--- /dev/null
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+/*
+ * Returns the analog of "cd path" from a directory "cwd".
+ * The root is defined as empty path (instead of "/")
+ * An attempt to go past the root (with "..") leaves the path at root.
+ * The cwd is not expanded.
+ */
+char *pathexpand(const char *cwd, const char *path)
+{
+ static const char SEP[] = "/";
+ if (!*path) /* empty path -> "." (don't move) */
+ path = ".";
+ if (!*cwd || *SEP == *path) /* no cwd, or path begins with "/" */
+ cwd = "";
+
+ while (*cwd && *SEP == *cwd)
+ ++cwd;
+
+ size_t len = strlen(cwd);
+ char *out = malloc(len + 1 + strlen(path) + 1);
+ char *p = strcpy(out, cwd) + len;
+
+ for (; *path; ++path)
+ {
+ char *pl;
+ if (p > out && p[-1] != *SEP)
+ *p++ = *SEP;
+ pl = p;
+ while (*path && *SEP != *path)
+ *p++ = *path++;
+ *p = '\0';
+ /* ..."//"... */
+ if (p == pl)
+ ; /* just ignore */
+ /* ..."/./"... */
+ else if ( p - pl == 1 && '.' == *pl )
+ --p; /* just ignore */
+ /* ..."/../"... */
+ else if ( p - pl == 2 && '.' == pl[0] && '.' == pl[1] )
+ {
+ /* drop the last element of the resulting path */
+ if (pl > out && --pl > out)
+ for (--pl; pl > out && *SEP != *pl; --pl)
+ ;
+ p = pl > out ? ++pl: out;
+ }
+ /* ..."/path/"... */
+ else if (*path)
+ *p++ = *path; /* just add the separator */
+
+ if (!*path)
+ break;
+ }
+ if (p > out+1 && *SEP == p[-1])
+ --p;
+ *p = '\0';
+ return out;
+}
+
+#ifdef CHECK_PATHEXPAND
+static void check(const char *cwd, const char *path, const char *good)
+{
+ static int n = 0;
+ printf("%-2d: %10s$ cd %s", ++n, cwd, path);
+ char *t = pathexpand(cwd, path);
+ if ( strcmp(t, good) )
+ printf(" ____________________failed(%s)\n", t);
+ else
+ printf(" \033[32m%s\033[0m\n", t);
+ free(t);
+}
+
+int main(int argc, char **argv)
+{
+ /* 1 */ check("/onelevel", "aa", "onelevel/aa");
+ /* 2 */ check("/", "..", "");
+ /* 3 */ check("/", "../..", "");
+ /* 4 */ check("/one", "aa/../bb", "one/bb");
+ /* 5 */ check("/one/two", "aa//bb", "one/two/aa/bb");
+ /* 6 */ check("", "/aa//bb", "aa/bb");
+ /* 7 */ check("/one/two", "", "one/two");
+ /* 8 */ check("/one/two", "aa/..bb/x/../cc/", "one/two/aa/..bb/cc");
+ /* 9 */ check("/one/two", "aa/x/././cc////", "one/two/aa/x/cc");
+ /* 10 */ check("/one/two", "../../../../aa", "aa");
+ /* 11 */ check("one/", "../one/two", "one/two");
+ /* 12 */ check("", "../../two", "two");
+ /* 13 */ check("a/b/c", "../../two", "a/two");
+ /* 14 */ check("a/b/", "../two", "a/two");
+ /* 15 */ check("///", "../two", "two");
+ return 0;
+}
+#endif
+