What exactly does `-rdynamic` do and when exactly is it needed?
Here is a simple example project to illustrate the use of -rdynamic.
bar.c
extern void foo(void);void bar(void){ foo();}main.c
#include <dlfcn.h>#include <stdio.h>#include <stdlib.h>void foo(void){ puts("Hello world");}int main(void){ void * dlh = dlopen("./libbar.so", RTLD_NOW); if (!dlh) { fprintf(stderr, "%s\n", dlerror()); exit(EXIT_FAILURE); } void (*bar)(void) = dlsym(dlh,"bar"); if (!bar) { fprintf(stderr, "%s\n", dlerror()); exit(EXIT_FAILURE); } bar(); return 0;}Makefile
.PHONY: all clean testLDEXTRAFLAGS ?=all: progbar.o: bar.c gcc -c -Wall -fpic -o $@ $<libbar.so: bar.o gcc -shared -o $@ $<main.o: main.c gcc -c -Wall -o $@ $<prog: main.o | libbar.so gcc $(LDEXTRAFLAGS) -o $@ $< -L. -lbar -ldlclean: rm -f *.o *.so progtest: prog ./$<Here, bar.c becomes a shared library libbar.so and main.c becomesa program that dlopens libbar and calls bar() from that library.bar() calls foo(), which is external in bar.c and defined in main.c.
So, without -rdynamic:
$ make testgcc -c -Wall -o main.o main.cgcc -c -Wall -fpic -o bar.o bar.cgcc -shared -o libbar.so bar.ogcc -o prog main.o -L. -lbar -ldl./prog./libbar.so: undefined symbol: fooMakefile:23: recipe for target 'test' failedAnd with -rdynamic:
$ make cleanrm -f *.o *.so prog$ make test LDEXTRAFLAGS=-rdynamicgcc -c -Wall -o main.o main.cgcc -c -Wall -fpic -o bar.o bar.cgcc -shared -o libbar.so bar.ogcc -rdynamic -o prog main.o -L. -lbar -ldl./progHello world
I use rdynamic to print out backtraces using the backtrace()/backtrace_symbols() of Glibc.
Without -rdynamic, you cannot get function names.
To know more about the backtrace() read it over here.
-rdynamic exports the symbols of an executable, this mainly addresses scenarios as described in Mike Kinghan's answer, but also it helps e.g. Glibc's backtrace_symbols() symbolizing the backtrace.
Here is a small experiment (test program copied from here)
#include <execinfo.h> #include <stdio.h>#include <stdlib.h>/* Obtain a backtrace and print it to stdout. */voidprint_trace (void){ void *array[10]; size_t size; char **strings; size_t i; size = backtrace (array, 10); strings = backtrace_symbols (array, size); printf ("Obtained %zd stack frames.\n", size); for (i = 0; i < size; i++) printf ("%s\n", strings[i]); free (strings);}/* A dummy function to make the backtrace more interesting. */voiddummy_function (void){ print_trace (); }intmain (void){ dummy_function (); return 0;}compile the program: gcc main.c and run it, the output:
Obtained 5 stack frames../a.out() [0x4006ca]./a.out() [0x400761]./a.out() [0x40076d]/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7f026597f830]./a.out() [0x4005f9]Now, compile with -rdynamic, i.e. gcc -rdynamic main.c, and run again:
Obtained 5 stack frames../a.out(print_trace+0x28) [0x40094a]./a.out(dummy_function+0x9) [0x4009e1]./a.out(main+0x9) [0x4009ed]/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7f85b23f2830]./a.out(_start+0x29) [0x400879]As you can see, we get a proper stack trace now!
Now, if we investigate ELF's symbol table entry (readelf --dyn-syms a.out):
without -rdynamic
Symbol table '.dynsym' contains 9 entries: Num: Value Size Type Bind Vis Ndx Name 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND 1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND free@GLIBC_2.2.5 (2) 2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts@GLIBC_2.2.5 (2) 3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace_symbols@GLIBC_2.2.5 (2) 4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace@GLIBC_2.2.5 (2) 5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail@GLIBC_2.4 (3) 6: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@GLIBC_2.2.5 (2) 7: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (2) 8: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__with -rdynamic, we have more symbols, including the executable's:
Symbol table '.dynsym' contains 25 entries: Num: Value Size Type Bind Vis Ndx Name 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND 1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND free@GLIBC_2.2.5 (2) 2: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab 3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts@GLIBC_2.2.5 (2) 4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace_symbols@GLIBC_2.2.5 (2) 5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace@GLIBC_2.2.5 (2) 6: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail@GLIBC_2.4 (3) 7: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@GLIBC_2.2.5 (2) 8: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.2.5 (2) 9: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__ 10: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable 11: 0000000000601060 0 NOTYPE GLOBAL DEFAULT 24 _edata 12: 0000000000601050 0 NOTYPE GLOBAL DEFAULT 24 __data_start 13: 0000000000601068 0 NOTYPE GLOBAL DEFAULT 25 _end 14: 00000000004009d8 12 FUNC GLOBAL DEFAULT 14 dummy_function 15: 0000000000601050 0 NOTYPE WEAK DEFAULT 24 data_start 16: 0000000000400a80 4 OBJECT GLOBAL DEFAULT 16 _IO_stdin_used 17: 0000000000400a00 101 FUNC GLOBAL DEFAULT 14 __libc_csu_init 18: 0000000000400850 42 FUNC GLOBAL DEFAULT 14 _start 19: 0000000000601060 0 NOTYPE GLOBAL DEFAULT 25 __bss_start 20: 00000000004009e4 16 FUNC GLOBAL DEFAULT 14 main 21: 00000000004007a0 0 FUNC GLOBAL DEFAULT 11 _init 22: 0000000000400a70 2 FUNC GLOBAL DEFAULT 14 __libc_csu_fini 23: 0000000000400a74 0 FUNC GLOBAL DEFAULT 15 _fini 24: 0000000000400922 182 FUNC GLOBAL DEFAULT 14 print_traceI hope that helps!