Android NDK: undefined reference to 'stderr'

Question

I want to use the ASI SDK (prebuilt binary) in an Android application. I am using Android Studio 2.1.3 on Windows 10 with the gradle "experimental plugin" and Android NDK r12b.

My JNI test method that calls a basic SDK function looks as follows:

#include <jni.h>
#include <stdio.h>
#include <ASICamera2.h>

JNIEXPORT jstring JNICALL
Java_at_wana_androguide_MainActivity_getMsgFromJni(JNIEnv *env, jobject instance)
{
   char str[512];

   int numCams = ASIGetNumOfConnectedCameras();
   snprintf(str, sizeof(str), "Connected cameras: %d", numCams);

   return env->NewStringUTF(str);
}

The build.gradle for the app module looks like this:

apply plugin: 'com.android.model.application'

model {
    android {
        compileSdkVersion 24
        buildToolsVersion "24.0.2"

        defaultConfig {
            applicationId "at.wana.androguide"
            minSdkVersion.apiLevel 19
            targetSdkVersion.apiLevel 24
            versionCode 1
            versionName "0.1"
        }
        buildTypes {
            release {
                minifyEnabled false
                proguardFiles.add(file('proguard-android.txt'))
            }
        }
        ndk {
            moduleName "hello-android-jni"
            abiFilters.add("arm64-v8a")
            abiFilters.add("x86")
            stl "gnustl_static"
        }
        sources {
            main {
                jni {
                    dependencies {
                        library "ASISDK" linkage "static"
                        project ":usb" linkage "shared"
                    }
                }
            }
        }
    }
    repositories {
        libs(PrebuiltLibraries) {
            ASISDK {
                headers.srcDir "src/main/jni/drivers/ASI/include"
                binaries.withType(StaticLibraryBinary) {
                    staticLibraryFile = file("src/main/jni/drivers/ASI/lib/${targetPlatform.getName()}/libASICamera2.a")
                }
            }
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:24.2.0'
    compile 'com.squareup:otto:1.3.8'
}

Since the ASI SDK depends on libusb, I also added the libusb sources as a separate module and imported is as a dependency into the app module. This is the build.gradle file for the libusb module (called "usb"):

apply plugin: "com.android.model.native"

model {
    android {
        compileSdkVersion 24
        buildToolsVersion "24.0.2"

        ndk {
            moduleName "usb"

            CFlags.add("-I${file("src/main/jni")}".toString())
            CFlags.add("-I${file("src/main/jni/os")}".toString())

            ldLibs.addAll(["log"])
        }
    }
}

Problem:

This code builds OK, but when I run it in Genymotion (x86), Android 5, it crashes with a runtime exception:

java.lang.UnsatisfiedLinkError: dlopen failed: cannot locate symbol "stderr" referenced by "libhello-android-jni.so"..

I tried adding supc++ and/or stdc++ to ldFlags, I tried changing the filename extension to cpp and back to c, and I already tried setting the platformVersion to 19 or even 9 in the build.gradle files, but this only resulted in a compile time error saying that stderr is not defined:

C:\Users\greuff\Documents\devel\AndroGuide\app\src\main\jni\drivers\ASI\lib\x86\libASICamera2.a(ASICamera2.o):ASICamera2.cpp:function ASIGetNumOfConnectedCameras: error: undefined reference to 'stderr' collect2.exe: error: ld returned 1 exit status

Curiously, this error only occurs for the x86 platform target.

I read that stderr etc were removed from Android NDK Headers at a certain point in time because they became real functions in libc, but I'm not quite sure how to solve this problem since setting platformVersion to a lower value (as suggested in some questions) did not help. I would be very grateful for pointers in the right direction.

Answer 1

Turns out I was chasing the wrong thing. The ASI SDK was statically compiled against the glibc which contains stderr as globally available symbols, whereas bionic (the Android libc) doesn't contain them.

All I had to do was to undefine stderr and created it myself as a linker symbol, so the linker can find it:

#include <jni.h>
#include <stdio.h>
#include <ASICamera2.h>

#undef stderr
FILE *stderr = &__sF[2];

...