[libc-commits] [libc] [libc] Fix modular printf attributes (PR #194003)

[Volodymyr Turanskyy via libc-commits]

https://github.com/voltur01 created https://github.com/llvm/llvm-project/pull/194003

This fixes declarations of modular printf attributes by adding the format attribute required by clang and documented here
https://github.com/llvm/llvm-project/blob/deb84db5b4056eed1457a6b03148a486bbadd8ea/libc/docs/dev/modular_format.rst?plain=1#L26

Assisted-by: codex, reviewed and cross checked, also tested with ATfE, by me. Modular printf reduced code size from ~37K to ~13K for int-only printf sample.

>From e36c9d6b9c1f0df50a577442a1c7460327efd1e3 Mon Sep 17 00:00:00 2001
From: Volodymyr Turanskyy <volodymyr.turanskyy at arm.com>
Date: Fri, 24 Apr 2026 17:04:26 +0100
Subject: [PATCH] Fix modular printf attributes

This fixes declarations of modular printf attributes by adding the format
attribute required by clang and documented here
https://github.com/llvm/llvm-project/blob/deb84db5b4056eed1457a6b03148a486bbadd8ea/libc/docs/dev/modular_format.rst?plain=1#L26
---
 libc/include/llvm-libc-macros/CMakeLists.txt     |  2 +-
 .../_LIBC_MODULAR_FORMAT_PRINTF-disable.h        |  3 ++-
 .../_LIBC_MODULAR_FORMAT_PRINTF.h                |  4 +++-
 libc/include/stdio.yaml                          | 16 ++++++++--------
 4 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/libc/include/llvm-libc-macros/CMakeLists.txt b/libc/include/llvm-libc-macros/CMakeLists.txt
index 1f34257c57e01..a923b979e71c9 100644
--- a/libc/include/llvm-libc-macros/CMakeLists.txt
+++ b/libc/include/llvm-libc-macros/CMakeLists.txt
@@ -400,7 +400,7 @@ add_macro_header(
     sysexits-macros.h
 )
 
-if (LIBC_CONF_MODULAR_FORMAT)
+if (LIBC_CONF_PRINTF_MODULAR)
   add_macro_header(
     _LIBC_MODULAR_FORMAT_PRINTF
     HDR
diff --git a/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF-disable.h b/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF-disable.h
index e3238161b3808..ecec03c2142ce 100644
--- a/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF-disable.h
+++ b/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF-disable.h
@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
 #define LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
 
-#define _LIBC_MODULAR_FORMAT_PRINTF(MODULAR_IMPL_FN)
+#define _LIBC_MODULAR_FORMAT_PRINTF(MODULAR_IMPL_FN, FORMAT_INDEX,             \
+                                    FIRST_TO_CHECK)
 
 #endif // LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
diff --git a/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF.h b/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF.h
index 918241ab8f2ec..e7349afe94371 100644
--- a/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF.h
+++ b/libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF.h
@@ -9,7 +9,9 @@
 #ifndef LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
 #define LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
 
-#define _LIBC_MODULAR_FORMAT_PRINTF(MODULAR_IMPL_FN)                           \
+#define _LIBC_MODULAR_FORMAT_PRINTF(MODULAR_IMPL_FN, FORMAT_INDEX,             \
+                                    FIRST_TO_CHECK)                            \
+  __attribute__((format(printf, FORMAT_INDEX, FIRST_TO_CHECK)))                \
   __attribute__((modular_format(MODULAR_IMPL_FN, "__printf", "float")))
 
 #endif // LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
diff --git a/libc/include/stdio.yaml b/libc/include/stdio.yaml
index 24890b4b25670..ce7738e8e0648 100644
--- a/libc/include/stdio.yaml
+++ b/libc/include/stdio.yaml
@@ -51,7 +51,7 @@ functions:
       - type: const char *__restrict
       - type: '...'
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__asprintf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__asprintf_modular, 2, 3)
   - name: clearerr
     standards:
       - stdc
@@ -304,7 +304,7 @@ functions:
       - type: const char *__restrict
       - type: '...'
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__printf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__printf_modular, 1, 2)
   - name: putc
     standards:
       - stdc
@@ -377,7 +377,7 @@ functions:
       - type: const char *__restrict
       - type: '...'
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__snprintf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__snprintf_modular, 3, 4)
   - name: sprintf
     standards:
       - stdc
@@ -387,7 +387,7 @@ functions:
       - type: const char *__restrict
       - type: '...'
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__sprintf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__sprintf_modular, 2, 3)
   - name: sscanf
     standards:
       - stdc
@@ -412,7 +412,7 @@ functions:
       - type: const char *__restrict
       - type: va_list
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__vasprintf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vasprintf_modular, 2, 0)
   - name: vfprintf
     standards:
       - stdc
@@ -429,7 +429,7 @@ functions:
       - type: const char *__restrict
       - type: va_list
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__vprintf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vprintf_modular, 1, 0)
   - name: vsnprintf
     standards:
       - stdc
@@ -440,7 +440,7 @@ functions:
       - type: const char *__restrict
       - type: va_list
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__vsnprintf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vsnprintf_modular, 3, 0)
   - name: vsprintf
     standards:
       - stdc
@@ -450,7 +450,7 @@ functions:
       - type: const char *__restrict
       - type: va_list
     attributes:
-      - _LIBC_MODULAR_FORMAT_PRINTF(__vsprintf_modular)
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vsprintf_modular, 2, 0)
   - name: vsscanf
     standards:
       - stdc