undefined reference to `main' in C

问题

Hi I am getting below error while compiling a c code using gcc

/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../lib64/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status

I am trying to import the fftw() function into SystemVerilog. Here is my code

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <math.h>
#include <fftw3.h> 

void fftw(double FFT_in[],int size)
{

    double *IFFT_out;
    int i;

    fftw_complex *middle;

    fftw_plan fft;
    fftw_plan ifft;
    middle = (fftw_complex*) fftw_malloc(sizeof(fftw_complex)*size);
    IFFT_out = (double *) malloc(size*sizeof(double));

    fft = fftw_plan_dft_r2c_1d(size, FFT_in, middle, FFTW_ESTIMATE);  //Setup fftw plan for fft (real 1D data)
    ifft = fftw_plan_dft_c2r_1d(size, middle, IFFT_out, FFTW_ESTIMATE);   //Setup fftw plan for ifft

    fftw_execute(fft);
    fftw_execute(ifft);

    printf("Input:    \tFFT_coefficient[i][0]      \tFFT_coefficient[i][1]   \tRecovered Output:\n");

    for(i=0;i<size;i++)
        printf("%f\t%f\t\t\t%f\t\t\t%f\n",FFT_in[i],middle[i][0],middle[i][1],IFFT_out[i]/size);

    fftw_destroy_plan(fft);
    fftw_destroy_plan(ifft);
    fftw_free(middle);
    free(IFFT_out);

    //return IFFT_out;
}

Here is a system Verilog code from where I am trying to call fftw

module top;
import "DPI-C" function void fftw(real FFT_in[0:11], int size);
real j [0:11];
integer i,size;
real FFT_in [0:11]; 

initial begin
    size = 12;
    FFT_in[0] = 0.1;
     FFT_in[1] = 0.6;
     FFT_in[2] = 0.1;
     FFT_in[3] = 0.4;
     FFT_in[4] = 0.5;
     FFT_in[5] = 0.0;
     FFT_in[6] = 0.8;
     FFT_in[7] = 0.7;
     FFT_in[8] = 0.8;
     FFT_in[9] = 0.6;
     FFT_in[10] = 0.1;
     FFT_in[11] = 0.0;

    $display("Entering in SystemVerilog Initial Block\n");
    #20
     fftw(FFT_in,size);

    $display("Printing recovered output from system verilog\n"); 
    //for(i=0;i<size;i++)
        //$display("%f\t\n",(j[i])/size);

    $display("Exiting from SystemVerilog Initial Block");
    #5 $finish;

end

endmodule

Here is an irun command to compile both systemverilg and C files

# Compile the SystemVerilog files
fftw_test.sv 
-access +rwc
# Generate a header file called _sv_export.h
-dpiheader _sv_export.h
# Delay compilation of fftw_test.c until after elaboration
#-cpost fftw_test_DPI.c -end
-I/home/fftw/local/include -L/home/ss69/fftw/local/lib fftw_test_DPI.c -lfftw3 -lm 
# Redirect output of ncsc_run to a log file called ncsc_run.log
-log_ncsc_run ncsc_run.log

while running this command give below error: building library run.so ld: /home/fftw/local/lib/libfftw3.a(mapflags.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC /homefftw/local/lib/libfftw3.a: could not read symbols: Bad value collect2: ld returned 1 exit status make: * [/home/ss69/DPI/./INCA_libs/irun.lnx8664.12.20.nc/librun.so] Error 1 ncsc_run: *E,TBBLDF: Failed to build test library /home/DPI/./INCA_libs/irun.lnx8664.12.20.nc/librun.so

irun: *E,CCERR: Error during cc compilation (status 1), exiting.

When I simply try to compile C using gcc with below command:
gcc -g -Wall -Werror -I/home/fftw/local/include -L/home/ss69/fftw/local/lib \ fftw_test_DPI.c -lfftw3 -lm -o fftw_test_DPI

I get this error:

/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../lib64/crt1.o: In function _start': (.text+0x20): undefined reference tomain' collect2: ld returned 1 exit status

回答1:

Exactly how are you using the function void fftw(double FFT_in[],int size) from your comments it sounds like you are coding routine that is called as DLL or as part of a static library.

If this is the case then adding main() isn't going to help, at all.

What you have written is ABSOLUTELY 100% OK, if it is to be used as a callable routine.

What you might need to do is compile this routine into a library, even a static lib. is probably ok. If this is the case then consult your GCC documentation on how to create a static or dynamic lib.

Finally, I have written Verilog code myself, so you can also provide any lines or references to Verilog documentation that you have read and whose instructions you are following. I assume that at some point you are invoking Verilog and supplying it with a list of libraries it can/should use. Your lib should be included in that list.

Am including comments from jxh per his request: To import the function into SystemVerilog, you need to compile your function into a shared object. Then, you would point SystemVerilog at the shared object. (I don't use SystemVerilog, but that is what I gather from its web page.)

gcc -shared -fPIC -g -Wall -Werror \
-I/home/ss69/fftw/local/include -L/home/ss69/fftw/local/lib \
fftw_test_DPI.c -lfftw3 -lm -o libfftw_test_DPI.so

回答2:

• Your are missing #include "svdpi.h" in the fftwc.c file (or maybe you are not showing it because it is in fftwc.h). This include is needed for DPI.
• You are compiling a DPI library to be used with a SystemVerilog simulator. Therefore, you do not need a main() method.
• I prefer to always compile all DPI methods outside of the SystemVerilog compiler. The include the DPI library to the simulation phase. My flow looks something like the following:
  

${SVTOOL} -compile -f svfiles.f -dpi_header gen_dpi_header.h
gcc -fPIC -pipe -O2 -c -g \
    -I${SVTOOL_PATH}/include -Imy_dpi_dir -I. \
    -o fftw_test_DPI.o \
    fftw_test_DPI.c
gcc -shared -o libdpi.so \
    fftw_test_DPI.o [other object files]
# then call ${SVTOOL} again for simulation with libdpi.so

If you cannot get past the first gcc stage then your issue is clearly on the C side.

I do not have access to the fftw3 library at the moment. I'm wondering your void fftw(double FFT_in[],int size) might be clobbering a library function. Try renaming it void dpi_fftw(double FFT_in[],int size)

回答3:

You have no main function. Every binary must define main. If it doesn't, you don't have a null region of memory that _start defines in the binary, which means your program can't start!

Add a function:

int main(){

   fftw(doubleArgumentsArray, intArgument); //Or whatever function calls this function
   return 1; //Needed for C89, C99 will automatically return 1
}

回答4:

Have found the following tutorial on Dynamic Programming Interface (DPI) :

    http://www.doulos.com/knowhow/sysverilog/tutorial/dpi/

Specifically, scroll down to the "Including Foreign Language Code".

It should help with background information about how to construct a C modules for SystemVerilog.

Also, the tutorial has the following import statement:

  import "DPI" function void slave_write(input int address, input int data);

This SystemVerilog statement obviously has input defs on the parameters, is this required? Your import does NOT identify input vs. output??

回答5:

I believe this is an issue with some gcc linkers. I added the following linker flag:

irun ... -Wld,-B/usr/lib/x86_64-linux-gnu

And it fixed the issue.

来源：https://stackoverflow.com/questions/18086992/undefined-reference-to-main-in-c