printf output is null when combined with an int C

Question

I have a problem with my log function, I tried to create a log method, which prints the current time in brackets before, but it don't work. Every time null is printed instead of the string.

This is my log function:

void log(char *szDebugString)
{
    printf("%s", szDebugString); //only for debug

    time_t currentTime;

    time(&currentTime);

    printf("%s", szDebugString); //only for debug
    printf("[%d] %s\n", currentTime, szDebugString);
}

Now when i'm calling the function:

log("test\n");

I get the following output on the console(time variies):

test
test
[1414078074] (null)

So my question is, why the string in the third printf is null?

Last test

change the name of your function `log` it is already defined in `math.h` — ani627, Oct 23 '14 at 15:39
If you call `printf("%s", szDebugString)` after `printf("[%d] %s\n", currentTime, szDebugString);` what does it output? Is it null as well? — Grice, Oct 23 '14 at 15:42
What do you get if you print time and string in reverse order? And what happens when you write `(int)currentTime` instead? — meaning-matters, Oct 23 '14 at 15:45
My compiler warned my when I used the function name log() warning: conflicting types for built-in function \u2018log\u2019 — Gopi, Oct 23 '14 at 15:46
what should i say? It don't work at my code, i'm using VStudio Express 2013. — NFoerster, Oct 23 '14 at 15:50
The name `log` is reserved whether you've included `` or not. Change the name of the function and change the `printf` call to something like `printf("[%ld] %s\n", (long)currentTime, szDebugString)`. If you still see the problem after doing that, let us know, and show us a complete self-contained program that exhibits the problem. What OS and compiler are you using? — Keith Thompson, Oct 23 '14 at 15:53
@1336087: Using `%ld` is no better than using `%d`. `%d` requires an `int` argument; `%ld` requires a `long int` argument. `time_t` is not necessarily either. — Keith Thompson, Oct 23 '14 at 15:53
your code works with log function in ideone : http://ideone.com/p7yRff — utarid, Oct 23 '14 at 15:55
@KeithThompson: Yes you are correct, OP should cast it to `long` before printing it. — ani627, Oct 23 '14 at 15:55
@1336087: "cast", not "typecast". Typecasting is something that happens to actors. — Keith Thompson, Oct 23 '14 at 15:56
Screenshots are not particularly useful. You've already shown us the output as text. — Keith Thompson, Oct 23 '14 at 15:57
@KeithThompson: Thanks for correcting me :) However on many sites I see it as typecast. — ani627, Oct 23 '14 at 15:58
@KeithThompson: Thank you :) I now know what is correct term for it. — ani627, Oct 23 '14 at 16:00
to use the time() function, the code needs to have the line: #include which will also define the 'time_t' type. — user3629249, Oct 23 '14 at 16:28

Shafik Yaghmour · Accepted Answer · 2014-10-30T15:54:46.467

3

The type of time_t is unspecified and it not always int in a test I just did it is actually long, and clang gives me this warning (see it live):

warning: format specifies type 'int' but the argument has type 'time_t' (aka 'long') [-Wformat]
printf("[%d] %s\n", currentTime, szDebugString);
         ~~         ^~~~~~~~~~~
         %ld

Passing an invalid specifier to printf is undefined behavior, possibly printf is using the extra bytes from currentTime when processing the %s format specifier.

As Keith Thompson points out there is no format specifier for time_t but you can convert it to a known type such as long:

printf("[%ld] %s\n", (long)currentTime, szDebugString);
         ^^^         ^^^^^^

Note, that log is used in the standard library and you should not be using that name.

edited Oct 30 '14 at 15:54

answered Oct 23 '14 at 15:43

Shafik Yaghmour

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There's no built-in format for printing a `time_t` value, but you can convert it to a known type. Try this: `printf("[%ld] %s\n", (long)currentTime, szDebugString);` – Keith Thompson Oct 23 '14 at 15:50
I ran this program and [%d] prints an integer and %s prints a string. So is this an undefined behavior? – Gopi Oct 23 '14 at 15:51
@KeithThompson I was wondering about that, that is one reasonable solution. – Shafik Yaghmour Oct 23 '14 at 15:52
@Gopi: Yes, passing an argument whose type is inconsistent with the format string has undefined behavior. – Keith Thompson Oct 23 '14 at 15:55
@KeithThompson ,Shafik thank you both, the solution casting currentTime to long worked – NFoerster Oct 23 '14 at 15:59

score 1 · Answer 2 · edited May 23 '17 at 11:57

1

The below link might help you:

What is ultimately a time_t typedef to?

Since the time_t doesn't specify any particular type or range this will lead to undefined behavior.

You also need to change your function name because compiler may throw below warning:

warning: conflicting types for built-in function 'log'

edited May 23 '17 at 11:57

Community

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answered Oct 23 '14 at 16:00

Gopi

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printf output is null when combined with an int C

2 Answers2