libc: Implement realpath

specified here:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/realpath.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Ie52dcd1c5c5faa4b033901eedd7182bbb9473f7a

include/stdlib.h

@@ -167,6 +167,8 @@ static inline void _Exit(int s)
 int       system(FAR const char *cmd);
 #endif
 
+FAR char *realpath(FAR const char *path, FAR char *resolved);
+
 /* String to binary conversions */
 
 long      strtol(FAR const char *nptr, FAR char **endptr, int base);

libs/libc/stdlib/Make.defs

@@ -21,7 +21,7 @@
 # Add the stdlib C files to the build
 
 CSRCS += lib_abs.c lib_abort.c lib_div.c lib_ldiv.c lib_lldiv.c
-CSRCS += lib_itoa.c lib_labs.c lib_llabs.c
+CSRCS += lib_itoa.c lib_labs.c lib_llabs.c lib_realpath.c
 CSRCS += lib_bsearch.c lib_rand.c lib_qsort.c lib_srand.c
 CSRCS += lib_strtol.c lib_strtoll.c lib_strtoul.c lib_strtoull.c
 CSRCS += lib_strtod.c lib_strtof.c lib_strtold.c lib_checkbase.c

libs/libc/stdlib/lib_realpath.c

@@ -0,0 +1,235 @@
+/****************************************************************************
+ * libs/libc/stdlib/lib_realpath.c
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <sys/stat.h>
+
+#include <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+FAR char *realpath(FAR const char *path, FAR char *resolved)
+{
+#ifdef CONFIG_PSEUDOFS_SOFTLINKS
+  char wbuf[2][PATH_MAX];
+  int nlnk = 0;
+  int idx = 0;
+  ssize_t n;
+#endif
+  FAR const char *q;
+  FAR char *fres = NULL;
+  FAR char *p;
+  struct stat sb;
+  size_t len;
+
+  if (path == NULL)
+    {
+      errno = EINVAL;
+      return NULL;
+    }
+
+  if (*path == '\0')
+    {
+      errno = ENOENT;
+      return NULL;
+    }
+
+  if (resolved == NULL)
+    {
+      fres = resolved = malloc(PATH_MAX);
+      if (resolved == NULL)
+        {
+          return NULL;
+        }
+    }
+
+  /* Build real path one by one with paying an attention to .,
+   * .. and symbolic link.
+   */
+
+  /* `p' is where we'll put a new component with prepending
+   * a delimiter.
+   */
+
+  p = resolved;
+
+  /* If relative path, start from current working directory. */
+
+  if (*path != '/')
+    {
+      /* check for resolved pointer to appease coverity */
+
+      if (getcwd(resolved, PATH_MAX) == NULL)
+        {
+          goto out;
+        }
+
+      len = strlen(resolved);
+      if (len > 1)
+        {
+          p += len;
+        }
+    }
+
+loop:
+
+  /* Skip any slash. */
+
+  while (*path == '/')
+    {
+      path++;
+    }
+
+  if (*path == '\0')
+    {
+      if (p == resolved)
+        {
+          *p++ = '/';
+        }
+
+      *p = '\0';
+      return resolved;
+    }
+
+  /* Find the end of this component. */
+
+  q = path;
+  do
+    {
+      q++;
+    }
+  while (*q != '/' && *q != '\0');
+
+  /* Test . or .. */
+
+  if (path[0] == '.')
+    {
+      if (q - path == 1)
+        {
+          path = q;
+          goto loop;
+        }
+
+      if (path[1] == '.' && q - path == 2)
+        {
+          /* Trim the last component. */
+
+          if (p != resolved)
+            {
+              while (*--p != '/')
+                {
+                  continue;
+                }
+            }
+
+          path = q;
+          goto loop;
+        }
+    }
+
+  /* Append this component. */
+
+  if (p - resolved + 1 + q - path + 1 > PATH_MAX)
+    {
+      errno = ENAMETOOLONG;
+      goto out;
+    }
+
+  p[0] = '/';
+  memcpy(&p[1], path, q - path);
+  p[1 + q - path] = '\0';
+
+  /* If this component is a symlink, toss it and prepend link
+   * target to unresolved path.
+   */
+
+  if (lstat(resolved, &sb) == -1)
+    {
+      goto out;
+    }
+
+#ifdef CONFIG_PSEUDOFS_SOFTLINKS
+  if (S_ISLNK(sb.st_mode))
+    {
+      if (nlnk++ >= SYMLOOP_MAX)
+        {
+          errno = ELOOP;
+          goto out;
+        }
+
+      n = readlink(resolved, wbuf[idx], sizeof(wbuf[0]) - 1);
+      if (n <= 0)
+        {
+          if (n == 0)
+            {
+              errno = ENOENT;
+            }
+
+          goto out;
+        }
+
+      /* Append unresolved path to link target and switch to it. */
+
+      if (n + (len = strlen(q)) + 1 > sizeof(wbuf[0]))
+        {
+          errno = ENAMETOOLONG;
+          goto out;
+        }
+
+      memcpy(&wbuf[idx][n], q, len + 1);
+      path = wbuf[idx];
+      idx ^= 1;
+
+      /* If absolute symlink, start from root. */
+
+      if (*path == '/')
+        {
+          p = resolved;
+        }
+
+      goto loop;
+    }
+#endif
+
+  if (*q == '/' && !S_ISDIR(sb.st_mode))
+    {
+      errno = ENOTDIR;
+      goto out;
+    }
+
+  /* Advance both resolved and unresolved path. */
+
+  p += 1 + q - path;
+  path = q;
+  goto loop;
+
+out:
+  free(fres);
+  return NULL;
+}