Viewing environmental variables in a running program in gdb

Question

In a core dump file, I am able to view environmental variables with:

$ gdb file core
>>> p __environ[0]
$1 = 0x7ffe6420dcbd "SHELL=/bin/bash"
>>> p __environ[1]
$2 = 0x7ffe6420dccd "TMUX=/tmp/tmux-1001/default,13871,0"
# basically the same as doing $ printenv    

However, when I try and debug a normal executable:

# file.s
.globl main
main:
    ret

$ gcc file.s -ggdb3 -o file2
$ gdb -q file2
Reading symbols from file...
(No debugging symbols found in file)
>>> starti
>>> p __environ
No symbol "__environ" in current context.

How can I get access to the environ and argv ? One possible way is to examine its process file:

>>> python print(open('/proc/48011/environ').read().replace('\x00','\n'))
SHELL=/bin/bash
TMUX=/tmp/tmux-1001/default,13871,0
SSH_AUTH_SOCK=/tmp/ssh-RtmbGp8HSFcN/agent.14219
SSH_AGENT_PID=14220
EDITOR=/usr/bin/vim

Where 48011 is from:

─── Threads ───────────────────────────────────────────────────────────────────
[1] id 48011 name file from 0x0000000000401005 in _start 

Answer 1

According to its documentation, GDB has a show environment command. On Linux, read also gdb(1), gcore(1), core(5), proc(5), signal(7) and gcc(1) and conventions documented in environ(7). See also getenv(3) and putenv(3). You could also play with objdump(1) and readelf(1) and nm(1). Be sure to read more documentation about ELF, perhaps in the Linkers and loaders book.

But if you compile on LInux an assembler file using gcc file.s -ggdb3 -o file2 that assembler file lacks DWARF debug information, nearly required by GDB.

So try writing a small hello-world.c program, compile it using gcc -Wall -Wextra -fverbose-asm -g -S hello-world.c and examine (e.g. with less(1)) the emitted hello-world.s assembler file. You'll see there a lot of DWARF related assembler directives. Compile it again with gcc -v -Wall -Wextra -g -O hello-world.c -o helloworld to understand what programs GCC is starting for you. Read also the documentation about invoking GCC.

Be of course aware of crt0 startup code. On Linux, study the source code of both GNU libc (or musl-libc) and of GCC and binutils. All of them are free software or at least open source software, and you are allowed to download, study and improve their source code.

Read also the specification of Linux ABI. See this answer. Consider also studying the source code of your Linux kernel. See also kernelnewbies.

You might be interested by linuxfromscratch.org which gives detailed step by step instructions (to build an entire Linux distribution from scratch). Read Advanced Linux Programming and Linux Assembler HowTo. Budget then several weeks of work.

Read also execve(2), elf(5), syscalls(2) and study the source code of GNU bash.

Read wikipages about processes, system calls, computer files, file systems and executables. Consider reading a good textbook on operating systems and the Modern C book and perhaps the C11 standard n1570.