Creating a shared library with main() [duplicate] - c

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How to make a linux shared object (library) runnable on its own?
(2 answers)
Closed 5 years ago.
In glibc (and also EGLIBC I believe), the libc.so library has a main() method:
$ /lib/i386-linux-gnu/libc.so.6
GNU C Library (Debian GLIBC 2.19-18+deb8u1) stable release version 2.19, by Roland McGrath et al.
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.8.4.
Compiled on a Linux 3.16.7 system on 2015-08-30.
Available extensions:
crypt add-on version 2.1 by Michael Glad and others
GNU Libidn by Simon Josefsson
Native POSIX Threads Library by Ulrich Drepper et al
BIND-8.2.3-T5B
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<http://www.debian.org/Bugs/>.
Now I want to do the same with the following minimalistic example (i. e. I want my own SO to also have a main()):
#include <stdio.h>
int main(int argc, char *argv[]) {
printf("Hello, World!\n");
return 0;
}
Linking as a regular executable:
$ gcc -g -O0 -Wall -c test.c
$ gcc -o test test.o
$ ./test
Hello, World!
Now linking as a shared object:
$ gcc -g -O0 -Wall -c test.c
$ gcc -shared -o libtest.so test.o
$ ./libtest.so
Segmentation fault
nm shows that main symbol is present (000004f5 T main)
When debugging, gdb doesn't show any meaningful backtrace.
Adding -fpic or -fPIC to the gcc command line doesn't help.
What am I doing wrong?

Answer 1

the libc.so library has a main() method
No, the implementation of C library of gcc has an entry point not a main symbol.
You can find an example of how to do this here.