title: Undefined reference to crcsum(unsigned char const*, unsigned long, unsigned short)
body: <p>I am trying to implement a CRC16 in an arduino sketch. I got the crc.c file from online and I wanted to try it out. I created the other files to allow crc.c to run correctly. Here are my files.</p>
  
  <p>crc.h:</p>
  
  <pre><code>#ifndef CRC_C_
  #define CRC_C_
  
  unsigned short crcsum(const unsigned char* message, unsigned long length, unsigned short crc);
  int crcverify(const unsigned char* message, unsigned long length);
  void crcappend(unsigned char* message, unsigned long length);
  
  
  #endif // CRC_C_
  </code></pre>
  
  <p>crc.c:</p>
  
  <pre><code>/*
   * Derived from CRC algorithm for JTAG ICE mkII, published in Atmel
   * Appnote AVR067.  Converted from C++ to C.
   */
  
  #include "crc.h"
  
  /* CRC16 Definitions */
  static const unsigned short crc_table[256] = {
    0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
    0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
    0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
    0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
    0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
    0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
    0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
    0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
    0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
    0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
    0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
    0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
    0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
    0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
    0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
    0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
    0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
    0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
    0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
    0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
    0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
    0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
    0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
    0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
    0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
    0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
    0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
    0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
    0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
    0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
    0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
    0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
   };
  
  /* CRC calculation macros */
  #define CRC_INIT 0xFFFF
  #define CRC(crcval,newchar) crcval = (crcval &gt;&gt; 8) ^ \
      crc_table[(crcval ^ newchar) &amp; 0x00ff]
  
  unsigned short crcsum(const unsigned char* message, unsigned long length,
     unsigned short crc)
  {
    unsigned long i;
  
    for(i = 0; i &lt; length; i++)
      {
    CRC(crc, message[i]);
  }
     return crc;
  }
  
  int crcverify(const unsigned char* message, unsigned long length)
  {
     /*
     * Returns true if the last two bytes in a message is the crc of the
     * preceding bytes.
     */
    unsigned short expected;
  
    expected = crcsum(message, length - 2, CRC_INIT);
     return (expected &amp; 0xff) == message[length - 2] &amp;&amp;
      ((expected &gt;&gt; 8) &amp; 0xff) == message[length - 1];
  }
  
  void crcappend(unsigned char* message, unsigned long length)
  {
    unsigned long crc;
  
    crc = crcsum(message, length, CRC_INIT);
    message[length] = (unsigned char)(crc &amp; 0xff);
    message[length+1] = (unsigned char)((crc &gt;&gt; 8) &amp; 0xff);
  }
  </code></pre>
  
  <p>I call the function "crcsum" in the following file.</p>
  
  <p>crc.ino:</p>
  
  <pre><code>#include "crc.h"
  
  void setup() {
  
  crcsum((const unsigned char*)"0123456789", 50000, 4);  
  
  
  }
  
  void loop() {
    // put your main code here, to run repeatedly:
  
  }
  </code></pre>
  
  <p>The problem is that I get the following error.</p>
  
  <pre><code>crc.cpp.o: In function `setup':
  /Users/SuperDuperBro/crc.ino:10: undefined reference to `crcsum(unsigned char const*, unsigned long, unsigned short)'
  collect2: error: ld returned 1 exit status
  Error compiling.
  </code></pre>
  
  <p>I know that my header file and functions are operating correctly because when I comment out the function call in crc.ino, everything compiles just fine.</p>
  
  <p>Why am I getting this error? I have looked online and have been trying to fix it for a while, but I haven't had any luck. My guess is that the problem has to be something with the function parameters. I tried adjusting the parameters, but to no avail. Any input is greatly appreciated. Thank you all in advance!</p>