posix: unistd: support for confstr()

Support querying POSIX string configuration values (similar to
sysconf()).

confstr() is required by the POSIX_SINGLE_PROCESS Option
Group as detailed in Section E.1 of IEEE-1003.1-2017 and has
been part of the specification since POSIX-2.

The POSIX_SINGLE_PROCESS Option Group is required for PSE51,
PSE52, PSE53, and PSE54 conformance, and is otherwise mandatory
for any POSIX conforming system as per Section A.2.1.3 of
IEEE-1003-1.2017.

With this, we have complete support for the POSIX_SINGLE_PROCESS
Option Group.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

include/zephyr/posix/sys/confstr.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2024, Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef ZEPHYR_INCLUDE_POSIX_SYS_CONFSTR_H_
+#define ZEPHYR_INCLUDE_POSIX_SYS_CONFSTR_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+enum {
+	_CS_PATH,
+	_CS_POSIX_V7_ILP32_OFF32_CFLAGS,
+	_CS_POSIX_V7_ILP32_OFF32_LDFLAGS,
+	_CS_POSIX_V7_ILP32_OFF32_LIBS,
+	_CS_POSIX_V7_ILP32_OFFBIG_CFLAGS,
+	_CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS,
+	_CS_POSIX_V7_ILP32_OFFBIG_LIBS,
+	_CS_POSIX_V7_LP64_OFF64_CFLAGS,
+	_CS_POSIX_V7_LP64_OFF64_LDFLAGS,
+	_CS_POSIX_V7_LP64_OFF64_LIBS,
+	_CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS,
+	_CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS,
+	_CS_POSIX_V7_LPBIG_OFFBIG_LIBS,
+	_CS_POSIX_V7_THREADS_CFLAGS,
+	_CS_POSIX_V7_THREADS_LDFLAGS,
+	_CS_POSIX_V7_WIDTH_RESTRICTED_ENVS,
+	_CS_V7_ENV,
+	_CS_POSIX_V6_ILP32_OFF32_CFLAGS,
+	_CS_POSIX_V6_ILP32_OFF32_LDFLAGS,
+	_CS_POSIX_V6_ILP32_OFF32_LIBS,
+	_CS_POSIX_V6_ILP32_OFFBIG_CFLAGS,
+	_CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS,
+	_CS_POSIX_V6_ILP32_OFFBIG_LIBS,
+	_CS_POSIX_V6_LP64_OFF64_CFLAGS,
+	_CS_POSIX_V6_LP64_OFF64_LDFLAGS,
+	_CS_POSIX_V6_LP64_OFF64_LIBS,
+	_CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS,
+	_CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS,
+	_CS_POSIX_V6_LPBIG_OFFBIG_LIBS,
+	_CS_POSIX_V6_WIDTH_RESTRICTED_ENVS,
+	_CS_V6_ENV,
+};
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_INCLUDE_POSIX_SYS_CONFSTR_H_ */

include/zephyr/posix/unistd.h

@@ -19,6 +19,7 @@
 #ifdef CONFIG_POSIX_SYSCONF
 #include <zephyr/posix/signal.h>
 #endif
+#include <zephyr/posix/sys/confstr.h>
 #include <zephyr/posix/sys/stat.h>
 #include <zephyr/posix/sys/sysconf.h>
 
@@ -266,6 +267,9 @@ int usleep(useconds_t useconds);
 #ifdef CONFIG_POSIX_SYSCONF_IMPL_FULL
 long sysconf(int opt);
 #endif
+#if _POSIX_C_SOURCE >= 2
+size_t confstr(int name, char *buf, size_t len);
+#endif
 
 #ifdef __cplusplus
 }

lib/posix/options/CMakeLists.txt

@@ -40,6 +40,7 @@ zephyr_library_sources_ifdef(CONFIG_POSIX_API perror.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_CLOCK clock.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_CLOCK nanosleep.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_CLOCK sleep.c)
+zephyr_library_sources_ifdef(CONFIG_POSIX_CONFSTR confstr.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_ENV env.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_FS fs.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_MQUEUE mqueue.c)

lib/posix/options/Kconfig

@@ -27,6 +27,7 @@ endif # POSIX_CLOCK
 rsource "Kconfig.barrier"
 rsource "Kconfig.clock"
 rsource "Kconfig.cond"
+rsource "Kconfig.confstr"
 rsource "Kconfig.env"
 rsource "Kconfig.eventfd"
 rsource "Kconfig.fdtable"

lib/posix/options/Kconfig.confstr

@@ -0,0 +1,9 @@
+# Copyright (c) 2024, Meta
+#
+# SPDX-License-Identifier: Apache-2.0
+
+config POSIX_CONFSTR
+	bool "Retrieve string system configuration"
+	default y if POSIX_API
+	help
+	  This enables the POSIX confstr() function.

lib/posix/options/confstr.c

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2024, Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/posix/unistd.h>
+
+size_t confstr(int name, char *buf, size_t len)
+{
+	if (name < 0 || name > _CS_V6_ENV) {
+		errno = EINVAL;
+		return 0;
+	}
+
+	if (buf != NULL && len > 0) {
+		buf[0] = '\0';
+	}
+
+	return 1;
+}