Parsing command-line entries in C: implementing a shell - c

Question

I'm implementing a shell in C, and I ran into some problems parsing command-line entries. I want my parser method to separate command-line entries that are delimited by the whitespace character, and return the result as a double char pointer. ie, say I have "ls -l >ls.txt", my parser should return a char **r with r[0]="ls", r[1]="-l", and r[2]=">ls.txt".
Here is the code for my current parse method, which is, by the way, segfaulting, and I'm out of ideas as to how to fix that:
char **parser(int *argc, char *s)
{
char **r;
char *t, *m;
int i,n,size;
t = malloc(strlen(s)); // firs i used this instead of *r, but i run
// into trouble when i have more than two
// argc. ( You see why, right?)
//strcpy(t,s);
i = 0;
size = 5;
r = malloc(size*sizeof(char *));
while (( m = strchr(s, ' '))) {
n = ((int)m) - ((int)s);
if (i==0) {
*r = malloc(n);
} else {
*r = realloc(*r, n);
}
strncpy(*r, s, n);
*r[n]= '\0';
s = (char*)(s+n+1);
if (i == size)
r = realloc(r, (size = 2*size)*sizeof(char*));
i++;
r = (char **)(r + sizeof(char*));
}
s[strlen(s)-1] = '\0';
if ((i<1) || (strlen(s)>1)) {
*r = s;
}
*argcp = ++i;
return r;
}
I know my code isn't ideal. It could be made better using strsep, but my main concer is how to manage memory for the double char pointer I want to return.
Thanks for the help!

Answer 1

This is a quick stab.
My C is so rusty, all of the hinges are stuck, so.
The premise is that you will end up with a pointer to an array of pointers. The key detail though, is at the end of that list of pointers, is the argument data itself. So when you're done, you simply need to free the returned pointer.
Untested. There may well be a one off error sneaking in here.
Edit, I compiled and quickly tested it.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char **parser(int *argc, char *s) {
char **r, **rp;
char *t, *p, *w;
void *vp;
int l;
l = strlen(s); // size of cmd line
vp = malloc(l + (*argc * sizeof(char *))); // total buffer size
t = (char *)(vp + (*argc * sizeof(char *))); // offset into buffer for argument copy
r = (char **)vp; // start of buffer, start of pointer array to arguments
strcpy(t, s); // copy arguments in to buffer
p = t; // parsing pointer
w = t; // word pointer for each argument
rp = r; // storage for first pointer
while(*p) { // while not at end of string
if (*p == ' ') { // if we find a space
if (w) { // if we have a word pointer assigned
*rp++ = w; // store the word pointer
w = NULL; // set word pointer to null
*p = '\0'; // terminate argument with a 0
} // else do nothing continue to skip spaces
} else {
if (w == NULL) { // If we haven't got a new arg yet
w = p; // set it
} // otherwise, just keep scanning
}
p++; // move along the string
}
if (w) { // clean up at the end if we have an arg
*rp++ = w;
w = NULL; // no reason to set 0 at the end, it's already there from strcpy
}
return r;
}
int main() {
char *cmd = "arg1 arg2";
int argc = 2;
char **r = parser(&argc, cmd);
printf("%s\n",r[0]);
printf("%s\n",r[1]);
}