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POTrouble with C unit conversion
 

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  1. POTrouble with C unit conversion
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    <p>I am currently developing an <code>Android</code> application which uses a <code>native C library.</code> I am very unfamiliar with C, and am having some conversion problems. I know that the problem has something to do with signed/unsigned numbers, but cannot figure out what I need to do to fix this.</p> <p>The C library works fine on its own, and produces the correct results. However when I port it back to Android, only the positive values are correct. The negative values are returned as 256 minus the value. </p> <p>For example:</p> <pre><code> 8 -&gt; 8 5 -&gt; 5 1 -&gt; 1 0 -&gt; 0 -3 -&gt; 253 -5 -&gt; 251 -8 -&gt; 248 </code></pre> <p>This is what the C code looks like when it assigns the value:</p> <pre><code>BYTE *parms; WORD __stdcall GetValue(BYTE what, long *val){ char myStringParmsOne[255]; sprintf( myStringParmsOne, "parms[1]=%d", parms[1] ); LOG_INFO( myStringParmsOne ); char myStringIsG[255]; sprintf( myStringIsG, "API_isG=%d",API_isG ); LOG_INFO( myStringIsG ); char myStringCharParmsB[255]; sprintf( myStringCharParmsB, "(char)parms[1]=%d", (char)parms[1] ); LOG_INFO( myStringCharParmsB ); *val=(char)parms[1]-(API_isG?11:0); } </code></pre> <p>The <code>LOG_INFO</code> statements print the values to <code>LogCat</code>. Below I have values when the program is run and a value of <code>-3</code> is expected.</p> <pre><code>The first **parms[1]=%d** statement returns **parms[1]=253** The second **API_isG=%d** statement returns **API_isG=0** The third **(char)parms[1]=%d** statement returns **(char)parms[1]=253** </code></pre> <p>The BYTE and WORD types are defined as follows:</p> <pre><code>typedef unsigned char BYTE; typedef unsigned short WORD; </code></pre> <p>The wrapper function I use to send this value back to Java is shown below:</p> <pre><code>long Java_my_package_Model_00024NativeCalls_GetValueJava( JNIEnv *env, jobject obj, WORD what) { long c; int j = GetValue( what, &amp;c ); if( j == 0 ) { return c; } else { return -1; } } </code></pre> <p><strong>EDIT</strong> </p> <p>The <code>GetValue</code> method is used all the time, but only this individual call gives me problems with negative numbers, it works with other calls. I have tried the two following things for my wrapper function:</p> <pre><code>long Java_my_package_Model_00024NativeCalls_GetValueJava( JNIEnv *env, jobject obj, WORD what) { long c; int j = GetValue( what, &amp;c ); if( j == 0 ) { return (signed char)c; } else { return -1; } } </code></pre> <p>The casting of the return value above provides me with a proper return value. However, all of the other calls to this function for other values are completely haywire. </p> <pre><code>long Java_my_package_Model_00024NativeCalls_GetValueJava( JNIEnv *env, jobject obj, WORD what) { long c; int j = GetValue( what, &amp;c ); if( j == 0 ) { return (signed long)c; } else { return -1; } } </code></pre> <p>The above casting of the return value changed nothing.</p>
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    1. COThat's because `BYTE` is unsigned, and the hardware uses two's compliment representation of negatives.
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    2. COCan you tell for which hardware architecture our are building your application for?
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    3. CO@mathk It is custom, nothing that is available on the open market.
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