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C programs using PyMem_MALLOC in pymem.h generate a warning when -Wint-in-bool-context is enabled (typically through -Wall). In places where -Werror is enabled (such as RPM build), this causes the build to fail with an error that looks like:
...
In file included from /usr/include/python2.7/Python.h:78:0,
 from netnsmodule.c:16:
netnsmodule.c: In function 'netns_nsexecvp':
/usr/include/python2.7/pymem.h:97:30: error: '*' in boolean context, suggest '&&' instead [-Werror=int-in-bool-context]
 ( (type *) PyMem_MALLOC((n) * sizeof(type)) ) )
 ^
/usr/include/python2.7/pymem.h:75:15: note: in definition of macro 'PyMem_MALLOC'
 : malloc((n) ? (n) : 1))
 ^
netnsmodule.c:61:10: note: in expansion of macro 'PyMem_NEW'
 argv = PyMem_NEW(char *, argc + 1);
 ^~~~~~~~~
cc1: all warnings being treated as errors
error: command 'gcc' failed with exit status 1
...
I'm attaching a patch that fixes the issue, please consider applying!
Thanks,
--Gabriel

This attachment illustrates how the problem is triggered. The file is part of the CORE network emulator (github.com/coreemu/core). Compile with "gcc -Wall -I/usr/include/python2.7 -c netnsmodule.c".

> /usr/include/python2.7/pymem.h:97:30: error: '*' in boolean context, suggest '&&' instead [-Werror=int-in-bool-context]
I don't understand this error. I would be curious to see the output of the preprocessor. Try maybe to use gcc -E?

output of "gcc -E -Wall -I/usr/include/python2.7 -c netnsmodule.c > foo.c"
I think gcc7 is a bit more paranoid about whether some expression evaluating to an int can/should in itself be used as a Boolean (i.e., without being compared to 0).

> malloc(((argc + 1) * sizeof(char *)) ? ((argc + 1) * sizeof(char *)) : 1)) ) );
Ah ok, the "(n)?" expression is "((argc + 1) * sizeof(char *)) ? " and yes it contains "*".

This is a duplicate of https://bugs.python.org/issue31474
which was fixed in https://github.com/python/cpython/pull/3581
It should be included in the next Python 2.7.x release.