getline_concept.c

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

// comically small buffer to prove the recursion concept
#define BUF 16

static ssize_t innerget(char **lineptr, size_t *n, int delim, FILE *stream, size_t offset) {
	char buf[BUF];
	for(size_t i=0; i<BUF; i++) {
		int c = getc(stream);
		if(c != EOF)
			buf[i] = c;
		if(c == EOF || c == delim) {
			*n = offset+i;
			*lineptr = realloc(*lineptr, *n+1);
			if(!*lineptr) return -1;
			(*lineptr)[*n] = '\0';
			memcpy(*lineptr+offset, buf, i);
			if(c == EOF) return -1;
			else return *n;
		}
	}
	ssize_t r = innerget(lineptr, n, delim, stream, offset+BUF);
	if(*lineptr) {
		memcpy(*lineptr+offset, buf, BUF);
	}
	return r;
}
ssize_t getdelim(char **lineptr, size_t *n, int delim, FILE *stream) {
	return innerget(lineptr, n, delim, stream, 0);
}
ssize_t getline(char **lineptr, size_t *n, FILE *stream) {
	return getdelim(lineptr, n, '\n', stream);
}

int main() {
	size_t n;
	char *p = 0;
	getline(&p, &n, stdin);
	printf("got %zu '%s'\n", n, p);
	free(p);
	n=0; p=0;
	getline(&p, &n, stdin);
	printf("got %zu '%s'\n", n, p);
}

// extending the concept to asprintf is left as an exercise for the reader