Ada: Gnat generates bad .o file - ada

Question

My code is:
with Ada.Text_IO; use Ada.Text_IO;
procedure Hello is
begin
Put_Line ("Hello, world!");
end Hello;
GNAT makes a .o and .ali.
I type
gcc hello.o -o hello
GCC complains:
/usr/lib/gcc/i686-linux-gnu/6/../../../i386-linux-gnu/crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
hello.o: In function `_ada_hello':
hello.adb:(.text+0x26): undefined reference to `ada__text_io__put_line__2'
collect2: error: ld returned 1 exit status

Answer 1

You shouldn't try to compile Ada programs by hand with the gcc command. There are two sensible alternatives depending on your preferences:
Use the gnatmake command as Brian suggests in his comment. This works well as long as all the relevant source files are stored in the same directory.
Create a project file and use gprbuild. Projects can handle source files distributed in multiple directories, include other projects, contain compiler and linker flags, etc.
On the "Hello_World" level, gnatmake works okay, but you should be aware that gnatmake doesn't behave like a real Ada compiler, unless you pass it some specific flags. I've managed this with an alias in my shell, but in reality it is yet another argument in favour of using gprbuild even for tiny projects, as you can put the flags which make GCC behave like a real Ada compiler in the project files, and not worry more about it.
project Hello_World is
for Source_Dirs use ("src");
for Object_Dir use "obj";
for Exec_Dir use "bin";
for Main use ("hello.adb");
package Builder is
for Default_Switches ("Ada")
use ("-m");
end Builder;
package Compiler is
for Default_Switches ("Ada")
use ("-fstack-check", -- Generate stack checking code (part of Ada)
"-gnata", -- Enable assertions (part of Ada)
"-gnato13", -- Overflow checking (part of Ada)
"-gnatf", -- Full, verbose error messages
"-gnatwa", -- All optional warnings
"-gnatVa", -- All validity checks
"-gnaty3abcdefhiklmnoOprstux", -- Style checks
"-gnatwe", -- Treat warnings as errors
"-gnat2012", -- Use Ada 2012
"-Wall", -- All GCC warnings
"-O2"); -- Optimise (level 2/3)
end Compiler;
end Hello_World;
If you use GNAT Programming Studio (GPS) as your IDE, you may want to remove the -gnatf flag, as I have experienced that can prevent GPS from parsing the error messages from the compiler.