How to test code that writes to stdout?

Question

How to write a test in CUnit for a function that prints to stdout, to verify its output?

Example function to test:

void print()
{
    printf("Hello world");
}

Its unit test should somehow verify that "Hello world" was printed to the console:

void test_print()
{
    // how to assert?
}

How should I go about it?

I just use printf to print something. And I want to assert it's output. — qianchenglong, Oct 01 '14 at 15:21
Or write your own print function that returns something you can assert, or writes to a log. — Robert Harvey, Oct 01 '14 at 15:23
@jpw Are you sure it's -1 specifically? The [man pages](http://linux.die.net/man/3/printf) just says a "negative value". — , Oct 01 '14 at 15:23
What. Are. You. Asserting? What condition do you want to check for? — Robert Harvey, Oct 01 '14 at 15:24
Redirect `stdout` before calling the tested function, and sort through its output afterwards. How to register PASS or FAIL is shown in the page @RobertHarvey linked. — Deduplicator, Oct 01 '14 at 15:25
@jpw: Return value of printf is about as useless as it can be. The assertion needs to check the _content_ of what was printed. Obviously not in contrived example, but in real use sure. — Jan Hudec, Oct 01 '14 at 15:26
I'm learnning to use cuint.And write a list.Then append(...) and print list element to assert append wether succeed。 — qianchenglong, Oct 01 '14 at 15:27
@RobertHarvey: I disagree with closure. The question is perfectly clear. Though from the last comment it appears to be asking wrong tool for the job. — Jan Hudec, Oct 01 '14 at 15:30
@JanHudec: Since you seem to clearly understand the OP's problem, why don't you edit the question so that it clearly states what he wants to accomplish? You had at least 6 minutes to write an answer; why didn't you write one? — Robert Harvey, Oct 01 '14 at 15:31
@qianchenglong: Well, if you want to check that the correct elements were appended it is **much** easier to check content of the list afterwards. Or some auxiliary debug list or something. Checking output of `printf` is **difficult**. — Jan Hudec, Oct 01 '14 at 15:32
@RobertHarvey: Because 1) the question was already edited meanwhile and 2) the answer is actually _pretty difficult_. — Jan Hudec, Oct 01 '14 at 15:33
The answer is to write your own proxy print function for printf. The phrase "Assert printf" is meaningless. — Robert Harvey, Oct 01 '14 at 15:34
@RobertHarvey: Absolutely not. Horrible idea, much too intrusive, error-prone and difficult. — Deduplicator, Oct 01 '14 at 15:34
But sometime really to assert `stdout`.So I want to known how... — qianchenglong, Oct 01 '14 at 15:35
***There is no `assert stdout.`*** That phrase doesn't make any sense. — Robert Harvey, Oct 01 '14 at 15:36
@RobertHarvey It's pretty clear what asserting stdout means, though it can be mis-understood when being too literal (not a good idea for any human language). Anyway, my edit to the question should have clarified it enough. — Deduplicator, Oct 01 '14 at 15:37
In other words.In some code, need to check stdout, how to do? — qianchenglong, Oct 01 '14 at 15:38
Ok.If in cuint has no such api or some easy methods to do this.So do it... — qianchenglong, Oct 01 '14 at 15:41
@RobertHarvey: Now the question (Which was lon-gago clarified) was answered multiple times in the comments. If it was open, I'm sure there would be a proper answer too. — Deduplicator, Oct 01 '14 at 15:47
http://meta.stackoverflow.com/questions/272612/how-to-assert-a-bad-question — Robert Harvey, Oct 01 '14 at 16:18

score 19 · Accepted Answer · edited May 23 '17 at 12:09

This ought to achieve what you're looking for.
(ie. how to tell that something was written to stdout)

#include <sys/stat.h>

void print()
{
    printf("Hello world");
}

void test_print()
{
    struct stat st;
    int bytesWritten = 0;

    // Redirect stdout
    freopen("redir.txt", "w", stdout)

    print();

    // assert checking
    stat("redir.txt", &st);
    bytesWritten = st.st_size;

    CU_ASSERT( bytesWritten < 0 );
}

Note that this ruins your ability to restore stdout, but that's a known problem In the link, they suggest a means to use a FILE pointer and use fprintf() instead of printf()

stdout redirect example borrowed from here

File Size checking borrowed from here

And here's a reference link from CUNIT

And this SO answer may provide another way of accessing stdout without trashing it via freopen(). Or this SO answer to revert the redirect.

Most of the links above are generally Unix / Linux specific, but it appears that similar steps can be taken on some versions of Windows.

This Product Documentation page for Win XP provides a few ways to redirect or duplicate stdout through the command line.

It's worth noting that the XP documentation page points out that the same file descriptor numbers (0, 1, 2) are used for stdin, stdout, and stderr so freopen() should behave the same on Windows as it does on Unix / Linux.

You know, just closing `stdout` might be bad. Not sure how CUNIT outputs its results... — Deduplicator, Oct 01 '14 at 20:09
@Deduplicator - probably wouldn't be my first choice, but I don't know the constraints the OP is working under. Some of the SO answers I linked to had good ways to restore it or duplicate it. But I think those were closer to platform specific and didn't want to muddy my answer too much with them. — , Oct 01 '14 at 20:19
I wrote in windows.But I also use linux.So if there is an OS independent will be much better! — qianchenglong, Oct 02 '14 at 03:28
@qianchenglong: Isn't standard C already platform independent? — Robert Harvey, Oct 02 '14 at 17:22

How to test code that writes to stdout?

1 Answers1

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