c++ - gdb error not in executable format: File format not recognized

Question

I am trying to debug a simple "hello world" C++ program on Ubuntu 16.04 but gdb is not able to recognize the executable file format. However, I am able to successfully run the executable on the command line. Here is the code

#include <iostream>
using namespace std;

int main() {
    cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!
    return 0;
}

I compile the program file TestProject.cpp using the command

g++ -g TestProject.cpp -o hello

Then to debug, I give the command

gdb ./hello

I get the following error message

GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
"/home/<home>/./hello": not in executable format: File format not recognized

Something seems to be corrupt with the Ubuntu machine. Because I am able to debug the same program on another Ubuntu 16.04 virtual machine.

Answer 1

It is almost certain that ks1322's comment is correct one:

1. You've installed a 64-bit GCC, so your ./hello is a 64-bit binary (use file ./hello to confirm).
2. You've installed a 32-bit only GDB, so it doesn't know how to debug x86_64 binaries.

The fix is simple: install 64-bit GDB (which is capable of debugging both 32 and 64-bit binaries), or build hello in 32-bit mode (with g++ -m32 ...).