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pathsplit.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
	char *env = getenv("PATH");
	int cnt = 1, i = 0;
	// just a fancy way of counting the number of ':' characters in a string.
	// we start off with the char pointer `p`, pointing to the beggining of `env`
	// and we do the third part of the for loop (cnt+=*p++==':'), for as long as
	// `*p`, which also means `*p != '\0'`, because strings are terminated by '\0'.
	// The third part may be more readable when we add parens to it:
	// `cnt += (*(p++) == ':')`
	// so. we increment `cnt` by `(*(p++) == ':')`, which means 1 when `*p == ':'`,
	// and 0 when it doesn't. `*p++` is just a way of saying: "give me the value
	// at `p`, and then increment"
	for (char *p = env; *p; cnt+=*p++==':');
	// ok, we now have the number of ':' characters + 1 in the variable `cnt`.

	// strok is explained here: https://stackoverflow.com/a/23456549/12954733
	char *ptr = strtok(env, ":");

	// we reserve an array of strings for every subpath in $PATH
	char **arr = malloc(cnt * sizeof(char*));
	// the string ptr looks something like this:
	// "/usr/bin\0/usr/local/sbin:/bin:..."
	// notice the NULL byte '\0' which was a ':', but was replaced by strtok.

	// now we are just assigning a pointer into the original `env` string
	// to the i-th index of the array, and calling strtok to replace another ':'
	// with a '\0'
	do {
		arr[i++] = ptr;
	} while(ptr = strtok(NULL, ":"));

	// now, our `env` string, and the `arr` array looks something like this:
	// env: "/usr/bin\0/usr/local/sbin\0/home/foo/.local/bin\0/bin"
	//       ^         ^                ^                     ^
	//       arr[0]    arr[1]           arr[2]                arr[3]
	// the string hasn't moved anywhere, and we are just using the fact that a
	// string must be NULL terminated, so when we do something like
	// `printf("%s", arr[0])`, it goes only to the closes null byte to the right

	// print out the array to confirm.
	for (int j = 0; j < i; j++) {
		printf("arr[%d] = '%s'\n", j, arr[j]);
	}

	return 0;
}