dlsym函数返回null的解决办法

dlsym是什么？

dlsym是一个计算机函数，功能是根据动态链接库操作句柄与符号，返回符号对应的地址，不但可以获取函数地址，也可以获取变量地址

见：采用dlopen、dlsym、dlclose加载动态链接库【总结】

出现问题

给Android添加system_service的代码时，在HAL层出现问题；

源代码

/* Get the address of the struct hal_module_info. */ const char *sym = HAL_MODULE_INFO_SYM_AS_STR; hmi = (struct hw_module_t *)dlsym(handle, sym); if (hmi == NULL) { ALOGE("load: couldn't find symbol %s", sym); status = -EINVAL; goto done; }

错误日志打印：

2021-06-14 21:23:58.084 1828-1828/system_process I/HAL: 1.1 hardware load() path:/system/lib64/hw/freg.default.so 2021-06-14 21:23:58.084 1828-1828/system_process I/HAL: 2 hardware load() 2021-06-14 21:23:58.084 1828-1828/system_process E/HAL: load: couldn't find symbol HMI

结果表明，库“freg.default.s”没有HMI符号。

排查步骤

（1）首先使用grep查看“.os”是否有HMI，如下：

gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$ ls  audio.a2dp.default.so camera.default.so camera.usb.default.so freg.default.so input.evdev.default.so gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$ grep -rn HMI Binary file input.evdev.default.so matches Binary file camera.default.so matches Binary file audio.a2dp.default.so matches Binary file camera.usb.default.so matches gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$

上面的结果现实，“freg.default.s”没有HMI符号。

（2）使用查看符号表命令；

查看符号表的命令：linux 下查看 so库 符号表

gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$ nm -D freg.default.so U __android_log_print 0000000000002008 A __bss_start U close U __cxa_atexit U __cxa_finalize 0000000000002008 A _edata 0000000000002008 A _end U __errno U free U malloc U __open_2 U read U __register_atfork U __stack_chk_fail U strcmp U strerror U __write_chk gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$ gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$ nm -D input.evdev.default.so U __android_log_assert U __android_log_print 0000000000004118 A __bss_start U __cxa_atexit U __cxa_finalize 0000000000004118 A _edata 0000000000004120 A _end 0000000000004010 D HMI U memcpy U pthread_create U pthread_setspecific U __register_atfork U __stack_chk_fail U strcmp U _ZdlPv 0000000000001530 T _ZN7android11EvdevModule12notifyReportEP12input_report 0000000000001410 T _ZN7android11EvdevModule4initEv 0000000000001510 T _ZN7android11EvdevModule4loopEv 0000000000001300 T _ZN7android11EvdevModuleC1EPNS_18InputHostInterfaceE 0000000000001300 T _ZN7android11EvdevModuleC2EPNS_18InputHostInterfaceE U _ZN7android8InputHubC1ERKNSt3__110shared_ptrINS_22InputCallbackInterfaceEEE U _ZNKSt3__119__shared_weak_count13__get_deleterERKSt9type_info 0000000000001840 W _ZNSt3__114__thread_proxyINS_5tupleIJNS_10unique_ptrINS_15__thread_structENS_14default_deleteIS3_EEEEMN7android11EvdevModuleEFvvEPS8_EEEEEPvSD_ U _ZNSt3__115__thread_structC1Ev U _ZNSt3__115__thread_structD1Ev U _ZNSt3__119__shared_weak_count14__release_weakEv U _ZNSt3__119__shared_weak_countD2Ev U _ZNSt3__119__thread_local_dataEv 0000000000001750 W _ZNSt3__120__shared_ptr_emplaceIN7android18InputDeviceManagerENS_9allocatorIS2_EEE16__on_zero_sharedEv 0000000000001760 W _ZNSt3__120__shared_ptr_emplaceIN7android18InputDeviceManagerENS_9allocatorIS2_EEE21__on_zero_shared_weakEv U _ZNSt3__120__throw_system_errorEiPKc U _ZNSt3__16threadD1Ev U _Znwm U _ZSt9terminatev U _ZTVN7android18InputDeviceManagerE U _ZTVN7android9InputHostE 0000000000003c60 V _ZTVNSt3__120__shared_ptr_emplaceIN7android18InputDeviceManagerENS_9allocatorIS2_EEEE gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$

（3）查看代码；

查看代码，对比了 hardware/libhardware/modules/ 目录下的其他模块的 Android.bp 文件，发现多了个  “-fvisibility=hidden”,  如下面所示，删除掉这句。

这是从其他的文件中拷贝过来的，因此引入了这个问题。

 cflags: [ "-Wall", "-Wextra", "-Werror", "-fvisibility=hidden", ],

（4）验证问题，编译后，重新查看符号表。如下：

gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$ nm -D freg.default.so U __android_log_print 0000000000003108 A __bss_start U close U __cxa_atexit U __cxa_finalize 0000000000003108 A _edata 000000000000310c A _end U __errno U free 0000000000003010 D HMI U malloc U __open_2 U read U __register_atfork U __stack_chk_fail U strcmp U strerror U __write_chk gs@aigo:~/nvme_1T/android9.0.0_35/out/target/product/generic_x86_64/system/lib64/hw$ 

可以看到，有 “HMI” 了，至此，问题解决。