C code.
I have a Linux shell-like program in C. The code is 250 lines long, so I can't be posting it here.
In short, I have this function for non-built in commands:
int not_builtin(char** args, int background, char* line) {
int status;
pid_t pid;
pid = fork();
if (pid < 0) {
printf("fork failed\n");
}
else if (pid == 0) {
if (execvp(args[0], args) == -1) {
printf("exec failed\n");
}
return 0;
}
else {
if (!background) {
if (waitpid(pid, &status, 0) < 0) {
printf("An error occurred\n");
}
}
add_job(line, pid, background);
}
return 1;
}
When I run my shell on my PC (Macbook), everything works as expected. BUT, when I run this on my university server (Linux), I get a bug. I cannot properly debug this since on my computer everything works fine. The bug is it outputs: "exec failed" for any command (line 9).
By printing through the code, I was able to detect the following behavior:
The char** args are good throughout the code, I printed them until just before the "if (pid == 0)".
Once you enter the if statement mentioned above, printing the args returns some garbage.
Specifically, for an input: $ ls -l
I got printed ls -l before the if, and ?\?? after the if.
Like this for any other command string.
What could be the reason?
The whole day the program worked fine. It started to happen once I changed my "parse-line" function, to copy the line before editing. It now looks like this:
char** parse_line(char *line, int *background) {
char line_copy[BUFF_SIZE];
strcpy(line_copy, line);
char **tokens = malloc(BUFF_SIZE*sizeof(char*));
if (!tokens) {
printf("An error occurred\n");
exit(EXIT_FAILURE);
}
int position = 0;
char *token;
token = strtok(line_copy, DELIM);
while (token) {
tokens[position] = token;
position++;
token = strtok(NULL, DELIM);
}
// change the background option
if (position > 0 && strcmp(tokens[position-1], "&")==0) {
*background = 1;
tokens[position-1]=NULL;
line[strlen(line)-2] = '\0';
} else {
*background = 0;
tokens[position] = NULL;
}
return tokens;
}
Before the change I would perform the strtok directly on line, but now I figured I need that value intact. But how is this connect in any way?
Any help would be MUCH appreciated