Using libfplutil_print

libfplutil_print implements a set of wrappers for functions that write to the standard output stream. This enables applications that write to the terminal using printf to have their output redirected to the Android log.

Using both libfplutil_main and libfplutil_print makes it possible to compile and run non-interactive command line applications on Android devices.

It's possible to use libfplutil_print without libfplutil_main in conjunction with NativeActivity by implementing an android_main entry point or using printf in libraries that are called from Java modules. However, the typical use case is to use libfplutil_print in conjunction with libfplutil_main so that C/C++ applications can easily be cross compiled to multiple platforms including Android.

For example, the canonical C "Hello World" application will compile and run on Android simply by linking with both libfplutil_print and libfplutil_main:

#include <stdio.h>

int main(int argc, char *argv[]) {
  printf("Hello World\n");
  return 0;
}

The above application, when compiled and linked against libfplutil_print and libfplutil_main (see Linking) will print "Hello World" to the Android log which can be viewed using adb's logcat command.

Customizing the Log Tag

By default, the tag used to annontate log messages is "main", this can be modified using the SetAndroidLogWrapperTag() function. For example:

#include "fplutil/print.h"
#include "fplutil/main.h"

int main(int argc, char **argv) {
  SetAndroidLogWrapperTag("my_application");
  printf("Print to the log from printf\n");
  std::cout << "Print to the log from a stream" << std::endl;
  return 0;
}

Prints the following to Android's log:

 Print to the log from printf
 Print to the log from a stream

All log messages from the application will be prefixed with the tag, "my_application" so they can optionally extracted from adb logcat using

adb logcat -s "my_application"

In addition to changing the tag, it's possible to modify the log priority SetAndroidLogWrapperPriority().

Configuring Buffering

Each Android log message is terminated with a newline, so in order to simulate printf behavior where newlines are explicit, libfplutil_print will buffer messages up to 256 bytes before writing a line to the log. The size of this buffer can be modified using SetAndroidLogWrapperBufferSize().

For example, after increasing the buffer size to 1KB, the following will print a line longer than 256 bytes to a single log message:

#include "fplutil/print.h"
#include "fplutil/main.h"

int main(int argc, char **argv) {
  SetAndroidLogWrapperBufferSize(1024);
  for (i = 0; i < 256; i++) {
    printf("*");
  }
  printf("A long line\n");
  return 0;
}

Redirecting Output

libfplutil_print captures all output to the standard output stream and by default sends it to the Android log. Applications can capture data sent to the standard output stream and perform further processing before sending it to the log. This can be useful when logging to files or annotating log messages with custom logic.

For example:

#include <stdarg.h>
#include "fplutil/print.h"
#include "fplutil/main.h"

int CaptureLog(int priority, const char *tag, const char *format,
               va_list list) {
  // Do something with the message.
  return 0;
}

int main(int argc, char **argv) {
  SetAndroidStdioOutputFunction(CaptureLog);
  printf("A message to capture\n");
  return 0;
}