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

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 ...).