[PATCH v2 0/7] Replace "__auto_type" with "auto"

[PATCH v2 0/7] Replace "__auto_type" with "auto"

[H. Peter Anvin]

"auto" was defined as a keyword back in the K&R days, but as a storage
type specifier.  No one ever used it, since it was and is the default
storage type for local variables.

C++11 recycled the keyword to allow a type to be declared based on the
type of an initializer.  This was finally adopted into standard C in
C23.

gcc and clang provide the "__auto_type" alias keyword as an extension
for pre-C23, however, there is no reason to pollute the bulk of the
source base with this temporary keyword; instead define "auto" as a
macro unless the compiler is running in C23+ mode.

This macro is added in <linux/compiler_types.h> because that header is
included in some of the tools headers, wheres <linux/compiler.h> is
not as it has a bunch of very kernel-specific things in it.

Changes in v2:

- Restore indentation of macro backslashes (David Laight)
- arch/nios2: Replace an adjacent typeof() with a similar "auto" construct
  (Linus Torvalds)
- fs/proc/inode.c: change "__auto_type" to "const auto" (Alexey Dobriyan)

--- 
 arch/nios2/include/asm/uaccess.h                        |  8 ++++----
 arch/x86/include/asm/bug.h                              |  2 +-
 arch/x86/include/asm/string_64.h                        |  6 +++---
 arch/x86/include/asm/uaccess_64.h                       |  2 +-
 fs/proc/inode.c                                         | 16 ++++++++--------
 include/linux/cleanup.h                                 |  6 +++---
 include/linux/compiler.h                                |  2 +-
 include/linux/compiler_types.h                          | 13 +++++++++++++
 include/linux/minmax.h                                  |  6 +++---
 tools/testing/selftests/bpf/prog_tests/socket_helpers.h |  9 +++++++--
 tools/virtio/linux/compiler.h                           |  2 +-
 11 files changed, 45 insertions(+), 27 deletions(-)

[PATCH v2 1/7] compiler_types.h: add "auto" as a macro for "__auto_type"

[H. Peter Anvin]

"auto" was defined as a keyword back in the K&R days, but as a storage
type specifier.  No one ever used it, since it was and is the default
storage type for local variables.

C++11 recycled the keyword to allow a type to be declared based on the
type of an initializer.  This was finally adopted into standard C in
C23.

gcc and clang provide the "__auto_type" alias keyword as an extension
for pre-C23, however, there is no reason to pollute the bulk of the
source base with this temporary keyword; instead define "auto" as a
macro unless the compiler is running in C23+ mode.

This macro is added in <linux/compiler_types.h> because that header is
included in some of the tools headers, wheres <linux/compiler.h> is
not as it has a bunch of very kernel-specific things in it.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 include/linux/compiler_types.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 2b77d12e07b2..c8b1ee37934e 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -13,6 +13,19 @@
 
 #ifndef __ASSEMBLY__
 
+/*
+ * C23 introduces "auto" as a standard way to define type-inferred
+ * variables, but "auto" has been a (useless) keyword even since K&R C,
+ * so it has always been "namespace reserved."
+ *
+ * Until at some future time we require C23 support, we need the gcc
+ * extension __auto_type, but there is no reason to put that elsewhere
+ * in the source code.
+ */
+#if __STDC_VERSION__ < 202311L
+# define auto __auto_type
+#endif
+
 /*
  * Skipped when running bindgen due to a libclang issue;
  * see https://github.com/rust-lang/rust-bindgen/issues/2244.
-- 
2.50.1

Re: [PATCH v2 1/7] compiler_types.h: add "auto" as a macro for "__auto_type"

[Miguel Ojeda]

On Sun, Jul 20, 2025 at 8:52 AM H. Peter Anvin <hpa@zytor.com> wrote:
>
> gcc and clang provide the "__auto_type" alias keyword as an extension
> for pre-C23, however, there is no reason to pollute the bulk of the
> source base with this temporary keyword; instead define "auto" as a
> macro unless the compiler is running in C23+ mode.
>
> This macro is added in <linux/compiler_types.h> because that header is
> included in some of the tools headers, wheres <linux/compiler.h> is
> not as it has a bunch of very kernel-specific things in it.

Sounds good. I guess we could need a workaround if someone happened to
invent an attribute which requires using "auto" in it, since it is not
reserved there in C23 AFAIU. So FWIW:

Acked-by: Miguel Ojeda <ojeda@kernel.org>

> + * so it has always been "namespace reserved."

Not sure what this means (could we just say reserved?).

Thanks!

Relatedly, there are some proposed, further changes to `auto` for C2y:

    https://www.open-std.org/jtc1/sc22/WG14/www/docs/n3579.htm

Cheers,
Miguel

[PATCH v2 2/7] include/linux: change "__auto_type" to "auto"

[H. Peter Anvin]

Replace instances of "__auto_type" with "auto" in include/linux.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 include/linux/cleanup.h  | 6 +++---
 include/linux/compiler.h | 2 +-
 include/linux/minmax.h   | 6 +++---
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 7093e1d08af0..c7f3031ae400 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -199,10 +199,10 @@
 
 #define __free(_name)	__cleanup(__free_##_name)
 
-#define __get_and_null(p, nullvalue)   \
+#define __get_and_null(p, nullvalue)	    \
 	({                                  \
-		__auto_type __ptr = &(p);   \
-		__auto_type __val = *__ptr; \
+		auto __ptr = &(p);	    \
+		auto __val = *__ptr;	    \
 		*__ptr = nullvalue;         \
 		__val;                      \
 	})
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 6f04a1d8c720..7fef5cd41dfc 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -186,7 +186,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #define data_race(expr)							\
 ({									\
 	__kcsan_disable_current();					\
-	__auto_type __v = (expr);					\
+	auto __v = (expr);						\
 	__kcsan_enable_current();					\
 	__v;								\
 })
diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index eaaf5c008e4d..a0158db54a04 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -89,7 +89,7 @@
 	__cmp_once_unique(op, type, x, y, __UNIQUE_ID(x_), __UNIQUE_ID(y_))
 
 #define __careful_cmp_once(op, x, y, ux, uy) ({		\
-	__auto_type ux = (x); __auto_type uy = (y);	\
+	auto ux = (x); auto uy = (y);			\
 	BUILD_BUG_ON_MSG(!__types_ok(ux, uy),		\
 		#op"("#x", "#y") signedness error");	\
 	__cmp(op, ux, uy); })
@@ -129,7 +129,7 @@
 	__careful_cmp(max, (x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull)
 
 #define __careful_op3(op, x, y, z, ux, uy, uz) ({			\
-	__auto_type ux = (x); __auto_type uy = (y);__auto_type uz = (z);\
+	auto ux = (x); auto uy = (y); auto uz = (z);			\
 	BUILD_BUG_ON_MSG(!__types_ok3(ux, uy, uz),			\
 		#op"3("#x", "#y", "#z") signedness error");		\
 	__cmp(op, ux, __cmp(op, uy, uz)); })
@@ -203,7 +203,7 @@
  * This macro checks @val/@lo/@hi to make sure they have compatible
  * signedness.
  */
-#define clamp(val, lo, hi) __careful_clamp(__auto_type, val, lo, hi)
+#define clamp(val, lo, hi) __careful_clamp(auto, val, lo, hi)
 
 /**
  * clamp_t - return a value clamped to a given range using a given type
-- 
2.50.1

[PATCH v2 3/7] fs/proc: replace "__auto_type" with "const auto"

[H. Peter Anvin]

Replace use of "__auto_type" in fs/proc/inode.c with "const auto".

Suggested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
---
 fs/proc/inode.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 3604b616311c..1afa2dd2285b 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -303,7 +303,7 @@ static ssize_t proc_reg_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 
 static ssize_t pde_read(struct proc_dir_entry *pde, struct file *file, char __user *buf, size_t count, loff_t *ppos)
 {
-	__auto_type read = pde->proc_ops->proc_read;
+	const auto read = pde->proc_ops->proc_read;
 	if (read)
 		return read(file, buf, count, ppos);
 	return -EIO;
@@ -325,7 +325,7 @@ static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count,
 
 static ssize_t pde_write(struct proc_dir_entry *pde, struct file *file, const char __user *buf, size_t count, loff_t *ppos)
 {
-	__auto_type write = pde->proc_ops->proc_write;
+	auto write = pde->proc_ops->proc_write;
 	if (write)
 		return write(file, buf, count, ppos);
 	return -EIO;
@@ -347,7 +347,7 @@ static ssize_t proc_reg_write(struct file *file, const char __user *buf, size_t
 
 static __poll_t pde_poll(struct proc_dir_entry *pde, struct file *file, struct poll_table_struct *pts)
 {
-	__auto_type poll = pde->proc_ops->proc_poll;
+	auto poll = pde->proc_ops->proc_poll;
 	if (poll)
 		return poll(file, pts);
 	return DEFAULT_POLLMASK;
@@ -369,7 +369,7 @@ static __poll_t proc_reg_poll(struct file *file, struct poll_table_struct *pts)
 
 static long pde_ioctl(struct proc_dir_entry *pde, struct file *file, unsigned int cmd, unsigned long arg)
 {
-	__auto_type ioctl = pde->proc_ops->proc_ioctl;
+	auto ioctl = pde->proc_ops->proc_ioctl;
 	if (ioctl)
 		return ioctl(file, cmd, arg);
 	return -ENOTTY;
@@ -392,7 +392,7 @@ static long proc_reg_unlocked_ioctl(struct file *file, unsigned int cmd, unsigne
 #ifdef CONFIG_COMPAT
 static long pde_compat_ioctl(struct proc_dir_entry *pde, struct file *file, unsigned int cmd, unsigned long arg)
 {
-	__auto_type compat_ioctl = pde->proc_ops->proc_compat_ioctl;
+	auto compat_ioctl = pde->proc_ops->proc_compat_ioctl;
 	if (compat_ioctl)
 		return compat_ioctl(file, cmd, arg);
 	return -ENOTTY;
@@ -414,7 +414,7 @@ static long proc_reg_compat_ioctl(struct file *file, unsigned int cmd, unsigned
 
 static int pde_mmap(struct proc_dir_entry *pde, struct file *file, struct vm_area_struct *vma)
 {
-	__auto_type mmap = pde->proc_ops->proc_mmap;
+	auto mmap = pde->proc_ops->proc_mmap;
 	if (mmap)
 		return mmap(file, vma);
 	return -EIO;
@@ -497,7 +497,7 @@ static int proc_reg_open(struct inode *inode, struct file *file)
 	if (!use_pde(pde))
 		return -ENOENT;
 
-	__auto_type release = pde->proc_ops->proc_release;
+	auto release = pde->proc_ops->proc_release;
 	if (release) {
 		pdeo = kmem_cache_alloc(pde_opener_cache, GFP_KERNEL);
 		if (!pdeo) {
@@ -534,7 +534,7 @@ static int proc_reg_release(struct inode *inode, struct file *file)
 	struct pde_opener *pdeo;
 
 	if (pde_is_permanent(pde)) {
-		__auto_type release = pde->proc_ops->proc_release;
+		auto release = pde->proc_ops->proc_release;
 		if (release) {
 			return release(inode, file);
 		}
-- 
2.50.1

[PATCH v2.1 3/7] fs/proc: replace "__auto_type" with "const auto"

[H. Peter Anvin]

Replace use of "__auto_type" in fs/proc/inode.c with "const auto".

Suggested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
---
 fs/proc/inode.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

[ v2 of this patch was an obvious thinko, this is the correct one ]

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 3604b616311c..8b90ab9b9cfc 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -303,7 +303,7 @@ static ssize_t proc_reg_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 
 static ssize_t pde_read(struct proc_dir_entry *pde, struct file *file, char __user *buf, size_t count, loff_t *ppos)
 {
-	__auto_type read = pde->proc_ops->proc_read;
+	const auto read = pde->proc_ops->proc_read;
 	if (read)
 		return read(file, buf, count, ppos);
 	return -EIO;
@@ -325,7 +325,7 @@ static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count,
 
 static ssize_t pde_write(struct proc_dir_entry *pde, struct file *file, const char __user *buf, size_t count, loff_t *ppos)
 {
-	__auto_type write = pde->proc_ops->proc_write;
+	const auto write = pde->proc_ops->proc_write;
 	if (write)
 		return write(file, buf, count, ppos);
 	return -EIO;
@@ -347,7 +347,7 @@ static ssize_t proc_reg_write(struct file *file, const char __user *buf, size_t
 
 static __poll_t pde_poll(struct proc_dir_entry *pde, struct file *file, struct poll_table_struct *pts)
 {
-	__auto_type poll = pde->proc_ops->proc_poll;
+	const auto poll = pde->proc_ops->proc_poll;
 	if (poll)
 		return poll(file, pts);
 	return DEFAULT_POLLMASK;
@@ -369,7 +369,7 @@ static __poll_t proc_reg_poll(struct file *file, struct poll_table_struct *pts)
 
 static long pde_ioctl(struct proc_dir_entry *pde, struct file *file, unsigned int cmd, unsigned long arg)
 {
-	__auto_type ioctl = pde->proc_ops->proc_ioctl;
+	const auto ioctl = pde->proc_ops->proc_ioctl;
 	if (ioctl)
 		return ioctl(file, cmd, arg);
 	return -ENOTTY;
@@ -392,7 +392,7 @@ static long proc_reg_unlocked_ioctl(struct file *file, unsigned int cmd, unsigne
 #ifdef CONFIG_COMPAT
 static long pde_compat_ioctl(struct proc_dir_entry *pde, struct file *file, unsigned int cmd, unsigned long arg)
 {
-	__auto_type compat_ioctl = pde->proc_ops->proc_compat_ioctl;
+	const auto compat_ioctl = pde->proc_ops->proc_compat_ioctl;
 	if (compat_ioctl)
 		return compat_ioctl(file, cmd, arg);
 	return -ENOTTY;
@@ -414,7 +414,7 @@ static long proc_reg_compat_ioctl(struct file *file, unsigned int cmd, unsigned
 
 static int pde_mmap(struct proc_dir_entry *pde, struct file *file, struct vm_area_struct *vma)
 {
-	__auto_type mmap = pde->proc_ops->proc_mmap;
+	const auto mmap = pde->proc_ops->proc_mmap;
 	if (mmap)
 		return mmap(file, vma);
 	return -EIO;
@@ -497,7 +497,7 @@ static int proc_reg_open(struct inode *inode, struct file *file)
 	if (!use_pde(pde))
 		return -ENOENT;
 
-	__auto_type release = pde->proc_ops->proc_release;
+	const auto release = pde->proc_ops->proc_release;
 	if (release) {
 		pdeo = kmem_cache_alloc(pde_opener_cache, GFP_KERNEL);
 		if (!pdeo) {
@@ -534,10 +534,9 @@ static int proc_reg_release(struct inode *inode, struct file *file)
 	struct pde_opener *pdeo;
 
 	if (pde_is_permanent(pde)) {
-		__auto_type release = pde->proc_ops->proc_release;
-		if (release) {
+		const auto release = pde->proc_ops->proc_release;
+		if (release)
 			return release(inode, file);
-		}
 		return 0;
 	}
 
-- 
2.50.1

[PATCH v2 4/7] arch/nios2: replace "__auto_type" and adjacent equivalent with "auto"

[H. Peter Anvin]

Replace uses of "__auto_type" in arch/nios2/include/asm/uaccess.h with
"auto", and equivalently convert an adjacent cast to the analogous
form.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 arch/nios2/include/asm/uaccess.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/nios2/include/asm/uaccess.h b/arch/nios2/include/asm/uaccess.h
index b8299082adbe..6ccc9a232c23 100644
--- a/arch/nios2/include/asm/uaccess.h
+++ b/arch/nios2/include/asm/uaccess.h
@@ -172,15 +172,15 @@ do {									\
 
 #define __put_user(x, ptr)						\
 ({									\
-	__auto_type __pu_ptr = (ptr);					\
-	typeof(*__pu_ptr) __pu_val = (typeof(*__pu_ptr))(x);		\
+	auto __pu_ptr = (ptr);						\
+	auto __pu_val = (typeof(*__pu_ptr))(x);				\
 	__put_user_common(__pu_val, __pu_ptr);				\
 })
 
 #define put_user(x, ptr)						\
 ({									\
-	__auto_type __pu_ptr = (ptr);					\
-	typeof(*__pu_ptr) __pu_val = (typeof(*__pu_ptr))(x);		\
+	auto __pu_ptr = (ptr);						\
+	auto __pu_val = (typeof(*__pu_ptr))(x);				\
 	access_ok(__pu_ptr, sizeof(*__pu_ptr)) ?			\
 		__put_user_common(__pu_val, __pu_ptr) :			\
 		-EFAULT;						\
-- 
2.50.1

Re: [PATCH v2 4/7] arch/nios2: replace "__auto_type" and adjacent equivalent with "auto"

[Dinh Nguyen]

On 7/20/25 01:50, H. Peter Anvin wrote:
> Replace uses of "__auto_type" in arch/nios2/include/asm/uaccess.h with
> "auto", and equivalently convert an adjacent cast to the analogous
> form.
> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> ---
>   arch/nios2/include/asm/uaccess.h | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 

Acked-by: Dinh Nguyen <dinguyen@kernel.org>

[PATCH v2 5/7] arch/x86: replace "__auto_type" with "auto"

[H. Peter Anvin]

Replace instances of "__auto_type" with "auto" in:

	arch/x86/include/asm/bug.h
	arch/x86/include/asm/string_64.h
	arch/x86/include/asm/uaccess_64.h

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 arch/x86/include/asm/bug.h        | 2 +-
 arch/x86/include/asm/string_64.h  | 6 +++---
 arch/x86/include/asm/uaccess_64.h | 2 +-
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index f0e9acf72547..05e166faf9a6 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -94,7 +94,7 @@ do {								\
  */
 #define __WARN_FLAGS(flags)					\
 do {								\
-	__auto_type __flags = BUGFLAG_WARNING|(flags);		\
+	auto __flags = BUGFLAG_WARNING|(flags);			\
 	instrumentation_begin();				\
 	_BUG_FLAGS(ASM_UD2, __flags, ANNOTATE_REACHABLE(1b));	\
 	instrumentation_end();					\
diff --git a/arch/x86/include/asm/string_64.h b/arch/x86/include/asm/string_64.h
index 79e9695dc13e..4635616863f5 100644
--- a/arch/x86/include/asm/string_64.h
+++ b/arch/x86/include/asm/string_64.h
@@ -31,7 +31,7 @@ KCFI_REFERENCE(__memset);
 #define __HAVE_ARCH_MEMSET16
 static inline void *memset16(uint16_t *s, uint16_t v, size_t n)
 {
-	const __auto_type s0 = s;
+	const auto s0 = s;
 	asm volatile (
 		"rep stosw"
 		: "+D" (s), "+c" (n)
@@ -44,7 +44,7 @@ static inline void *memset16(uint16_t *s, uint16_t v, size_t n)
 #define __HAVE_ARCH_MEMSET32
 static inline void *memset32(uint32_t *s, uint32_t v, size_t n)
 {
-	const __auto_type s0 = s;
+	const auto s0 = s;
 	asm volatile (
 		"rep stosl"
 		: "+D" (s), "+c" (n)
@@ -57,7 +57,7 @@ static inline void *memset32(uint32_t *s, uint32_t v, size_t n)
 #define __HAVE_ARCH_MEMSET64
 static inline void *memset64(uint64_t *s, uint64_t v, size_t n)
 {
-	const __auto_type s0 = s;
+	const auto s0 = s;
 	asm volatile (
 		"rep stosq"
 		: "+D" (s), "+c" (n)
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index c8a5ae35c871..b0e4533ce625 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -72,7 +72,7 @@ static inline void __user *mask_user_address(const void __user *ptr)
 	return ret;
 }
 #define masked_user_access_begin(x) ({				\
-	__auto_type __masked_ptr = (x);				\
+	auto __masked_ptr = (x);				\
 	__masked_ptr = mask_user_address(__masked_ptr);		\
 	__uaccess_begin(); __masked_ptr; })
 
-- 
2.50.1

[PATCH v2 6/7] selftests/bpf: replace "__auto_type" with "auto"

[H. Peter Anvin]

Replace instances of "__auto_type" with "auto" in:

	tools/testing/selftests/bpf/prog_tests/socket_helpers.h

This file does not seem to be including <linux/compiler_types.h>
directly or indirectly, so copy the definition but guard it with
!defined(auto).

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 tools/testing/selftests/bpf/prog_tests/socket_helpers.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/socket_helpers.h b/tools/testing/selftests/bpf/prog_tests/socket_helpers.h
index e02cabcc814e..0d59503a0c73 100644
--- a/tools/testing/selftests/bpf/prog_tests/socket_helpers.h
+++ b/tools/testing/selftests/bpf/prog_tests/socket_helpers.h
@@ -17,11 +17,16 @@
 #define VMADDR_CID_LOCAL 1
 #endif
 
+/* include/linux/compiler_types.h */
+#if __STDC_VERSION__ < 202311L && !defined(auto)
+# define auto __auto_type
+#endif
+
 /* include/linux/cleanup.h */
 #define __get_and_null(p, nullvalue)                                           \
 	({                                                                     \
-		__auto_type __ptr = &(p);                                      \
-		__auto_type __val = *__ptr;                                    \
+		auto __ptr = &(p);					       \
+		auto __val = *__ptr;                                           \
 		*__ptr = nullvalue;                                            \
 		__val;                                                         \
 	})
-- 
2.50.1

Re: [PATCH v2 6/7] selftests/bpf: replace "__auto_type" with "auto"

[Eduard Zingerman]

On Sat, 2025-07-19 at 23:50 -0700, H. Peter Anvin wrote:
> Replace instances of "__auto_type" with "auto" in:
> 
> 	tools/testing/selftests/bpf/prog_tests/socket_helpers.h
> 
> This file does not seem to be including <linux/compiler_types.h>
> directly or indirectly, so copy the definition but guard it with
> !defined(auto).
> 
> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

[PATCH v2 7/7] tools/virtio: replace "__auto_type" with "auto"

[H. Peter Anvin]

Replace one instance of "__auto_type" with "auto" in:

	tools/virtio/linux/compiler.h

This file *does* include <linux/compiler_types.h> directly, so there
is no need to duplicate the definition.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 tools/virtio/linux/compiler.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/virtio/linux/compiler.h b/tools/virtio/linux/compiler.h
index 204ef0e9f542..725b93bfeee1 100644
--- a/tools/virtio/linux/compiler.h
+++ b/tools/virtio/linux/compiler.h
@@ -31,7 +31,7 @@
  */
 #define data_race(expr)							\
 ({									\
-	__auto_type __v = (expr);					\
+	auto __v = (expr);						\
 	__v;								\
 })
 
-- 
2.50.1

Re: [PATCH v2 0/7] Replace "__auto_type" with "auto"

[Kees Cook]

On July 19, 2025 11:50:37 PM PDT, "H. Peter Anvin" <hpa@zytor.com> wrote:
>"auto" was defined as a keyword back in the K&R days, but as a storage
>type specifier.  No one ever used it, since it was and is the default
>storage type for local variables.
>
>C++11 recycled the keyword to allow a type to be declared based on the
>type of an initializer.  This was finally adopted into standard C in
>C23.
>
>gcc and clang provide the "__auto_type" alias keyword as an extension
>for pre-C23, however, there is no reason to pollute the bulk of the
>source base with this temporary keyword; instead define "auto" as a
>macro unless the compiler is running in C23+ mode.

Yeah, this is good. We have typeof() used extensively in macros all over. I'll try this for fortify macros and see if we see any binary output changes...

-- 
Kees Cook