floor of double(time_t)

Question

I cannot understand why this throws undefined reference to `floor'":

double curr_time = (double)time(NULL);
return floor(curr_time);

Hasn't it been casted to double, which is what floor receives?

Answer 1

Maybe because you don't link with the math library? The error has nothing to do with casts and data types, btw.

Answer 2

You possibly have run in to the infamous -lm problem: Compile as:

gcc yourfile.c -o out -lm 

This is C FAQ 14.3 item as well.

Answer 3

You probably have to link explicitly to the library. On an UNIX-like system this would typically be "/usr/lib/libm.a". The C standard library should be linked by default, but the math library is, depending on your system, not linked and you may have to link explicitly. (e.g. on Mac OS X it is also linked by default on my ubuntu system it is not).

Note that this has nothing to do with your include path. If you are on something UNIX-like you will probably find the header with the prototype declaration under "/usr/include/math.h", where your compiler will always look for headers.

If you are using gcc, you can either link directly with:

gcc yourfile.c /usr/lib/libm.a -o out

or with "-l*nameroflibrary*" like this:

gcc yourfile.c -lm -o out

this will look for a library in the same directory as the C standard library with the name "lib*nameoflibrary*.a"