From 60c096eb0433f7573c768a2c8523abd3c11e0720 Mon Sep 17 00:00:00 2001
From: jvoisin
Date: Wed, 1 Apr 2026 20:37:02 +0200
Subject: Make do even if PATH_MAX isn't defined

As explained in `man realpath(3)`:

> The POSIX.1-2001 standard version of this function is broken by design,
> since it is impossible  to determine  a  suitable  size  for  the  output
> buffer, resolved_path.  According to POSIX.1-2001 a buffer of size PATH_MAX
> suffices, but PATH_MAX need not be a defined constant, and may have to  be
> obtained  using  pathconf(3).  And asking pathconf(3) does not really help,
> since, on the one hand POSIX warns that the result of pathconf(3) may be huge
> and unsuitable for mallocing memory, and on the other hand pathconf(3) may
> return -1 to  signify  that  PATH_MAX  is  not  bounded.   The  re‐ solved_path
> == NULL  feature,  not standardized in POSIX.1-2001, but standardized in
> POSIX.1-2008, allows this design problem to be avoided.

So we can either not compile, or be pragmatic, and define PATH_MAX to a sane
value, like 4096, which is the one used on Linux and some/most BSD.

This commit also adds two tests to ensure that things aren't catastrophically
broken by this change.
---
 tests/Makefile             |  2 ++
 tests/test_realpath.c      | 11 +++++++++++
 tests/test_realpath_null.c | 11 +++++++++++
 3 files changed, 24 insertions(+)
 create mode 100644 tests/test_realpath.c
 create mode 100644 tests/test_realpath_null.c

(limited to 'tests')

diff --git a/tests/Makefile b/tests/Makefile
index 81f7f6e..9c2dede 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -61,6 +61,8 @@ RUNTIME_TARGETS= \
 	test_read_static              \
 	test_readlink_dynamic         \
 	test_readlink_static          \
+	test_realpath_null            \
+	test_realpath                 \
 	test_recv_dynamic             \
 	test_recv_static              \
 	test_recvfrom_dynamic         \
diff --git a/tests/test_realpath.c b/tests/test_realpath.c
new file mode 100644
index 0000000..2a17708
--- /dev/null
+++ b/tests/test_realpath.c
@@ -0,0 +1,11 @@
+#include "common.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(int argc, char** argv) {
+  char buf[PATH_MAX];
+  char* rpath = realpath("/root/../", buf);
+  printf("%s\n", rpath);
+  return 0;
+}
diff --git a/tests/test_realpath_null.c b/tests/test_realpath_null.c
new file mode 100644
index 0000000..b62cee8
--- /dev/null
+++ b/tests/test_realpath_null.c
@@ -0,0 +1,11 @@
+#include "common.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(int argc, char** argv) {
+  char* rpath = realpath("/root/../", NULL);
+  printf("%s\n", rpath);
+  free(rpath);
+  return 0;
+}
-- 
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