yushuailong opened a new pull request, #19016:
URL: https://github.com/apache/nuttx/pull/19016
readdir() returned NULL at end-of-directory without ensuring errno was
clean. POSIX requires errno to be unchanged at EOF, but the underlying read()
path may leave a stale errno value. Callers that follow the POSIX idiom (set
errno to 0 before the call, test it after a NULL return), such as readdir_r()
and scandir(), then misread this as a readdir() failure.
Save errno on entry and restore it on the end-of-directory return so that
EOF no longer reports a spurious error, while genuine errors (read() returning
a negative value) still propagate.
## Summary
`readdir()` returns NULL both at end-of-directory and on error, and per
POSIX a caller distinguishes the two by setting `errno` to 0 before the call
and checking it after a NULL return. The current implementation does not
guarantee `errno` is left clean at end-of-directory: the underlying `read()`
path can leave a stale `errno` value behind, so a normal end-of-directory looks
like a failure to such callers (`readdir_r()`, `scandir()`, and the libc++
`std::filesystem::directory_iterator`).
This patch saves `errno` on entry and restores it on the end-of-directory
(`read() `== 0) return, mirroring glibc's `readdir()`. Genuine errors (`read()`
returning a negative value, with errno already set) are left untouched and
still propagate to the caller.
## Impact
Affected: `readdir() `only, and only its end-of-directory path
(`read()`==0), which now restores errno. The error path and normal path are
unchanged.
## Testing
Build Host: Ubuntu 22.04, arm-none-eabi-gcc 10.3.1 (2021.07)
Target: ARM Cortex-M55 (ARMv8-M) custom board
```c
static void test_readdir_eof_errno(FAR const char *path)
{
FAR DIR *dirp;
FAR struct dirent *ent;
int count = 0;
int eof_errno;
dirp = opendir(path);
if (dirp == NULL)
{
printf("readdir_test: opendir(%s) failed: errno=%d (%s)\n",
path, errno, strerror(errno));
return;
}
/* POSIX idiom: clear errno before the loop so that a non-zero errno
* after a NULL return unambiguously means an error (not end-of-dir).
*/
errno = 0;
while ((ent = readdir(dirp)) != NULL)
{
count++;
}
eof_errno = errno;
printf("readdir_test: path=%s entries=%d eof_errno=%d (%s) -> %s\n",
path, count, eof_errno, strerror(eof_errno),
eof_errno == 0 ? "PASS" : "FAIL");
closedir(dirp);
}
```
readdir_test: path=/font entries=17 eof_errno=0 (Unknown error 0) -> PASS
## PR verification Self-Check
* [x] This PR introduces only one functional change.
* [x] I have updated all required description fields above.
* [x] My PR adheres to Contributing
[Guidelines](https://github.com/apache/nuttx/blob/master/CONTRIBUTING.md) and
[Documentation](https://nuttx.apache.org/docs/latest/contributing/index.html)
(git commit title and message, coding standard, etc).
* [ ] My PR is still work in progress (not ready for review).
* [x] My PR is ready for review and can be safely merged into a codebase.
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