我正试图在Cygwin下构建OCRmyPDF，但遇到了一个难题。 虽然我的整个职业生涯都是在开发，但我的工作主要是在Java中进行的，对Python内部和C++了解甚少。
Linux上的OCRmyPDF是作为一组 "轮子 "包来安装的。 我认为awheel是一个预构建的依赖包。 出于某种原因，在Cygwin下，pip安装程序认为它不能使用轮子包，所以想从源码重建。这个问题发生在试图重建pikepdf包的时候。
下面是错误信息。
building 'pikepdf._qpdf' extension
creating build/temp.cygwin-3.1.4-x86_64-3.7
creating build/temp.cygwin-3.1.4-x86_64-3.7/src
creating build/temp.cygwin-3.1.4-x86_64-3.7/src/qpdf
gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -ggdb -O2 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4 -fdebug-prefix-map=/cygdrive/d/cyg_pub/devel/python/python37/python37-3.7.7-1.x86_64/build=/usr/src/debug/python37-3.7.7-1 -fdebug-prefix-map=/cygdrive/d/cyg_pub/devel/python/python37/python37-3.7.7-1.x86_64/src/Python-3.7.7=/usr/src/debug/python37-3.7.7-1 -ggdb -O2 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4 -fdebug-prefix-map=/cygdrive/d/cyg_pub/devel/python/python37/python37-3.7.7-1.x86_64/build=/usr/src/debug/python37-3.7.7-1 -fdebug-prefix-map=/cygdrive/d/cyg_pub/devel/python/python37/python37-3.7.7-1.x86_64/src/Python-3.7.7=/usr/src/debug/python37-3.7.7-1 -D__MISC_VISIBLE=1 -I/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include -I/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include -I/usr/include/python3.7m -c src/qpdf/annotation.cpp -o build/temp.cygwin-3.1.4-x86_64-3.7/src/qpdf/annotation.o -DVERSION_INFO="1.11.2" -std=c++14 -fvisibility=hidden
In file included from src/qpdf/annotation.cpp:18:
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h: In member function ‘void pybind11::cpp_function::initialize_generic(pybind11::detail::function_record*, const char*, const std::type_info* const*, pybind11::size_t)’:
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:197:21: error: ‘strdup’ was not declared in this scope; did you mean ‘strcmp’?
197 | rec->name = strdup(rec->name ? rec->name : "");
| ^~~~~~
| strcmp
In file included from src/qpdf/annotation.cpp:18:
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h: In member function ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property_static(const char*, const pybind11::cpp_function&, const pybind11::cpp_function&, const Extra& ...)’:
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1284:31: error: there are no arguments to ‘strdup’ that depend on a template parameter, so a declaration of ‘strdup’ must be available [-fpermissive]
1284 | rec_fget->doc = strdup(rec_fget->doc);
| ^~~~~~
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1284:31: note: (if you use ‘-fpermissive’, G++ will accept your code, but allowing the use of an undeclared name is deprecated)
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1292:33: error: there are no arguments to ‘strdup’ that depend on a template parameter, so a declaration of ‘strdup’ must be available [-fpermissive]
1292 | rec_fset->doc = strdup(rec_fset->doc);
| ^~~~~~
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h: In instantiation of ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property_static(const char*, const pybind11::cpp_function&, const pybind11::cpp_function&, const Extra& ...) [with Extra = {pybind11::is_method, pybind11::return_value_policy}; type_ = QPDFAnnotationObjectHelper; options = {}]’:
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1263:80: required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property(const char*, const pybind11::cpp_function&, const pybind11::cpp_function&, const Extra& ...) [with Extra = {pybind11::return_value_policy}; type_ = QPDFAnnotationObjectHelper; options = {}]’
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1252:91: required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property(const char*, const Getter&, const Setter&, const Extra& ...) [with Getter = pybind11::cpp_function; Setter = std::nullptr_t; Extra = {pybind11::return_value_policy}; type_ = QPDFAnnotationObjectHelper; options = {}]’
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1234:58: required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property_readonly(const char*, const pybind11::cpp_function&, const Extra& ...) [with Extra = {pybind11::return_value_policy}; type_ = QPDFAnnotationObjectHelper; options = {}]’
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1228:87: required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property_readonly(const char*, const Getter&, const Extra& ...) [with Getter = std::basic_string<char> (QPDFAnnotationObjectHelper::*)(); Extra = {}; type_ = QPDFAnnotationObjectHelper; options = {}]’
src/qpdf/annotation.cpp:28:82: required from here
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1284:37: error: ‘strdup’ was not declared in this scope; did you mean ‘strcmp’?
1284 | rec_fget->doc = strdup(rec_fget->doc);
| ~~~~~~^~~~~~~~~~~~~~~
| strcmp
/tmp/pip-build-env-fdjj5aaz/overlay/lib/python3.7/site-packages/pybind11/include/pybind11/pybind11.h:1292:39: error: ‘strdup’ was not declared in this scope; did you mean ‘strcmp’?
1292 | rec_fset->doc = strdup(rec_fset->doc);
| ~~~~~~^~~~~~~~~~~~~~~
| strcmp
error: command 'gcc' failed with exit status 1
----------------------------------------
ERROR: Failed building wheel for pikepdf
Failed to build pikepdf
ERROR: Could not build wheels for pikepdf which use PEP 517 and cannot be installed directly
这样看来 pybind11.h 需要 strdup() 但无法使用。strdup() 被声明在 /usr/include/string.h，其中就包含了这一点。
#if __MISC_VISIBLE || __POSIX_VISIBLE >= 200809 || __XSI_VISIBLE >= 4
char *strdup (const char *) __malloc_like __result_use_check;
#endif
char *_strdup_r (struct _reent *, const char *);
#if __POSIX_VISIBLE >= 200809
char *strndup (const char *, size_t) __malloc_like __result_use_check;
#endif
所以我试着添加一个明确的 -D__MISC_VISIBLE 命令行中，看看是否能解决这个问题，但它没有工作（如上图所示）。 我在命令行中捕获了预处理程序的输出，用 -E 并发现即使 -D__MISC_VISIBLE 它的行为就像该宏没有被定义一样。只有 _strdup_r 出现。
Preprocessor output snippet
# 86 "/usr/include/string.h" 3 4
char *_strdup_r (struct _reent *, const char *);
这就是我被卡住的地方 我似乎无法使编译器的include strdup(). 我不知道下一步该怎么做，欢迎大家提出建议。
strdup是标准C语言的扩展。
Cygwin的头文件比其他系统更严格，而且范围都是报告的。
/usr/include/sys/features.h
通常的解决方案。
-std=c++11 与 -std=gnu++11 -std=xxx 一共 -D_GNU_SOURCE 后两者大致相当，因为默认的范围是最大的，而-std=xxx 减少了它。