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fix(iwyu): suggest C headers instead of C++ ones for C files #456

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fix(iwyu): suggest C headers instead of C++ ones for C files
breidenbach0 Aug 4, 2026
7914485
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Aug 4, 2026
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1 change: 1 addition & 0 deletions CHANGELOG.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@ TBA

* Fixed a whitespace/newline false positive for control conditions containing lambdas. (#410)
* We now error on relative include paths (``./``, ``../``). (#432)
* For C files, build/include_what_you_use now suggests the C header (e.g. ``<stdio.h>``) instead of its C++ counterpart (e.g. ``<cstdio>``). (#399)

@aaronliu0130 aaronliu0130 Aug 5, 2026

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https://github.com/cpplint/cpplint/compare/57db60e7ba2f234f856f7caacfff23f4595aafa6..6858cbead66f11a5d588358d7baaf45c836b10ac#diff-2c623f3c6a917be56c59d43279244996836262cb1e12d9d0786c9c49eef6b43c

* This makes ``#include "./foo.h"`` produce two separate errors: that foo.cpp should include foo.h and that relative paths are not allowed.

2.0.2 (2025-04-08)
Expand Down
61 changes: 55 additions & 6 deletions cpplint.py
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Expand Up @@ -916,6 +916,45 @@
r"vim?:\s*.*(\s*|:)filetype=c(\s*|:|$))"
)

# Maps the C++ C-library headers to their C equivalents so that, for C files,
# build/include_what_you_use suggests the C header (e.g. <stdio.h> rather than
# <cstdio>). See https://github.com/cpplint/cpplint/issues/399.

@aaronliu0130 aaronliu0130 Aug 5, 2026

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Suggested change
# <cstdio>). See https://github.com/cpplint/cpplint/issues/399.
# <cstdio>).

_C_LIBRARY_CPP_HEADERS = {
"<cassert>": "<assert.h>",
"<cctype>": "<ctype.h>",
"<cerrno>": "<errno.h>",
"<cfenv>": "<fenv.h>",
"<cfloat>": "<float.h>",
"<cinttypes>": "<inttypes.h>",
"<climits>": "<limits.h>",
"<clocale>": "<locale.h>",
"<cmath>": "<math.h>",
"<csetjmp>": "<setjmp.h>",
"<csignal>": "<signal.h>",
"<cstdarg>": "<stdarg.h>",
"<cstddef>": "<stddef.h>",
"<cstdint>": "<stdint.h>",
"<cstdio>": "<stdio.h>",
"<cstdlib>": "<stdlib.h>",
"<cstring>": "<string.h>",
"<ctime>": "<time.h>",
"<cuchar>": "<uchar.h>",
"<cwchar>": "<wchar.h>",
"<cwctype>": "<wctype.h>",
}


def _IsCFile(filename: str, lines: list[str]) -> bool:
"""Whether the file is a C file: a .c/.cu extension or a LINT_C_FILE marker.

Single source of truth shared by the C-specific error suppression and the
C-vs-C++ header suggestion, so the two never drift. See #399.
"""
return filename.lower().endswith((".c", ".cu")) or any(
_SEARCH_C_FILE.search(line) for line in lines
)


# Match string that indicates we're working on a Linux Kernel file.
_SEARCH_KERNEL_FILE = re.compile(r"\b(?:LINT_KERNEL_FILE)")

Expand Down Expand Up @@ -1206,10 +1245,10 @@ def ProcessGlobalSuppressions(filename: str, lines: list[str]) -> None:
last element being empty if the file is terminated with a newline.
filename: str, the name of the input file.
"""
if _IsCFile(filename, lines):
for category in _DEFAULT_C_SUPPRESSED_CATEGORIES:
_error_suppressions.AddGlobalSuppression(category)
for line in lines:
if _SEARCH_C_FILE.search(line) or filename.lower().endswith((".c", ".cu")):
for category in _DEFAULT_C_SUPPRESSED_CATEGORIES:
_error_suppressions.AddGlobalSuppression(category)
if _SEARCH_KERNEL_FILE.search(line):
for category in _DEFAULT_KERNEL_SUPPRESSED_CATEGORIES:
_error_suppressions.AddGlobalSuppression(category)
Expand Down Expand Up @@ -7119,7 +7158,9 @@ def FilesBelongToSameModule(filename_cc, filename_h):
return files_belong_to_same_module, common_path


def CheckForIncludeWhatYouUse(filename, clean_lines, include_state, error, io=codecs):
def CheckForIncludeWhatYouUse(
filename, clean_lines, include_state, error, io=codecs, is_c_file=False
):
"""Reports for missing stl includes.

This function will output warnings to make sure you are including the headers
Expand Down Expand Up @@ -7183,12 +7224,19 @@ def CheckForIncludeWhatYouUse(filename, clean_lines, include_state, error, io=co
if header_stripped not in include_dict and not (
header_stripped[0] == "c" and (header_stripped[1:] + ".h") in include_dict
):
# For C files, suggest the C header (e.g. <stdio.h>) rather than
# the C++ one (e.g. <cstdio>). See #399.

@aaronliu0130 aaronliu0130 Aug 5, 2026

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Suggested change
# the C++ one (e.g. <cstdio>). See #399.
# the C++ one (e.g. <cstdio>).

suggested = (
_C_LIBRARY_CPP_HEADERS[header]
if is_c_file and header in _C_LIBRARY_CPP_HEADERS
else header
)
error(
filename,
required[header][0],
"build/include_what_you_use",
4,
"Add #include " + header + " for " + template,
"Add #include " + suggested + " for " + template,
)


Expand Down Expand Up @@ -7578,7 +7626,8 @@ def ProcessFileData(filename, file_extension, lines, error, extra_check_function
"NONLINT block never ended",
)

CheckForIncludeWhatYouUse(filename, clean_lines, include_state, error)
is_c_file = _IsCFile(filename, clean_lines.raw_lines)
CheckForIncludeWhatYouUse(filename, clean_lines, include_state, error, is_c_file=is_c_file)

# Check that the .cc file has included its header if it exists.
if _IsSourceExtension(file_extension):
Expand Down
32 changes: 28 additions & 4 deletions cpplint_unittest.py
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Original file line number Diff line number Diff line change
Expand Up @@ -223,7 +223,8 @@ def PerformIncludeWhatYouUse(self, code, filename="foo.h", io=codecs):
error_collector = ErrorCollector(self.assertTrue)
include_state = cpplint._IncludeState()
nesting_state = cpplint.NestingState()
lines = code.split("\n")
raw_lines = code.split("\n")

@aaronliu0130 aaronliu0130 Aug 5, 2026

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What is the point of making a new variable here?

lines = raw_lines
Comment on lines +226 to +227

@coderabbitai coderabbitai Bot Aug 4, 2026

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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Keep raw_lines independent from lines.

lines = raw_lines aliases the same list. RemoveMultiLineComments() then mutates raw_lines before _IsCFile() reads it. This helper cannot reproduce detection of a LINT_C_FILE marker in a multi-line comment.

Proposed fix 
 raw_lines = code.split("\n")
-lines = raw_lines
+lines = raw_lines[:]
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
raw_lines = code.split("\n")
lines = raw_lines
raw_lines = code.split("\n")
lines = raw_lines[:]
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@cpplint_unittest.py` around lines 226 - 227, Update the setup in the affected
test helper so lines is an independent copy of raw_lines rather than an alias.
Preserve raw_lines unchanged for _IsCFile() after RemoveMultiLineComments()
mutates lines, ensuring LINT_C_FILE markers inside multi-line comments are
detected.

cpplint.RemoveMultiLineComments(filename, lines, error_collector)
lines = cpplint.CleansedLines(lines)
for i in range(lines.NumLines()):
Expand All @@ -235,7 +236,10 @@ def PerformIncludeWhatYouUse(self, code, filename="foo.h", io=codecs):
# have language problems.

# Second, look for missing includes.
cpplint.CheckForIncludeWhatYouUse(filename, lines, include_state, error_collector, io)
is_c_file = cpplint._IsCFile(filename, raw_lines)

@aaronliu0130 aaronliu0130 Aug 5, 2026

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This means we still run this twice for each file, which was exactly what we were trying to avoid with the move to state.

cpplint.CheckForIncludeWhatYouUse(
filename, lines, include_state, error_collector, io, is_c_file=is_c_file
)
return error_collector.Results()

# Perform lint and make sure one of the errors is what we want
Expand All @@ -259,8 +263,8 @@ def TestMultiLineLintRE(self, code, expected_message_re):
def TestLanguageRulesCheck(self, file_name, code, expected_message):
assert expected_message == self.PerformLanguageRulesCheck(file_name, code)

def TestIncludeWhatYouUse(self, code, expected_message):
assert expected_message == self.PerformIncludeWhatYouUse(code)
def TestIncludeWhatYouUse(self, code, expected_message, filename="foo.h"):
assert expected_message == self.PerformIncludeWhatYouUse(code, filename=filename)

def TestBlankLinesCheck(self, lines, start_errors, end_errors):
for extension in ["c", "cc", "cpp", "cxx", "c++", "cu"]:
Expand Down Expand Up @@ -1290,6 +1294,26 @@ def testIncludeWhatYouUse(self):
printf("hello world");""",
"",
) # Avoid false positives w/ c-style include
# C files should be told to include the C header (e.g. <stdio.h>) rather
# than its C++ counterpart (e.g. <cstdio>). See #399.

@aaronliu0130 aaronliu0130 Aug 3, 2026

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Suggested change
# than its C++ counterpart (e.g. <cstdio>). See #399.
# than the C++ counterpart (e.g. <cstdio>).

self.TestIncludeWhatYouUse(
'printf("hello world");',
"Add #include <stdio.h> for printf [build/include_what_you_use] [4]",
filename="foo.c",
)
# C++ files are unaffected.
self.TestIncludeWhatYouUse(
'printf("hello world");',
"Add #include <cstdio> for printf [build/include_what_you_use] [4]",
filename="foo.cpp",
)
# The "C header already included" behaviour is preserved for C files.
self.TestIncludeWhatYouUse(
"""#include <stdio.h>
printf("hello world");""",
"",
filename="foo.c",
)
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Comment on lines +1310 to +1316

@aaronliu0130 aaronliu0130 Aug 3, 2026

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This is already tested

self.TestIncludeWhatYouUse(
"void a(const string &foobar);",
"Add #include <string> for string [build/include_what_you_use] [4]",
Expand Down