Slightly modernize realpath(3).

SUSv4 requires that implementation returns EINVAL if supplied path is NULL,
and ENOENT if path is empty string [1].
Bring prototype in conformance with SUSv4, adding restrict keywords.
Allow the resolved path buffer pointer be NULL, in which case realpath(3)
allocates storage with malloc().

PR:	kern/121897 [1]
MFC after:	2 weeks

include/stdlib.h

@@ -201,7 +201,7 @@ int	 posix_openpt(int);
 char	*ptsname(int);
 int	 putenv(char *);
 long	 random(void);
-char	*realpath(const char *, char resolved_path[]);
+char	*realpath(const char * __restrict, char * __restrict);
 unsigned short
 	*seed48(unsigned short[3]);
 #ifndef _SETKEY_DECLARED

lib/libc/stdlib/realpath.3

@@ -31,7 +31,7 @@
 .\"     @(#)realpath.3	8.2 (Berkeley) 2/16/94
 .\" $FreeBSD$
 .\"
-.Dd February 16, 1994
+.Dd April 19, 2010
 .Dt REALPATH 3
 .Os
 .Sh NAME
@@ -43,7 +43,7 @@
 .In sys/param.h
 .In stdlib.h
 .Ft "char *"
-.Fn realpath "const char *pathname" "char resolved_path[PATH_MAX]"
+.Fn realpath "const char *pathname" "char *resolved_path"
 .Sh DESCRIPTION
 The
 .Fn realpath
@@ -64,7 +64,8 @@ argument
 .Em must
 refer to a buffer capable of storing at least
 .Dv PATH_MAX
-characters.
+characters, or be
+.Dv NULL .
 .Pp
 The
 .Fn realpath
@@ -82,6 +83,13 @@ The
 function returns
 .Fa resolved_path
 on success.
+If the function was supplied
+.Dv NULL
+as
+.Fa resolved_path ,
+and operation did not cause errors, the returned value is
+a null-terminated string in a buffer allocated by a call to
+.Fn malloc 3 .
 If an error occurs,
 .Fn realpath
 returns
@@ -89,6 +97,11 @@ returns
 and
 .Fa resolved_path
 contains the pathname which caused the problem.
+If
+.Fa resolved_path
+was
+.Dv NULL ,
+then information of the failed pathname component is lost.
 .Sh ERRORS
 The function
 .Fn realpath

lib/libc/stdlib/realpath.c

@@ -43,23 +43,37 @@ __FBSDID("$FreeBSD$");
 #include "un-namespace.h"
 
 /*
- * char *realpath(const char *path, char resolved[PATH_MAX]);
- *
  * Find the real name of path, by removing all ".", ".." and symlink
  * components.  Returns (resolved) on success, or (NULL) on failure,
  * in which case the path which caused trouble is left in (resolved).
  */
 char *
-realpath(const char *path, char resolved[PATH_MAX])
+realpath(const char * __restrict path, char * __restrict resolved)
 {
 	struct stat sb;
 	char *p, *q, *s;
 	size_t left_len, resolved_len;
 	unsigned symlinks;
-	int serrno, slen;
+	int serrno, slen, m;
 	char left[PATH_MAX], next_token[PATH_MAX], symlink[PATH_MAX];
 
+	if (path == NULL) {
+		errno = EINVAL;
+		return (NULL);
+	}
+	if (path[0] == '\0') {
+		errno = ENOENT;
+		return (NULL);
+	}
 	serrno = errno;
+	if (resolved == NULL) {
+		resolved = malloc(PATH_MAX);
+		if (resolved == NULL)
+			return (NULL);
+		m = 1;
+	} else
+		m = 0;
+
 	symlinks = 0;
 	if (path[0] == '/') {
 		resolved[0] = '/';
@@ -71,12 +85,19 @@ realpath(const char *path, char resolved[PATH_MAX])
 	} else {
 		if (getcwd(resolved, PATH_MAX) == NULL) {
 			strlcpy(resolved, ".", PATH_MAX);
+			if (m) {
+				serrno = errno;
+				free(resolved);
+				errno = serrno;
+			}
 			return (NULL);
 		}
 		resolved_len = strlen(resolved);
 		left_len = strlcpy(left, path, sizeof(left));
 	}
 	if (left_len >= sizeof(left) || resolved_len >= PATH_MAX) {
+		if (m)
+			free(resolved);
 		errno = ENAMETOOLONG;
 		return (NULL);
 	}
@@ -92,6 +113,8 @@ realpath(const char *path, char resolved[PATH_MAX])
 		p = strchr(left, '/');
 		s = p ? p : left + left_len;
 		if (s - left >= sizeof(next_token)) {
+			if (m)
+				free(resolved);
 			errno = ENAMETOOLONG;
 			return (NULL);
 		}
@@ -102,6 +125,8 @@ realpath(const char *path, char resolved[PATH_MAX])
 			memmove(left, s + 1, left_len + 1);
 		if (resolved[resolved_len - 1] != '/') {
 			if (resolved_len + 1 >= PATH_MAX) {
+				if (m)
+					free(resolved);
 				errno = ENAMETOOLONG;
 				return (NULL);
 			}
@@ -133,6 +158,8 @@ realpath(const char *path, char resolved[PATH_MAX])
 		 */
 		resolved_len = strlcat(resolved, next_token, PATH_MAX);
 		if (resolved_len >= PATH_MAX) {
+			if (m)
+				free(resolved);
 			errno = ENAMETOOLONG;
 			return (NULL);
 		}
@@ -141,16 +168,29 @@ realpath(const char *path, char resolved[PATH_MAX])
 				errno = serrno;
 				return (resolved);
 			}
+			if (m) {
+				serrno = errno;
+				free(resolved);
+				errno = serrno;
+			}
 			return (NULL);
 		}
 		if (S_ISLNK(sb.st_mode)) {
 			if (symlinks++ > MAXSYMLINKS) {
+				if (m)
+					free(resolved);
 				errno = ELOOP;
 				return (NULL);
 			}
 			slen = readlink(resolved, symlink, sizeof(symlink) - 1);
-			if (slen < 0)
+			if (slen < 0) {
+				if (m) {
+					serrno = errno;
+					free(resolved);
+					errno = serrno;
+				}
 				return (NULL);
+			}
 			symlink[slen] = '\0';
 			if (symlink[0] == '/') {
 				resolved[1] = 0;
@@ -171,6 +211,8 @@ realpath(const char *path, char resolved[PATH_MAX])
 			if (p != NULL) {
 				if (symlink[slen - 1] != '/') {
 					if (slen + 1 >= sizeof(symlink)) {
+						if (m)
+							free(resolved);
 						errno = ENAMETOOLONG;
 						return (NULL);
 					}
@@ -179,6 +221,8 @@ realpath(const char *path, char resolved[PATH_MAX])
 				}
 				left_len = strlcat(symlink, left, sizeof(left));
 				if (left_len >= sizeof(left)) {
+					if (m)
+						free(resolved);
 					errno = ENAMETOOLONG;
 					return (NULL);
 				}