execvp() - unsupported SysV option - c

Question

I am trying to write a simple shell in C which takes in a command and uses a child process to execute that command. For example, if I input:
ps -ael
my child process should execute that command along with its arguments. I print out my commands as they are stored in an array. This what I see:
Array[0] = ps
Array[1] = -ael
Array[2] = NULL
When I execute I get this:
error: unsupported SysV option
Usage:
ps [options]
Try 'ps --help <simple|list|output|threads|misc|all>'
or 'ps --help <s|l|o|t|m|a>'
for additional help text.
For more details see ps(1).
My code below.
int main(void)
{
char *args[MAX_LINE/2 +1]; // command line arguments
char *cmdLine;
int should_run = 1; // flag to determine when to exit the program
int i, x;
printf("osh> ");
fflush(stdout);
fgets(cmdLine, MAX_LINE, stdin);
char *token = strtok(cmdLine, " ");
int position = 0;
while (token != NULL)
{
args[position++] = token;
token = strtok(NULL, " ");
}
i = 0;
while (args[i] != NULL)
{
printf("Array[%d] = %s\n", i, args[i]);
i++;
}
if (args[i] == NULL) printf("Array[%d] = NULL", i);
x = 0;
pid_t pid;
/* fork a child process*/
pid = fork();
if (pid < 0)
{
/*Error occured*/
fprintf(stderr, "Fork failed.");
return 1;
}
else if (pid == 0)
{
/*child process*/
execvp(args[0], args); //error here
}
else
{
/*Parent process*/
wait(NULL);
printf("\nChild complete\n");
}
}

Answer 1

The string returned by fgets() includes the newline, but you're not removing it from the string. So you're setting args[1] to "-ael\n", and \n is not a valid option.
Include newline in your strtok() delimiters:
char *token = strtok(cmdLine, " \n");
int position = 0;
while (token != NULL)
{
args[position++] = token;
token = strtok(NULL, " \n");
}
and then it will not be included in the tokens.
You should have been able to see this in your output, I'll bet it printed:
Array[0] = ps
Array[1] = -ael
Array[2] = NULL
with a blank line there.
BTW, I don't see where you're ever setting the last argument to NULL. The while loop stops when strtok() returns NULL, so it never assigns that result to args[position++]. You need to add:
args[position] = NULL;
after the loop.
And there's no need for if (args[i] == NULL) -- the loop before that stops when that condition is met, so it's guaranteed to be true.