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#include <stdio.h> | |
#include <stdlib.h> | |
#include <string.h> | |
#include <dlfcn.h> | |
int main(int argc, char** argv) | |
{ | |
void *handle; | |
void (*func_print_name)(const char*); | |
if (argc != 2) { | |
fprintf(stderr, "Usage: %s animal_type\n", argv[0]); | |
return EXIT_FAILURE; | |
} | |
if (strcmp(argv[1], "dog") == 0) { | |
handle = dlopen("./libdog.so", RTLD_LAZY); | |
} else if (strcmp(argv[1], "cat") == 0) { | |
handle = dlopen("./libcat.so", RTLD_LAZY); | |
} else { | |
fprintf(stderr, "Error: unknown animal type: %s\n", argv[1]); | |
return EXIT_FAILURE; | |
} | |
if (!handle) { | |
/* fail to load the library */ | |
fprintf(stderr, "Error: %s\n", dlerror()); | |
return EXIT_FAILURE; | |
} | |
*(void**)(&func_print_name) = dlsym(handle, "print_name"); | |
if (!func_print_name) { | |
/* no such symbol */ | |
fprintf(stderr, "Error: %s\n", dlerror()); | |
dlclose(handle); | |
return EXIT_FAILURE; | |
} | |
func_print_name(argv[1]); | |
dlclose(handle); | |
return EXIT_SUCCESS; | |
} |
#pragma once | |
void print_name(const char* type); |
#include "animal.h" | |
#include <stdio.h> | |
void print_name(const char* type) | |
{ | |
printf("Tama is a %s.\n", type); | |
} |
#include "animal.h" | |
#include <stdio.h> | |
void print_name(const char* type) | |
{ | |
printf("Pochi is a %s.\n", type); | |
} |
app = dlopen_sample | |
lib = libcat.so libdog.so | |
CFLAGS = -Wall -ansi -pedantic | |
LDFLAGS = -ldl | |
all: $(app) $(lib) | |
lib%.so: %.c | |
$(CC) -shared -fPIC $(CFLAGS) -o $@ $< | |
clean: | |
$(RM) $(app) $(lib) | |
run: all | |
./$(app) cat | |
./$(app) dog | |
-./$(app) bear | |
$(lib): animal.h |
@cirosantili I like yours betters (not that this one is bad!)
Awesome! BTW, Tama and Pochi reminds me of an anime. Something like Death march kara hajimaru.
I am not able to compile the code. I am getting error. Please help.
[s]$ gcc dlopen_sample.c -o a
/tmp/cc3t29je.o: In function main': dlopen_sample.c:(.text+0x67): undefined reference to
dlopen'
dlopen_sample.c:(.text+0x98): undefined reference to dlopen' dlopen_sample.c:(.text+0xd8): undefined reference to
dlerror'
dlopen_sample.c:(.text+0x110): undefined reference to dlsym' dlopen_sample.c:(.text+0x121): undefined reference to
dlerror'
dlopen_sample.c:(.text+0x149): undefined reference to dlclose' dlopen_sample.c:(.text+0x170): undefined reference to
dlclose'
collect2: error: ld returned 1 exit status
[s]$
@apurba-101010 You need to link libdl.so, so add -ldl
(see LDFLAGS in my Makefile).
default LDFLAGS with wrong position, add code in Makefile like this:
dlopen_sample: dlopen_sample.c
Why need (void*)(&func_print_name) = dlsym(handle, "print_name");
When can use func_print_name = (void()(const char))(dlsym(handle, "print_name")); ?
Sorry for my English.
Why need (void*)(&func_print_name) = dlsym(handle, "print_name"); When can use func_print_name = (void()(const char))(dlsym(handle, "print_name")); ?
Sorry for my English.
@TKNgu From the man https://man7.org/linux/man-pages/man3/dlopen.3.html emphasis mine:
cosine = (double (*)(double)) dlsym(handle, "cos");
/* According to the ISO C standard, casting between function
pointers and 'void *', as done above, produces undefined results.
POSIX.1-2001 and POSIX.1-2008 accepted this state of affairs and
proposed the following workaround:
*(void **) (&cosine) = dlsym(handle, "cos");
This (clumsy) cast conforms with the ISO C standard and will
avoid any compiler warnings.
The 2013 Technical Corrigendum 1 to POSIX.1-2008 improved matters
by requiring that conforming implementations support casting
'void *' to a function pointer. Nevertheless, some compilers
(e.g., gcc with the '-pedantic' option) may complain about the
cast used in this program. */
Now you know a thing or two.
Thx for your example but just there is an addition; you can add the lines below to your Makefile to resolve the "undefined reference" error:
$(app):
$(CC) $(app).c -fPIC $(CFLAGS) $(LDFLAGS) -o $@ $<
Here is another minimal runnable example: https://github.com/cirosantilli/cpp-cheat/blob/81ab30c55634db24216e89ada1e0f271cac074e7/shared-library/basic/dlopen.c