.gitignore

*~
01-orig
02-global
03-struct

01-orig.c

#include <stdio.h>

/* First version: this does everything in a single call to count_to() */
void count_to(int target) {
    int i;
    /* using a loop */
    for(i = 1; i <= target; i++) {
        printf("%d\n", i);
        /* however there is no way to return this value:
         * you can only return one time, that ends the function */
    }
}

int main(int argc, char **argv) {
    count_to(10);
    return 0;
}

02-global.c

#include <stdio.h>

/* This is the easiest way to keep state: a global variable */
int i = 1; /* for(i = 1; ...) */

int count_to(int target) {
    if(i <= target) { /* for(...; i <= target; ...) */
        /* Now this function will print once per call. The function ends, but
         * the state is persisted for the next call */
        printf("%d\n", i);
        /* (we could return a value instead of printing) */
        i++; /* for(...; i++) */
        return 1; /* We tell the user of this function whether to keep going */
    } else {
        return 0; /* ... or stop */
    }
}

int main(int argc, char **argv) {
    int counting = 1;
    while(counting) {
        counting = count_to(10);
    }
    return 0;
}

03-struct.c

#include <stdio.h>

/* This is the same thing without global variables. It is idiomatic C code */

/* Use a struct because there are multiple variables. If there was only one, we
 * could get away with just passing a pointer to that.
 * This is what strtok_r() does for example (the "state" is the saveptr) */
struct count_state {
    /* Note that those are the local variables of our first implementation :) */
    int i;
    int target;
};

void count_init(struct count_state *state, int target) {
    state->i = 1; /* for(i = 1; ...) */
    state->target = target;
}

int count_next(struct count_state *state) {
    if(state->i <= state->target) { /* for(...; i <= target; ...) */
        printf("%d\n", state->i);
        state->i++; /* for(...; i++) */
        return 1;
    } else {
        return 0;
    }
}

int main(int argc, char **argv) {
    int counting = 1;
    struct count_state state;
    count_init(&state, 10);
    while(counting) {
        counting = count_next(&state);
    }
    return 0;
}

Makefile

.PHONY: all

all: 01-orig 02-global 03-struct

%: %.c
	cc -Wall -Wextra -Wno-unused-parameter -ansi -pedantic -o $@ $<