[New-bugs-announce] [issue38044] unittest causing segfaults with string malloc in c++ module

Jeremy report at bugs.python.org
Fri Sep 6 06:11:18 EDT 2019

New submission from Jeremy <jeremy.westhead456 at gmail.com>:

If a unittest is written which accesses a module written in C++ (I used Pybind11 to allow Python access) which uses malloc for a string, a segmentation fault is caused. This is not replicated when malloc is used for some other data types such as int, char or char*

C++ code:

#include <string>
#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
using namespace std;
void t(){
    string * data = (string*)malloc(100*sizeof(string));
    data[0] = "this is a test";
PYBIND11_MODULE(TestModule, m){
    m.def("editPointerString", &t);

Once compiled and imported this can be run fine with editPointerString() as expected. No errors occur when running this function from any Python functions or from within a class.

However, if this is run through a unittest such as:

class TestUnit(unittest.TestCase):
    def testStringPointer(self):

A segmentation fault occurs before the cout<<data[0]<<endl; is executed. Not having an assert statement does not affect this outcome.

However, running 3 or more tests which call this function allows the tests to run and pass - instead displaying there has been a segmentation fault after the OK message from unittest

This can be fixed by using calloc instead of malloc, so may be caused by processing of data which is not cleared when malloc is used?

Also, malloc can be used in functions in tests after a test running calloc has been run.

System info:
Python 3.7.3
Ubuntu 19.04
pybind11 2.3.0
g++ 8.3.0
C++ 8.3.0

components: Extension Modules, Library (Lib)
messages: 351243
nosy: ymerej
priority: normal
severity: normal
status: open
title: unittest causing segfaults with string malloc in c++ module
type: crash
versions: Python 3.7

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