#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <unistd.h>

static const char defshells[] = "/bin/sh\n/bin/csh\n";

static char *line;
static size_t linesize;
static FILE *f;

void endusershell(void)
{
	if (f) fclose(f);
	f = 0;
}

void setusershell(void)
{
	if (!f) f = fopen("/etc/shells", "rbe");
	if (!f) f = fmemopen((void *)defshells, sizeof defshells - 1, "rb");
}

char *getusershell(void)
{
	if (!f) setusershell();
	if (!f) return 0;
	do {
		ssize_t l = getline(&line, &linesize, f);
		if (l <= 0) return 0;
	} while (line[0] != '/');
	line[strcspn(line, "#\n")] = 0;
	return line;
}