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Editorial Team
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Asked: 2026-06-09T07:17:18+00:00

I am trying to just add print statements to my code to find errors,

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I am trying to just add print statements to my code to find errors, but cannot seem to get integers or bytes or jints or anything other than a String to show in LogCat. 

#define LOG_INFO( info ) __android_log_print( ANDROID_LOG_WARN, "NATIVE CODE", info );


LOG_INFO( "c = " + c );

The result for this is just 

c =

Does anyone know what I am doing wrong, or if this is possible? I have tried jints, ints, chars, jchars, and bytes ( as the type for c ).

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1 Answer

  1. Editorial Team

    The language is C. C does not have a builtin type for a string; strings are really arrays of char or pointers to char. You cannot add neither arrays nor pointers. So there’s no string concatenation by + operator, or by any other operator for that matter. C++ is different, but let’s not go there for now.

    The way __android_log_print() works is similar to Java’s String.format(). To output a character to LogCat, you use the following call:

    __android_log_print(ANDROID_LOG_WARN, "NATIVE CODE", "c=%c", (char)c);

    If you want to output several variables – say, a char and an int, the call would be:

    int n;
__android_log_print(ANDROID_LOG_WARN, "NATIVE CODE", "c=%c, n=%d", (char)c, n);

    __android_log_print is a variadic function – it takes a variable number of arguments (but no less than three), with unknown types beyond the first three. The types of the extra arguments are deduced from the format specifiers in the format string – the %-sequences. %c means char, %d means int, %s is char string and so forth, look it up.

    This is the printf pattern, used throughout the C world. It does not easily lend itself to macro-izing, unless you’re willing to introduce separate macros for outputting 0, 1, 2, etc. parameters. That way, it might look like this:

    #define LOG_INFO0( info ) __android_log_print( ANDROID_LOG_WARN, "NATIVE CODE", info );
#define LOG_INFO1( info, p1 ) __android_log_print( ANDROID_LOG_WARN, "NATIVE CODE", info, p1 );
#define LOG_INFO2( info, p1, p2 ) __android_log_print( ANDROID_LOG_WARN, "NATIVE CODE", info, p1, p2 );

//...
LOG_INFO("c=%c", (char)c);

    I’m deliberately avoiding the thorny issue of character type conversion here; for the %c format specifier, the function expects a parameter of type char which is not the same as jchar. For jchar’s that are non-ASCII characters, this snippet will print garbage.

    Just FYI: the correspondence between Java types and C types is not straightforward, and it’s actually somewhat platform-dependent. For the purposes of printing, you need to carefully watch. So:

    • jchar is not char; it’s unsigned short. Shortening to char loses data.
    • jlong is not long, it’s long long (64-bit integer)
    • jboolean is signed char (no boolean datatype in C)
    • jbyte is signed char (no byte datatype in C)
    • jint is int (on Android)
    • jfloat is float
    • jshort is short
    • jdouble is double
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