Prohibit implicit C function declarations

[yan12125]

BPO	27659
Nosy	@ronaldoussoren, @vstinner, @benjaminp, @ned-deily, @xdegaye, @vadmium, @moreati, @yan12125
PRs	[Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552
Files	check-crypt.patch check-crypt.patch: version 2, autoreconf necessary prohibit-implicit-function-declarations.patch prohibit-implicit-function-declarations.patch: Patch version 2 prohibit-implicit-function-declarations.patch: Patch version 3, autoreconf necessary

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-02-06.13:45:59.719>
created_at = <Date 2016-07-31.07:04:38.341>
labels = ['type-feature', '3.7', 'build']
title = 'Prohibit implicit C function declarations'
updated_at = <Date 2017-03-31.16:36:20.997>
user = 'https://github.com/yan12125'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:20.997>
actor = 'dstufft'
assignee = 'none'
closed = True
closed_date = <Date 2017-02-06.13:45:59.719>
closer = 'yan12125'
components = ['Build', 'Cross-Build']
creation = <Date 2016-07-31.07:04:38.341>
creator = 'yan12125'
dependencies = []
files = ['43954', '43955', '46033', '46036', '46302']
hgrepos = []
issue_num = 27659
keywords = ['patch']
message_count = 19.0
messages = ['271727', '271728', '271729', '271730', '271732', '278770', '278824', '278836', '283996', '284002', '284004', '285567', '285583', '286759', '287126', '287127', '287128', '287130', '287132']
nosy_count = 9.0
nosy_names = ['ronaldoussoren', 'vstinner', 'benjamin.peterson', 'ned.deily', 'xdegaye', 'python-dev', 'martin.panter', 'Alex.Willmer', 'yan12125']
pr_nums = ['552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27659'
versions = ['Python 3.7']

[yan12125]

On Android the crypt() function is missing, causing ugly linking errors when compiling the _crypt module. This patch handles it elegantly.

A question: should I include changes to configure and pyconfig.h.in in the patch?

[yan12125]

Version 2: correct the name added to missing

[yan12125]

Some references for crypt():

POSIX standard: http://pubs.opengroup.org/onlinepubs/9699919799/functions/crypt.html
Linux man page: http://man7.org/linux/man-pages/man3/crypt.3.html
FreeBSD man page: https://www.freebsd.org/cgi/man.cgi?crypt(3)
Mac OS X man page: https://developer.apple.com/library/ios/documentation/System/Conceptual/ManPages_iPhoneOS/man3/crypt.3.html

All requires <unistd.h>.

[xdegaye]

A question: should I include changes to configure and pyconfig.h.in in the patch?

You just need to mention that one should run autoreconf.

[yan12125]

Thanks, added to the patch description

[xdegaye]

Android does not have crypt, but the crypt module is cross-built nevertheless after this warning has been issued:
warning: implicit declaration of function 'crypt' is invalid in C99 [-Wimplicit-function-declaration]
And at runtime, importing the crypt module fails with:
ImportError: dlopen failed: cannot locate symbol "crypt" referenced by "_crypt.cpython-37m-i686-linux-android.so"

gcc and clang do not enforce the C99 rules and emit just a warning for implicit function declarations instead of the error that would be conforming to C99. This can be changed with the flag '-Werror=implicit-function-declaration' and the compilation of the crypt extension module rightly fails in that case.

I think this issue should be fixed by adding this flag to the Makefile.
Maybe in another issue.

[yan12125]

Agreed. Adding -Werror=implicit-function-declaration is much simpler. Feel free to close it as rejected.

[vadmium]

If there is an obscure platform where we don’t include the right header file for a function, changing the warning into an error would cause the build to fail. If we do make it an error, it should only be so for 3.7.

[yan12125]

(Re-use the existing issue)

Here's a patch that tries to add -Werror=implicit-function-declaration to $BASECFLAGS.

This is useful for cross-compiling. When a function is missing, the error jumps out during the build time rather than runtime.

Tested configurations:

• Arch Linux x86_64 native build
• macOS Sierra native build
• Android ARM, API 21 cross build

I'd like to hear some ideas from macOS experts as in my memory macOS's build system is fragile than that on Linux.