I'm trying to compile a program (called es3), but, when I write from terminal:
gcc es3.c -o es3
it appears this message:
/usr/lib/gcc/i686-linux-gnu/4.4.5/../../../../lib/crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
collect2: ld returned 1 exit status
What could I do?
It means that es3.c does not define a main function, and you are attempting to create an executable out of it. An executable needs to have an entry point, thereby the linker complains.
To compile only to an object file, use the -c option:
gcc es3.c -c
gcc es3.o main.c -o es3
The above compiles es3.c to an object file, then compiles a file main.c that would contain the main function, and the linker merges es3.o and main.o into an executable called es3.
You can just add a main function to resolve this problem.
Just like:
int main()
{
return 0;
}
Executable file needs a main function. See below hello world demo.
#include <stdio.h>
int main(void)
{
printf("Hello world!\n");
return 0;
}
As you can see there is a main function. if you don't have this main function, ld will report "undefined reference to main' "
check my result:
$ cat es3.c
#include <stdio.h>
int main(void)
{
printf("Hello world!\n");
return 0;
}
$ gcc -Wall -g -c es3.c
$ gcc -Wall -g es3.o -o es3
~$ ./es3
Hello world!
please use $ objdump -t es3.o to check if there is a main symbol. Below is my result.
$ objdump -t es3.o
es3.o: file format elf32-i386
SYMBOL TABLE:
00000000 l df *ABS* 00000000 es3.c
00000000 l d .text 00000000 .text
00000000 l d .data 00000000 .data
00000000 l d .bss 00000000 .bss
00000000 l d .debug_abbrev 00000000 .debug_abbrev
00000000 l d .debug_info 00000000 .debug_info
00000000 l d .debug_line 00000000 .debug_line
00000000 l d .rodata 00000000 .rodata
00000000 l d .debug_frame 00000000 .debug_frame
00000000 l d .debug_loc 00000000 .debug_loc
00000000 l d .debug_pubnames 00000000 .debug_pubnames
00000000 l d .debug_aranges 00000000 .debug_aranges
00000000 l d .debug_str 00000000 .debug_str
00000000 l d .note.GNU-stack 00000000 .note.GNU-stack
00000000 l d .comment 00000000 .comment
00000000 g F .text 0000002b main
00000000 *UND* 00000000 puts
One possibility which has not been mentioned so far is that you might not be editing the file you think you are. i.e. your editor might have a different cwd than you had in mind.
Run 'more' on the file you're compiling to double check that it does indeed have the contents you hope it does. Hope that helps!
I my case I found out the void for the main function declaration was missing.
I was previously using Visual Studio in Windows and this was never a problem, so I thought I might leave it out now too.
Perhaps your main function has been commented out because of e.g. preprocessing.
To learn what preprocessing is doing, try gcc -C -E es3.c > es3.i then look with an editor into the generated file es3.i (and search main inside it).
First, you should always (since you are a newbie) compile with
gcc -Wall -g -c es3.c
gcc -Wall -g es3.o -o es3
The -Wall flag is extremely important, and you should always use it. It tells the compiler to give you (almost) all warnings. And you should always listen to the warnings, i.e. correct your source code file es3.C till you got no more warnings.
The -g flag is important also, because it asks gcc to put debugging information in the object file and the executable. Then you are able to use a debugger (like gdb) to debug your program.
To get the list of symbols in an object file or an executable, you can use nm.
Of course, I'm assuming you use a GNU/Linux system (and I invite you to use GNU/Linux if you don't use it already).
In my case it was just because I had not Saved the source file and was trying to compile a empty file .
Source: Stackoverflow.com