1. Error reporting:

The error of Android C + + OpenSSL link is as follows:

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(armcap.o):armcap.c:function OPENSSL_cpuid_setup: error: undefined reference to 'sigfillset'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(armcap.o):armcap.c:function OPENSSL_cpuid_setup: error: undefined reference to 'sigdelset'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(armcap.o):armcap.c:function OPENSSL_cpuid_setup: error: undefined reference to 'sigdelset'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(armcap.o):armcap.c:function OPENSSL_cpuid_setup: error: undefined reference to 'sigdelset'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(armcap.o):armcap.c:function OPENSSL_cpuid_setup: error: undefined reference to 'sigdelset'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(ui_openssl.o):ui_openssl.c:function open_console: error: undefined reference to 'tcgetattr'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(ui_openssl.o):ui_openssl.c:function read_string_inner: error: undefined reference to 'signal'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(ui_openssl.o):ui_openssl.c:function read_string_inner: error: undefined reference to 'tcsetattr'

I:/webrtc/android/openssl-1.1.1k/output-armeabi-v7a/lib/libcrypto.a(ui_openssl.o):ui_openssl.c:function read_string_inner: error: undefined reference to 'tcsetattr'

clang++: error: linker command failed with exit code 1 (use -v to see invocation)

2. Reason:

Sigdelset, sigfillset and signal cannot be found in the SDK of Android

3. Solution:

Modify the minSdkVersion of build.grandle to a version number after 21:

apply plugin: 'com.android.application'
def LIBWEBRTC_HOME_PATH = "I:/webrtc/android/webrtc_m84_20201001/webrtc_android/src/"
//def LIBWEBRTC_HOME_PATH = "I:/webrtc/android/androidnativeapi/app/webrtc/"
android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "org.webrtc.examples.androidnativeapi"
        minSdkVersion 21
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                arguments "-DLIBWEBRTC_HOME_PATH=" + LIBWEBRTC_HOME_PATH,
                        "-DANDROID_STL=c++_static"
            }
        }
        ndk {
            abiFilters  "armeabi-v7a"
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    sourceSets {
        main {
            // 1. configure the root directory libs to load third-party so libraries, (it is best not to create jniLibs, in the many open source libraries may cause conflicts, not yet found)
            // 2. automatically copy the so libraries in the libs directory to the specified directory when running
            // 3. If you don't need to recompile the so you created, you can copy the so generated by (app/build/intermediates/transforms) to this directory
            jniLibs.srcDirs = ['libs']
            // If it is a single folder, you can directly configure it as follows
            // jniLibs.srcDir 'libs'
        }
    }
    buildToolsVersion '28.0.2'//ADD
}
repositories {
    flatDir{
        dirs'libs'
    }
}

dependencies {
    implementation fileTree(dir: "libs", include: ["*.aar"])

    //implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation 'com.android.support:appcompat-v7:28.0.0-rc02'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    //implementation(name: 'libwebrtc', ext: 'aar')
    //implementation 'org.webrtc:google-webrtc:1.0.+'
}