CODEWITHSUNDEEP
cgcccompilationubuntu-16.04
Alokananda
Alokananda

Reputation: 13

Unable to compile and link main.c to another source file using gcc

I am trying to compile the following source file using gcc -g -Wall -Wextra -pedantic -std=c99 -o main -lm.

source.h

void simple_sum(void)

source.c

    #include "source.h"
    #include <stdio.h>

    void simple_sum(void)
    {
        int a, b;
        scanf("%d %d", &a, &b);
        printf("%d + %d = %d\n",a, b, a + b);
    }

main.c

#include "source.h"                                                                                                     
#include <stdio.h>  

int main(void)                                                                                                          
{                                                                                                                           
    printf("\n");                                                                                                           
    simple_sum();     

    return 0;                                                                                                           
}

I get following error:

gcc -g -Wall -Wextra -pedantic -std=c99 -o main  -lm
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function _start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:6: recipe for target 'main' failed
make: *** [main] Error 1

Could someone please suggest how to fix this?

Edit

I get the following error when I run using gcc -g -Wall main.c -o main

/tmp/ccEAL4iG.o: In functionmain': /home/a/aalto_university/functions/calculation/main.c:7: undefined reference to simple_sum' collect2: error: ld returned 1 exit status

Upvotes: 0

Views: 705

Answers (2)

Achal
Achal

Reputation: 11921

Here

gcc -g -Wall -Wextra -pedantic -std=c99 -o main  -lm

you are not providing source file name to linker, hence it throw error like

undefined reference to `main'

While compiling provide source file main.c and source.c. For e.g first run this

gcc -g -Wall -Wextra -pedantic -std=c99 -c main.c source.c -lm

to create the object.o files & then create the executable by running

gcc source.o main.o -o my_exe

And finally run the executable. Also declaration of simple_sum() missing ; it should be

void simple_sum(void); /* you miss ;*/

Also learn how to use Makefile for compilation as @Basile pointed, there you don't have to create .o file manually, your Makefile will create .o file & compile if it's written correctly.

Upvotes: 3

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

Compile with

 gcc -g -Wall -Wextra -pedantic -std=c99 source.c main.c -o myprog -lm

(actually, -lm is not needed, you don't use <math.h> functions; but keeping -lm should not harm)

Later, learn to write your Makefile to do these things in several steps:

First, get the source.o object file with

gcc -g -Wall -Wextra -pedantic -std=c99 -c source.c

then get the main.o object file with

gcc -g -Wall -Wextra -pedantic -std=c99 -c main.c

At last, link both of them

gcc -g source.o main.o -lm -o myprog

Upvotes: 4

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