[PATCH] readlink.2: Document using st_size to allocate the buffer

Guillem Jover
2011-04-21T03:19:57Z

Signed-off-by: Guillem Jover <guillem-+FW4gsLVM0RAfugRpC6u6w@public.gmane.org>
---
man2/readlink.2 | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 files changed, 68 insertions(+), 1 deletions(-)

diff --git a/man2/readlink.2 b/man2/readlink.2
index 1327e08..2601e22 100644
--- a/man2/readlink.2
+++ b/man2/readlink.2
@@ -35,7 +35,7 @@
.\" Modified Tue Jul 9 23:55:17 1996 by aeb
.\" Modified Fri Jan 24 00:26:00 1997 by aeb
.\"
-.TH READLINK 2 2010-09-20 "Linux" "Linux Programmer's Manual"
+.TH READLINK 2 2011-04-20 "Linux" "Linux Programmer's Manual"
.SH NAME
readlink \- read value of a symbolic link
.SH SYNOPSIS
@@ -69,6 +69,21 @@ does not append a null byte to
It will truncate the contents (to a length of
.I bufsiz
characters), in case the buffer is too small to hold all of the contents.
+
+Using a statically sized buffer might not give enough room for the
+symbolic link contents, which might end up getting truncated. The
+needed size for the buffer can be obtained from the symbolic link's
+.I stat
+structure
+.I st_size
+field with
+.BR lstat (),
+but the number of written characters should be checked to make sure the
+symbolic link did not change between both calls, and no truncation
+happened. This also fixes a common portability problem when using
+.I PATH_MAX
+for the buffer size, as this is not guaranteed to be defined per POSIX
+if the system does not have such limit.
.SH "RETURN VALUE"
On success,
.BR readlink ()
@@ -130,6 +145,58 @@ was declared as
Nowadays, the return type is declared as
.IR ssize_t ,
as (newly) required in POSIX.1-2001.
+.SH EXAMPLE
+The following program allocates the space needed by
+.BR readlink ()
+dynamically from the information provided by
+.BR lstat (),
+making sure there's no race condition between both calls.
+.nf
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int
+main(int argc, char *argv[])
+{
+ struct stat sb;
+ char *linkname;
+ ssize_t r;
+
+ if (argc != 2) {
+ fprintf(stderr, "Usage: %s <pathname>\\n", argv[0]);
+ exit(EXIT_FAILURE);
+ }
+
+ if (lstat(argv[1], &sb) == \-1) {
+ perror("lstat");
+ exit(EXIT_FAILURE);
+ }
+
+ linkname = malloc(sb.st_size + 1);
+ if (linkname == NULL) {
+ fprintf(stderr, "insufficient memory\\n");
+ exit(EXIT_FAILURE);
+ }
+
+ r = readlink(argv[1], linkname, sb.st_size + 1);
+ if (r < 0) {
+ perror("lstat");
+ exit(EXIT_FAILURE);
+ } else if (r != sb.st_size) {
+ fprintf(stderr, "symlink changed between lstat and readlink\\n");
+ exit(EXIT_FAILURE);
+ }
+ linkname[sb.st_size] = '\\0';
+
+ printf("'%s' points to '%s'\\n", argv[1], linkname);
+
+ exit(EXIT_SUCCESS);
+}
+.fi
.SH "SEE ALSO"
.BR lstat (2),
.BR readlinkat (2),
--
1.7.4.4