I have the following C code:
#include <math.h>
int main(int argc, char ** argv)
{
double mydouble = 100.0;
double whatever = round(mydouble);
return (int) whatever;
}
When I compile this, I get the warnings:
round_test.c: In function ‘main’:
round_test.c:6: warning: implicit declaration of function ‘round’
round_test.c:6: warning: incompatible implicit declaration of built-in function ‘round’
I'm rusty with C, but I thought that the #include brought a declaration for round() into scope. I've checked my ANSI standard (C99 is the only copy I have) which confirms that the round() function exists in the math.h header. What am I missing here?
Edit: The compiler is GCC 4.3.2 on Ubuntu (intrepid, IIRC). Running gcc -E gives:
$ gcc -E round_test.c | grep round
# 1 "round_test.c"
# 1 "round_test.c"
# 2 "round_test.c" 2
double whatever = round(mydouble);
so the definition obviously isn't being found in the headers.
I see you're using gcc.
By default, gcc uses a standard similar to C89. You may want to "force" it to use the C99 standard (the parts it complies with)
gcc -std=c99 -pedantic ...
Quote from GCC Manual
By default, GCC provides some
extensions to the C language that on
rare occasions conflict with the C
standard. See Extensions to the C
Language Family. Use of the -std
options listed above will disable
these extensions where they conflict
with the C standard version selected.
You may also select an extended
version of the C language explicitly
with -std=gnu89 (for C89 with GNU
extensions) or -std=gnu99 (for C99
with GNU extensions). The default, if
no C language dialect options are
given, is -std=gnu89; this will change
to -std=gnu99 in some future release
when the C99 support is complete. Some
features that are part of the C99
standard are accepted as extensions in
C89 mode.
Something must be wrong with your gcc installation, system headers, or compilation options.
Try compiling with -E. That will show you what the preprocessor output -- including which headers are being included and what's in them. On my Ubuntu Linux system it's about 1000 lines of output, including this:
extern double round (double __x) __attribute__ ((__nothrow__)) __attribute__ ((__const__));
You need to tell gcc that you want C99, and that you want to link in libm:
gcc -std=c99 -lm round_test.c
The code you type compiles cleanly on MacOS X 10.5.8 with GCC 4.0.1. If prodded with options '-Wall -Wextra', it complains about unused parameters argc and argv - not material.
Have you looked in <math.h> on your machine?
Have you tried with options such as '-stc=c99'?
C99 was the answer, but the full story is a little more complicated. The reason I'd been playing with this at all was that I was trying to compile a library written for Windows, which had its own "optimised" definition of round(). I got a linker error telling me that the definition conflicted with the built-in, so I removed the definition (and declaration). Once I'd done that I started to get the "implicit declaration error".
It seems that the default compile mode (without the -std=c99 flag) is neither conforming C89 nor C99: if it were conforming C89, you should be able to provide a custom definition of round() without conflicting, and if it were conforming C99 the declaration should be in math.h.
you need to link with the math library. So when you compile, be sure to add the -lm flag.
