Add program to calculate the shortest relative path for symlinks

Makefile.am

@@ -124,13 +124,15 @@ tools_kmod_CFLAGS = $(AM_CFLAGS)
 tools_kmod_LDADD = libkmod/libkmod-util.la \
 		   libkmod/libkmod.la
 
-noinst_PROGRAMS = tools/kmod-nolib
+noinst_PROGRAMS = tools/kmod-nolib scripts/shortest_relpath
 tools_kmod_nolib_SOURCES = $(tools_kmod_SOURCES)
 tools_kmod_nolib_CPPFLAGS = $(tools_kmod_CPPFLAGS)
 tools_kmod_nolib_CFLAGS = $(tools_kmod_CFLAGS)
 tools_kmod_nolib_LDADD = libkmod/libkmod-util.la \
 			 libkmod/libkmod-private.la
 
+scripts_shortest_relpath_SOURCES = scripts/shortest_relpath.c
+
 ${noinst_SCRIPTS}: tools/kmod-nolib
 	$(AM_V_GEN) ($(RM) $@; \
 		$(LN_S) $(notdir $<) $@)

tools/.gitignore

@@ -7,3 +7,4 @@ modinfo
 depmod
 kmod
 kmod-nolib
+shortest-relpath

tools/shortest-relpath.c

@@ -0,0 +1,100 @@
+#include <stdio.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+
+static void usage(const char *progname)
+{
+	printf("%s <path> <relative-to-path>\n", progname);
+}
+
+/*
+ * path must be absolute path, as the result of calling realpath()
+ */
+static void split_path(const char *path, char d[PATH_MAX], size_t *dlen,
+						char n[NAME_MAX], size_t *nlen)
+{
+	size_t dirnamelen, namelen;
+	char *p;
+
+	p = strrchr(path, '/'); /* p is never NULL since path is abs */
+	dirnamelen = p - path;
+	if (dirnamelen > 0)
+		memcpy(d, path, dirnamelen);
+	d[dirnamelen] = '\0';
+
+	namelen = strlen(p + 1);
+	if (namelen > 0)
+		memcpy(n, p + 1, namelen + 1);
+
+	if (dlen)
+		*dlen = dirnamelen;
+	if (nlen)
+		*nlen = namelen;
+}
+
+int main(int argc, char *argv[])
+{
+	size_t sympathlen, namelen, pathlen, i, j;
+	char name[NAME_MAX], path[PATH_MAX];
+	char sympath[PATH_MAX];
+	char buf[PATH_MAX];
+	char *p;
+
+	if (argc < 3) {
+		usage(basename(argv[0]));
+		return EXIT_FAILURE;
+	}
+
+	p = realpath(argv[1], buf);
+	if (p == NULL) {
+		fprintf(stderr, "could not get realpath of %s: %m\n", argv[1]);
+		return EXIT_FAILURE;
+	}
+	split_path(buf, path, &pathlen, name, &namelen);
+
+	p = realpath(argv[2], sympath);
+	if (p == NULL) {
+		fprintf(stderr, "could not get realpath of %s: %m\n", argv[2]);
+		return EXIT_FAILURE;
+	}
+	sympathlen = strlen(sympath);
+
+	for (i = 1; i < sympathlen && i < pathlen; i++) {
+		if (sympath[i] == path[i])
+			continue;
+		break;
+	}
+
+	if (i < sympathlen) {
+		while (sympath[i - 1] != '/')
+			i--;
+	}
+
+	p = &path[i];
+
+	j = 0;
+	if (i < sympathlen) {
+		memcpy(buf + j, "../", 3);
+		j += 3;
+	}
+
+	for (; i < sympathlen; i++) {
+		if (sympath[i] != '/')
+			continue;
+
+		memcpy(buf + j, "../", 3);
+		j += 3;
+	}
+
+	if (p != path + pathlen) {
+		memcpy(buf + j, p, path + pathlen - p);
+		j += path + pathlen - p;;
+		buf[j++] = '/';
+	}
+	memcpy(buf + j, name, namelen + 1);
+
+	printf("%s\n", buf);
+	return 0;
+}