[英]Why this simple Linux C program loading .so at runtime crashes?
I am trying to write the tiniest program loading a shared object (.so) of mine too at runtime.我正在尝试编写在运行时加载我的共享对象 (.so) 的最小程序。
Unfortunately, it hangs at runtime despite doing error checking :-(不幸的是,尽管进行了错误检查,它还是在运行时挂起 :-(
I am very interested in what I overlooked on source code level.我对我在源代码级别忽略的内容非常感兴趣。
The source code and my shell session running my program follows.源代码和运行我的程序的 shell 会话如下。
File "libsample.c" :文件“libsample.c”:
#include <stdio.h>
void sample_check(void)
{
printf("INFO: Function sample_check() called.\n");
}
File "test.c" :文件“test.c”:
#include <stdio.h>
#include <dlfcn.h>
typedef void (*sample_func_t) (void);
int main(void)
{
setbuf(stdout, NULL);
setbuf(stderr, NULL);
void* h_lib = dlopen("./libsample.so.1", RTLD_LAZY); // RTLD_LAZY || RTLD_NOW
if (! h_lib)
{
fprintf(stderr, "ERROR(%d): %s\n", __LINE__, dlerror());
return 1;
}
sample_func_t* symver = NULL;
dlerror();
symver = dlsym(h_lib, "sample_check");
char* reter = dlerror();
if (reter)
{
fprintf(stderr, "ERROR(%d): %s\n", __LINE__, reter);
return 1;
}
printf("INFO(%d): Resolved library sample_check() symbol at %p\n", __LINE__, symver);
printf("INFO(%d): About to call library sample_check() ...\n", __LINE__);
(*symver)();
printf("INFO(%d): sample_check() called !\n", __LINE__);
int retcl = dlclose(h_lib);
if (retcl)
{
fprintf(stderr, "ERROR(%d): %s\n", __LINE__, dlerror());
return 1;
}
return 0;
}
File "build" :文件“构建”:
#! /bin/bash
echo "Begin of compilation ..."
rm test test.o libsample.so.1 libsample.so.1.0.1 libsample.o 2>/dev/null
gcc -fpic -c -o libsample.o libsample.c || exit 1
gcc -shared -Wl,-soname,libsample.so.1 -o libsample.so.1.0.1 libsample.o || exit 1
ln -s libsample.so.1.0.1 libsample.so.1 || exit 1
gcc -c -o test.o test.c || exit 1
gcc -o test test.o -ldl || exit 1
echo "Compilation successful !"
My shell session log :我的 shell 会话日志:
valentin@valentin-SATELLITE-L875-10G:~/PROGRAMMING/C/Libraries/libsample$ valentin@valentin-SATELLITE-L875-10G:~/PROGRAMMING/C/Libraries/libsample$ ./build Begin of compilation ... Compilation successful ! valentin@valentin-SATELLITE-L875-10G:~/PROGRAMMING/C/Libraries/libsample$ ./test INFO(27): Resolved library sample_check() symbol at 0x7f5e96df86f0 INFO(28): About to call library sample_check() ... Erreur de segmentation valentin@valentin-SATELLITE-L875-10G:~/PROGRAMMING/C/Libraries/libsample$
Any idea ?任何的想法 ?
Here这里
(*symver)();
the code dereferences what had been received as entry point for library function to be run.代码取消引用作为要运行的库函数的入口点接收到的内容。 This resolves to a random address, which, when being called, typically crashes the program.
这会解析为一个随机地址,当被调用时,通常会导致程序崩溃。
To fix this define为了解决这个定义
sample_func_t symver = NULL;
where samle_func_t
already is a pointer type because of其中
samle_func_t
已经是一个指针类型,因为
typedef void (*sample_func_t) (void);
(Mind the *
.) (注意
*
。)
Then there are two possibilities to assign symver
:然后有两种分配
symver
可能性:
The "dirty" one “脏”的
symver = dlsym(h_lib, "sample_check");
"dirty" as the compiler might issue a warning like this: “脏”,因为编译器可能会发出这样的警告:
ISO C forbids assignment between function pointer and 'void *'
The "cleaner" one “清洁工”之一
*(void**)(&symver) = dlsym(h_lib, "sample_check");
And finally call the function like this:最后像这样调用函数:
symver(); /* No need to dereference here. */
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