Android Source: Is fopen() in bionic libc only used for system internal apps?

问题

I created an NDK application that does the following successfully:

#include <string.h>
#include <stdio.h>

#include <android/log.h>


JNIEXPORT jstring JNICALL Java_my_package_NativeAccess_stringFromJNI(
        JNIEnv* env, jobject thiz) {

    char* str = "Native Code!";

    FILE* file = fopen("sdcard/hello.txt","w+");
    if (file == NULL) {
        file = fopen("mnt/sdcard/hello.txt","w+");
        if (file == NULL) {
            file = fopen("storage/sdcard/hello.txt","w+"); 
        }
    }

    if (file == NULL) {
        str = "Native Code! fopen() did not work!";
        __android_log_write(ANDROID_LOG_ERROR, "AAAAAAAA", "file null!");
    }
    else{
        str = "Native Code! fopen() worked!";
        __android_log_write(ANDROID_LOG_ERROR, "AAAAAAAA", "file exists!");
        fputs("HELLO WORLD!\n", file);
        fflush(file);
        fclose(file);
    }


    return (*env)->NewStringUTF(env, str);
}

I start the app in a custom built source of me. It is the latest KitKat version. There I only changed

/bionic/libc/upstream-freebsd/lib/libc/stdio/fopen.c

in the following way:

FILE *
fopen(const char *file, const char *mode)
{


//====================

    fputs ("XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n", stdout);

    int uid = getuid();
    int pid = getpid();
    int ppid = getppid();

    fprintf(stdout, "uid = %d", uid);
    fputs ("\n", stdout);
    fprintf(stdout, "pid = %d", pid);
    fputs ("\n", stdout);
    fprintf(stdout, "ppid = %d", ppid);
    fputs ("\n", stdout);
    fputs ("XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n", stdout);

//====================



// ... the usual fopen continues here


}

When launching the emulator with the custom built Android source, I only get the following output via LogCat:

01-15 06:18:18.400: I/stdout(1417): XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
01-15 06:18:18.400: I/stdout(1417): uid = 1001
01-15 06:18:18.400: I/stdout(1417): pid = 1417
01-15 06:18:18.400: I/stdout(1417): ppid = 1217
01-15 06:18:18.400: I/stdout(1417): XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

when doing

adb shell ps

I get

USER      PID   PPID  VSIZE  RSS     WCHAN    PC         NAME
...
root      1217  1     205812 40176 ffffffff b6f395ec S zygote
...
radio     1417  1217  238984 25788 ffffffff b6f3a650 S com.android.phone
...
u0_a1     1697  1217  217744 20236 ffffffff b6f3a650 S com.android.providers.calendar

The uid 1001 (== USER u0_a1 ) is therefore not my app.

My app had the uid 10055:

$ adb shell dumpsys package my.package | grep userId=
    userId=10055 gids=[3003, 1028, 1015]

My question is: Why does my native call of fopen not get logged, but system internal calls do get logged?

Thanks!

回答1:

For system processes, stdout/stderr are not redirected to logcat, i.e. anything you send there is lost (goes to /dev/null technically), that's why you don't see anything.

来源：https://stackoverflow.com/questions/21136622/android-source-is-fopen-in-bionic-libc-only-used-for-system-internal-apps

标签

android-ndk

android-source

libc