utils-std

offline_realpath.c (3354B)

// Based on the realpath(3) function from musl
// Copyright © 2005-2020 Rich Felker, et al.
// SPDX-License-Identifier: MIT
#define _POSIX_C_SOURCE 200809L
#include "fs.h"
#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
static size_t
slash_len(const char *s)
{
	const char *s0 = s;
	while(*s == '/')
		s++;
	return s - s0;
}
static char *
__strchrnul(const char *s, int c)
{
	c = (unsigned char)c;
	if(!c) return (char *)s + strlen(s);
	for(; *s && *(unsigned char *)s != c; s++)
		;
	return (char *)s;
}
// realpath(3) but without checking for symlinks
char *
offline_realpath(const char *restrict filename, char *restrict resolved)
{
	char stack[PATH_MAX + 1];
	char output[PATH_MAX];
	size_t p, q, l, l0, nup = 0;
	int check_dir = 0;
	if(!filename)
	{
		errno = EINVAL;
		return 0;
	}
	l = strnlen(filename, sizeof stack);
	if(!l)
	{
		errno = ENOENT;
		return 0;
	}
	if(l >= PATH_MAX) goto toolong;
	p = sizeof stack - l - 1;
	q = 0;
	memcpy(stack + p, filename, l + 1);
	/* Main loop. Each iteration pops the next part from stack of
	 * remaining path components and consumes any slashes that follow.
	 * If not a link, it's moved to output; if a link, contents are
	 * pushed to the stack. */
	for(;; p += slash_len(stack + p))
	{
		/* If stack starts with /, the whole component is / or //
		 * and the output state must be reset. */
		if(stack[p] == '/')
		{
			check_dir = 0;
			nup = 0;
			q = 0;
			output[q++] = '/';
			p++;
			/* Initial // is special. */
			if(stack[p] == '/' && stack[p + 1] != '/') output[q++] = '/';
			continue;
		}
		char *z = __strchrnul(stack + p, '/');
		l0 = l = z - (stack + p);
		if(!l && !check_dir) break;
		/* Skip any . component but preserve check_dir status. */
		if(l == 1 && stack[p] == '.')
		{
			p += l;
			continue;
		}
		/* Copy next component onto output at least temporarily, to
		 * call readlink, but wait to advance output position until
		 * determining it's not a link. */
		if(q && output[q - 1] != '/')
		{
			if(!p) goto toolong;
			stack[--p] = '/';
			l++;
		}
		if(q + l >= PATH_MAX) goto toolong;
		memcpy(output + q, stack + p, l);
		output[q + l] = 0;
		p += l;
		int up = 0;
		if(l0 == 2 && stack[p - 2] == '.' && stack[p - 1] == '.')
		{
			up = 1;
			/* Any non-.. path components we could cancel start
			 * after nup repetitions of the 3-byte string "../";
			 * if there are none, accumulate .. components to
			 * later apply to cwd, if needed. */
			if(q <= 3 * nup)
			{
				nup++;
				q += l;
				continue;
			}
		}
		check_dir = 0;
		if(up)
		{
			while(q && output[q - 1] != '/')
				q--;
			if(q > 1 && (q > 2 || output[0] != '/')) q--;
			continue;
		}
		if(l0) q += l;
		check_dir = stack[p];
	}
	output[q] = 0;
	if(output[0] != '/')
	{
		if(!getcwd(stack, sizeof stack)) return 0;
		l = strlen(stack);
		/* Cancel any initial .. components. */
		p = 0;
		while(nup--)
		{
			while(l > 1 && stack[l - 1] != '/')
				l--;
			if(l > 1) l--;
			p += 2;
			if(p < q) p++;
		}
		if(q - p && stack[l - 1] != '/') stack[l++] = '/';
		if(l + (q - p) + 1 >= PATH_MAX) goto toolong;
		memmove(output + l, output + p, q - p + 1);
		memcpy(output, stack, l);
		q = l + q - p;
	}
	if(resolved)
		return memcpy(resolved, output, q + 1);
	else
		return strdup(output);
toolong:
	errno = ENAMETOOLONG;
	return 0;
}