Call `atexit` when linking to libc dynamically on Linux - c

Question

If I have the following program written in C (compiled with GCC on Debian 8.7), I am able to call atexit() as you would expect:
#include <stdlib.h>
void exit_handler(void) {
return;
}
int main () {
atexit(exit_handler);
return 0;
}
And when I compile and run it:
$ gcc test.c
$ ./a.out
Outputs nothing, just as you would expect. In fact, when I run ldd, I get:
$ ldd a.out
linux-vdso.so.1 (0x00007fffbe592000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fe07d3a8000)
/lib64/ld-linux-x86-64.so.2 (0x00007fe07d753000)
However, libc does not seem to have any symbols for atexit, amd only has__cxa_atexit and __cxa_threaded_atexit_impl:
$ nm --dynamic /lib/x86_64-linux-gnu/libc.so.6 | grep 'atexit'
0000000000037d90 T __cxa_atexit
0000000000037fa0 T __cxa_thread_atexit_impl
As you would then expect, if I try to link to libc dynamically, I cannot actually call atexit(), such as in the following Racket program which links to libc and tries to find atexit:
#lang racket
(require ffi/unsafe)
(get-ffi-obj 'atexit (ffi-lib "libc" '("6")) (_fun (_fun -> _void) -> _int))
Giving the output:
$ racket findatexit.rkt
ffi-obj: couldn't get "atexit" from "libc.so.6" (/lib/x86_64-linux-gnu/libc.so.6: undefined symbol: atexit)
What I want to know here is:
If libc does not have any symbol for atexit on Linux, why can I still call it from a C program?
Is there any way I can call atexit or a similar function dynamically on Linux?
(I should note that atexit does appear to be a symbol on OS X, so its just Linux that seems unusual here.)
Edit:
At the suggestion of #Jonathan, I also ran:
$ gcc -c test.c
$ nm test.o
U atexit
0000000000000000 T exit_handler
0000000000000007 T main
Which seems to indicate the atexit symbol is there somewhere, but it does not appear in any of the libraries ldd is showing.

Answer 1

I did some poking around on a Centos 7 virtual machine, and I think I found it — but it was anything but obvious!
Found it!
In /usr/lib64/libc_nonshared.a:
$ nm /usr/lib64/libc_nonshared.a | grep -i atexit
atexit.oS:
0000000000000000 T atexit
U __cxa_atexit
$
Why look in that library? Good question — and a long story. Are you sitting comfortably? Then I'll begin…
Steps taken to get there
Use the test.c code from the question.
Compile it with gcc -v test.c:
$ gcc -v test.c
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin --enable-initfini-array --disable-libgcj --with-isl=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/isl-install --with-cloog=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/cloog-install --enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.5 20150623 (Red Hat 4.8.5-11) (GCC)
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/cc1 -quiet -v test.c -quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -version -o /tmp/ccPHTer7.s
GNU C (GCC) version 4.8.5 20150623 (Red Hat 4.8.5-11) (x86_64-redhat-linux)
compiled by GNU C version 4.8.5 20150623 (Red Hat 4.8.5-11), GMP version 6.0.0, MPFR version 3.1.1, MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=96 --param ggc-min-heapsize=124992
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include
/usr/local/include
/usr/include
End of search list.
GNU C (GCC) version 4.8.5 20150623 (Red Hat 4.8.5-11) (x86_64-redhat-linux)
compiled by GNU C version 4.8.5 20150623 (Red Hat 4.8.5-11), GMP version 6.0.0, MPFR version 3.1.1, MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=96 --param ggc-min-heapsize=124992
Compiler executable checksum: 356f86e67978d665416e07d560c8ba0d
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
as -v --64 -o /tmp/cc5WHEA4.o /tmp/ccPHTer7.s
GNU assembler version 2.25.1 (x86_64-redhat-linux) using BFD version version 2.25.1-22.base.el7
COMPILER_PATH=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/:/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/:/usr/libexec/gcc/x86_64-redhat-linux/:/usr/lib/gcc/x86_64-redhat-linux/4.8.5/:/usr/lib/gcc/x86_64-redhat-linux/
LIBRARY_PATH=/usr/lib/gcc/x86_64-redhat-linux/4.8.5/:/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-mtune=generic' '-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2 --build-id --no-add-needed --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../.. /tmp/cc5WHEA4.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o
$
The interesting part is the collect2 command line at the end. Written with one argument per line, that is:
/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2
--build-id
--no-add-needed
--eh-frame-hdr
--hash-style=gnu
-m
elf_x86_64
-dynamic-linker
/lib64/ld-linux-x86-64.so.2
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crti.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64
-L/lib/../lib64
-L/usr/lib/../lib64
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../..
/tmp/cc5WHEA4.o
-lgcc
--as-needed
-lgcc_s
--no-as-needed
-lc
-lgcc
--as-needed
-lgcc_s
--no-as-needed
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crtn.o
So, there are a bunch of cr*.o files, plus three libraries: -lc, -lgcc and -lgcc_s to look for, and a bunch of directories to look in:
-L/usr/lib/gcc/x86_64-redhat-linux/4.8.5, -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64, -L/lib/../lib64, -L/usr/lib/../lib64, -L/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../... The /tmp/cc5WHEA4.o is the object file created from test.c.
Applying some clean-up code to the path names, and then using ls to help find the libraries yields a list of files to examine further:
/lib64/ld-linux-x86-64.so.2
/usr/lib64/crt1.o
/usr/lib64/crti.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtbegin.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/crtend.o
/usr/lib64/crtn.o
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/libgcc.a
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/libgcc_s.so
/usr/lib64/libgcc_s.so.1
/lib64/libgcc_s.so.1
/usr/lib64/libgcc_s.so.1
/usr/lib64/libc.so
/usr/lib64/libc.so.6
/lib64/libc.so
/lib64/libc.so.6
/usr/lib64/libc.so
/usr/lib64/libc.so.6
That list of files was saved in a file yy (unimaginative name), and then used in:
$ nm -o $(<yy) | tee nm.log | grep -i atexit
nm: _trampoline.o: no symbols
nm: __main.o: no symbols
nm: _ctors.o: no symbols
nm: /usr/lib/gcc/x86_64-redhat-linux/4.8.5/libgcc_s.so: no symbols
nm: /usr/lib64/libgcc_s.so.1: no symbols
nm: /lib64/libgcc_s.so.1: no symbols
nm: /usr/lib64/libgcc_s.so.1: no symbols
nm: /usr/lib64/libc.so: File format not recognized
/usr/lib64/libc.so.6:00000000003bcc00 b added_atexit_handler.9157
/usr/lib64/libc.so.6:0000000000038c90 T __cxa_atexit
/usr/lib64/libc.so.6:0000000000038c90 t __cxa_atexit_internal
/usr/lib64/libc.so.6:00000000003b6838 d __elf_set___libc_atexit_element__IO_cleanup__
/usr/lib64/libc.so.6:0000000000038c40 t __internal_atexit
/usr/lib64/libc.so.6:00000000003b6838 d __start___libc_atexit
/usr/lib64/libc.so.6:00000000003b6840 d __stop___libc_atexit
nm: /lib64/libc.so: File format not recognized
/lib64/libc.so.6:00000000003bcc00 b added_atexit_handler.9157
/lib64/libc.so.6:0000000000038c90 T __cxa_atexit
/lib64/libc.so.6:0000000000038c90 t __cxa_atexit_internal
/lib64/libc.so.6:00000000003b6838 d __elf_set___libc_atexit_element__IO_cleanup__
/lib64/libc.so.6:0000000000038c40 t __internal_atexit
nm: /usr/lib64/libc.so: File format not recognized
/lib64/libc.so.6:00000000003b6838 d __start___libc_atexit
/lib64/libc.so.6:00000000003b6840 d __stop___libc_atexit
/usr/lib64/libc.so.6:00000000003bcc00 b added_atexit_handler.9157
/usr/lib64/libc.so.6:0000000000038c90 T __cxa_atexit
/usr/lib64/libc.so.6:0000000000038c90 t __cxa_atexit_internal
/usr/lib64/libc.so.6:00000000003b6838 d __elf_set___libc_atexit_element__IO_cleanup__
/usr/lib64/libc.so.6:0000000000038c40 t __internal_atexit
/usr/lib64/libc.so.6:00000000003b6838 d __start___libc_atexit
/usr/lib64/libc.so.6:00000000003b6840 d __stop___libc_atexit
$
There's no evidence of a plain atexit function there. Where's it hiding, and what's with those 'File format not recognized' messages?
$ file /usr/lib64/libc.so
/usr/lib64/libc.so: ASCII text
$
ASCII text? What?
$ cat /usr/lib64/libc.so
/* GNU ld script
Use the shared library, but some functions are only in
the static library, so try that secondarily. */
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /lib64/libc.so.6 /usr/lib64/libc_nonshared.a AS_NEEDED ( /lib64/ld-linux-x86-64.so.2 ) )
$
OK; what's in /usr/lib64/libc_nonshared.a?
$ nm /usr/lib64/libc_nonshared.a | grep -i atexit
atexit.oS:
0000000000000000 T atexit
U __cxa_atexit
$
Bingo! Found it!
So, it seems that the collect2 linker used by GCC is able to load files not listed on the command line, and that one of those files is /usr/lib64/libc_nonshared.a, and that this library has atexit() in it. Consequently, you should be able to invoke atexit() because it is statically linked into the executable … unless there's some more black magic hidden away here that I've not sussed out.