What does -fPIC Mean when compiling a shared library

Question

I am trying to create a shared library in linux I get the following error when trying to compile this :

relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC

If i remove the contents of GetCrc16 it Works. What am I doing wrong ?

/*
 * main.c
 *
 *  Created on: Jul 3, 2012
 *      Author: opc0de
 */


#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <resolv.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <pthread.h>
#include <errno.h>
#include <unistd.h>
#include <syslog.h>
#include <string.h>
#include <mysql/mysql.h>
#include <arpa/inet.h>
#include <curl/curl.h>

typedef struct param
{
    int * csock;
    char * IMEI;
}param;

#define DAEMON_NAME "MEGA_TRACKER"


static const unsigned short crctab16[] =
{
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,
};

unsigned char response[] = { 0x78, 0x78, 0x05 , 0x00 ,0x00, 0x00, 0x00, 0x00,0x0D, 0x0A};
char Hexars[] = {'0', '1', '2', '3', '4', '5','6', '7', '8', '9', 'a', 'b','c', 'd', 'e', 'f' };



unsigned short GetCrc16(const unsigned char* pData, int nLength)
{
unsigned short fcs = 0xffff; // initialize
while(nLength>0){
fcs = (fcs >> 8) ^ crctab16[(fcs ^ *pData) & 0xff];
nLength--;
pData++;
}
return ~fcs;
}

"... can not be used when making a shared object; **recompile with -fPIC**" — Mat, Jul 03 '12 at 10:54
@Mat could you be more specific I am new to linux.I will try recompiling with -fPIC but why ? — opc0de, Jul 03 '12 at 10:55

score 4 · Accepted Answer · edited May 23 '17 at 11:48

-fPIC instructs the compiler to compile your code as Position Independent Code. This is needed so that the machine code executed properly regardless of the absolute address.

These are needed mainly for shared libraries, as the same library code can be loaded in a location in each program's address space where it will not overlap with other uses of memory etc.

Following links can provide more info.

Wikipedia: http://en.wikipedia.org/wiki/Position_independent_code
SO: Why isn't all code compiled position independent?

score 1 · Answer 2 · answered Jul 03 '12 at 10:59

1

The command line option -fPIC means make position independent code. This is needed when compiling shared objects (.so) for 64bit. You were able to get away without using it on 32bit; even though it was recommended.

answered Jul 03 '12 at 10:59

Anya Shenanigans

91,618
3
107
122

What does -fPIC Mean when compiling a shared library

2 Answers2