C: compiling and linking component programs separately using 'ld'?

Question

I am trying to understand extern and linking. As a simple demonstration I have written two C files: a main file which uses a function and another file that defines the function:

/* main.c */
#include <stdio.h>
extern void print_nos(int, int);
int main(void) {
    print_nos(10, 20);
    return 0;
}

/* print_nos.c */
#include <stdio.h>
void print_nos(int lower, int higher) {
    int i;
    for (i = lower; i <= higher; i++)
        printf("%d ", i);
    printf("\n");
}

I have two questions pertaining to this:

1)

I know that I can simply pass both files as arguments to gcc at once:

gcc -o main main.c print_nos.c

And this works fine, but I wanted to do the linking explicitly using ld, so I did:

$ gcc -c main.c        # produces main.o
$ gcc -c print_nos.c   # produces print_nos.o
$ ld -o main main.o print_nos.o -lc
ld: warning: cannot find entry symbol _start; defaulting to 0000000000401050

As you can see, I get the above warning and when I try to run the output file :

$ ./main
-bash: ./main: No such file or directory

I do not have much experience in assembly so I do not where to begin troubleshooting this - any help is appreciated.

2)

Secondly, when I remove the extern specifier, and try running gcc -o main.c print_nos.c the program compiles just fine. Does a function need to be declared extern at all? When I peeked into all the standard include files like stdlib, stdio, etc I saw that all the functions have an extern declaration. Should I therefore use extern in my case (in actual code resembling the one above)?

You almost never use `ld` directly. C programs usually use a bunch of implementation-specific runtime libraries and startup files beyond the standard library (`-lc`). You would need to specify these things on the command line along with your own object files. Just use `gcc` instead: `gcc -o main main.o print_nos.o`. `gcc` will add these runtime libraries and startup files automatically. You can use `ld` if you build simple assembly-language programs with no runtime, or when you build C programs for embedded computers which do not use normal runtime. — n. m. could be an AI, Aug 07 '21 at 16:32
I'ld use `gcc main.c print_nos.c -o main` ... Additionally you may take a look here: https://stackoverflow.com/questions/14163208/how-to-link-c-object-files-with-ld — Sir Jo Black, Aug 07 '21 at 16:42

score 5 · Accepted Answer · answered Aug 07 '21 at 16:39

I wouldn't recommend using ld directly unless you know what you're doing and have a good reason. If you want to compile modules separately (which is a good idea, so you can recompile only the ones whose source files have changed) you can use gcc to do both the compiling and linking, like this:

gcc -c main.c        # produces main.o
gcc -c print_nos.c   # produces print_nos.o
gcc -o main main.o print_nos.o

As for the extern specifier, you don't need it for functions. Function declarations are implicitly extern by default.

C: compiling and linking component programs separately using 'ld'?

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1 Answers1