Strange error reading file

Question

I'm compiling in mingw on windows and using gdb to debug my application. I'm getting this output when trying to read a file from disk:

processfile (type=35633, source=0xec4d6c "î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_î_"...) at main.cpp:5

Here is my read file function:

const char* read_file_contents(const char* filename) { string ret = ""; string line; ifstream ifs(filename); if (ifs.is_open()) { while (getline(ifs, line)){ ret += line + '\n'; } } else { std::cout << "failed to open file: " << filename << std::endl; } return ret.c_str(); }

Here is my main:

#include <iostream> #include "FileOps.h" void test_func2(const char* test) { std::cout << strlen(test) << std::endl; std::cout << test << std::endl; } void test_func1(const char* test) { test_func2(test); } int main(int argc, char** argv) { test_func1(read_file_contents("test.txt")); return 0; }

Can someone explain this behavior? Thanks!

Sam Varshavchik · Accepted Answer · 2016-04-22 01:39:57Z

This is undefined behavior.

 return ret.c_str();

The object ret has a local function scope. This object gets destroyed when this function returns, and all of its internal memory gets deallocated.

It's c_str() method returns a pointer that's no longer valid, after the object gets destroyed. As soon as this function returns, the c_str() pointer is no longer valid.

The pointer that's returned by c_str() is only valid until the std::string object is modified, or it gets destroyed.

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