Undefined reference to log10?

Question

I have the following two functions and when compiling, I am face with the error in the title.

double getIdf(FileList fl, int D){
  double fileCount = countFiles(fl); //todo countfiles
  double temp = fileCount/D;
  double idf = log10(temp);
  return (fabs(idf));
}

double countFiles(FileList fl){
    if (fl == NULL){
        printf("countFiles FL does not exist\n");
        return 0;
    }
    double count = 0;
    FileList curr = fl;
    while (curr != NULL) {
        count++;
        curr = curr->next;
    }
    return count;
}

However, if I change the log10 function to log10(5.5) it would work.

double getIdf(FileList fl, int D){
  double fileCount = countFiles(fl); //todo countfiles
  double temp = fileCount/D;
  double idf = log10(5.5);
  return (fabs(idf));
}

I am compiling with -lm.

Whats wrong here?

Post the exact command line you are using to link. If it's like `gcc -lm foo.c`, that is your problem; the `-lm` needs to go at the end. See https://stackoverflow.com/questions/45135/why-does-the-order-in-which-libraries-are-linked-sometimes-cause-errors-in-gcc — Nate Eldredge, Oct 24 '20 at 15:09
The difference in your changed version is likely that when the argument is a constant, the compiler can compute `log10(5.5)` at compile time and not need a call to the library function at all. — Nate Eldredge, Oct 24 '20 at 15:11
Maybe there are some include files missing? (at least from the code you posted) — wildplasser, Oct 24 '20 at 15:13
@NateEldredge yes moving -lm to the end worked. Why is that? How do I change the code so that I do not have to move -lm to the back for it to compile? — REDBEAN, Oct 24 '20 at 15:18

score 3 · Answer 1 · answered Oct 24 '20 at 15:52

3

You typed

gcc -lm program.c

You want

gcc program.c -lm

Because libraries only pull in symbols that are referenced by the code already being linked in, and you tried to link in the library before adding any code.

I know, it's terrible. It would be nicer if it could index the libraries and use them to reference symbols needed only after the library loads. I've had to reference libraries twice.

answered Oct 24 '20 at 15:52

Joshua

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Is there no way to compile with -lm at the front? I'm doing a university project that will go through autotesting. The autotest program will compile with -lm at the front... – REDBEAN Oct 24 '20 at 16:28
@REDBEAN: the autotest program shouldn't do that unless it is running on a platform in which the linker scans all libraries. More details about the test environment would be useful (as an edit to your question, not as a comment here ) – rici Oct 24 '20 at 16:37

Undefined reference to log10?

1 Answers1