问题描述
-rdynamic(或在链接器级别的--export-dynamic)到底是做什么的，它与-fvisibility*标志或可见性pragma s和__attribute__ s定义的符号可见性有什么关系? >
对于--export-dynamic， ld(1)提到:
我不确定我是否完全理解这一点.您能否提供一个没有-rdynamic不能使用但可以使用的示例?
修改:我实际上尝试过使用-rdynamic和不使用-rdynamic编译几个虚拟库(单个文件，多个文件，各种-O级别，一些函数间调用，一些隐藏符号，一些可见). (在保持所有其他标志不变的情况下)获得与字节相同的输出，这非常令人困惑.
下面是一个简单的示例项目，用于说明-rdynamic的用法.
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;
}
制作文件
.PHONY: all clean test
LDEXTRAFLAGS ?=
all: prog
bar.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 $@ $< -L. -lbar -ldl
clean:
rm -f *.o *.so prog
test: prog
./$<
在这里，bar.c成为共享库libbar.so，而main.c成为共享库一个dlopen s libbar并从该库中调用bar()的程序.bar()调用foo()，它在bar.c中是外部的，并且是在main.c中定义的.
因此，没有-rdynamic:
$ make test
gcc -c -Wall -o main.o main.c
gcc -c -Wall -fpic -o bar.o bar.c
gcc -shared -o libbar.so bar.o
gcc -o prog main.o -L. -lbar -ldl
./prog
./libbar.so: undefined symbol: foo
Makefile:23: recipe for target 'test' failed
并使用-rdynamic:
$ make clean
rm -f *.o *.so prog
$ make test LDEXTRAFLAGS=-rdynamic
gcc -c -Wall -o main.o main.c
gcc -c -Wall -fpic -o bar.o bar.c
gcc -shared -o libbar.so bar.o
gcc -rdynamic -o prog main.o -L. -lbar -ldl
./prog
Hello world
What exactly does -rdynamic (or --export-dynamic at the linker level) do and how does it relate to symbol visibility as defined by the -fvisibility* flags or visibility pragmas and __attribute__s?
For --export-dynamic, ld(1) mentions:
I'm not sure I completely understand this. Could you please provide an example that doesn't work without -rdynamic but does with it?
Edit:I actually tried compiling a couple of dummy libraries (single file, multi-file, various -O levels, some inter-function calls, some hidden symbols, some visible), with and without -rdynamic, and so far I've been getting byte-identical outputs (when keeping all other flags constant of course), which is quite puzzling.
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 test
LDEXTRAFLAGS ?=
all: prog
bar.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 $@ $< -L. -lbar -ldl
clean:
rm -f *.o *.so prog
test: prog
./$<
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 test
gcc -c -Wall -o main.o main.c
gcc -c -Wall -fpic -o bar.o bar.c
gcc -shared -o libbar.so bar.o
gcc -o prog main.o -L. -lbar -ldl
./prog
./libbar.so: undefined symbol: foo
Makefile:23: recipe for target 'test' failed
And with -rdynamic:
$ make clean
rm -f *.o *.so prog
$ make test LDEXTRAFLAGS=-rdynamic
gcc -c -Wall -o main.o main.c
gcc -c -Wall -fpic -o bar.o bar.c
gcc -shared -o libbar.so bar.o
gcc -rdynamic -o prog main.o -L. -lbar -ldl
./prog
Hello world
这篇关于-rdynamic`的功能是什么?何时需要?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！