Why does gcc -ansi remove the declarations of cosf and the like?

Question

You probably think I am completely crazy and terribly bad at programming. One of those may be the case, but please read my findings.

Yes, I #include <math.h>

Full Code can be found here.( I was trying to make it ansi compliant to get it to compile on VS2010, It through an error about mixed code and declaration, and fminf() missing. I was surprised that VS2010 cared about mixed code and declaration with default warning levels. I recall 2008 not caring, but could be wrong. )

Here is the gcc output when using the c89/-ansi standard. note the implicit declarations of functions. There are a few others about unused parameters, but we don't care about those for now. ( needed for signature to register call backs with GLUT)

When I run the application using the c89 or ansi standard, it produces the wrong output, much like the math functions are not behaving as expected.

$ STANDARD=-std=c89 make -f Makefile.Unix
gcc -std=c89 -Wextra -Wall -pedantic  -c -o file-util.o file-util.c -I/usr/X11R6/include
gcc -std=c89 -Wextra -Wall -pedantic  -c -o gl-util.o gl-util.c -I/usr/X11R6/include
gcc -std=c89 -Wextra -Wall -pedantic  -c -o meshes.o meshes.c -I/usr/X11R6/include
In file included from meshes.c:12:
vec-util.h: In function ‘vec_length’:
vec-util.h:10: warning: implicit declaration of function ‘sqrtf’
meshes.c: In function ‘calculate_flag_vertex’:
meshes.c:48: warning: implicit declaration of function ‘sinf’
meshes.c:50: warning: implicit declaration of function ‘cosf’
gcc -std=c89 -Wextra -Wall -pedantic  -c -o flag.o flag.c -I/usr/X11R6/include
In file included from flag.c:18:
vec-util.h: In function ‘vec_length’:
vec-util.h:10: warning: implicit declaration of function ‘sqrtf’
flag.c: In function ‘update_p_matrix’:
flag.c:58: warning: implicit declaration of function ‘fminf’
flag.c: In function ‘mouse’:
flag.c:252: warning: unused parameter ‘x’
flag.c:252: warning: unused parameter ‘y’
flag.c: In function ‘keyboard’:
flag.c:261: warning: unused parameter ‘x’
flag.c:261: warning: unused parameter ‘y’
flag.c: At top level:
vec-util.h:1: warning: ‘vec_cross’ defined but not used
vec-util.h:13: warning: ‘vec_normalize’ defined but not used
gcc -o flag file-util.o gl-util.o meshes.o flag.o -L/usr/X11R6/lib -lGL -lglut -lGLEW

Now using the c99 standard the implicit declaration of function messages are gone.

$ STANDARD=-std=c99 make -f Makefile.Unix
gcc -std=c99 -Wextra -Wall -pedantic  -c -o file-util.o file-util.c -I/usr/X11R6/include
gcc -std=c99 -Wextra -Wall -pedantic  -c -o gl-util.o gl-util.c -I/usr/X11R6/include
gcc -std=c99 -Wextra -Wall -pedantic  -c -o meshes.o meshes.c -I/usr/X11R6/include
gcc -std=c99 -Wextra -Wall -pedantic  -c -o flag.o flag.c -I/usr/X11R6/include
flag.c: In function ‘mouse’:
flag.c:252: warning: unused parameter ‘x’
flag.c:252: warning: unused parameter ‘y’
flag.c: In function ‘keyboard’:
flag.c:261: warning: unused parameter ‘x’
flag.c:261: warning: unused parameter ‘y’
flag.c: At top level:
vec-util.h:1: warning: ‘vec_cross’ defined but not used
vec-util.h:13: warning: ‘vec_normalize’ defined but not used
gcc -o flag file-util.o gl-util.o meshes.o flag.o -L/usr/X11R6/lib -lGL -lglut -lGLEW

When using the c99 standard the program behaves as desired and expected.

The Question

Why would using the -ansi flag seemingly remove the declarations from math.h ?

Answer 1

If you check the GCC Builtins documentation, you'll see that sinf and cosf functions (and many more related ones) are introduced in the C99 standard.

Answer 2

Don't use -ansi for modern code. Despite the current version of ANSI C being aligned with ISO9899-1999 (C99), -ansi has been permanently assigned to mean "legacy mode" by gcc. Just use -std=c99 if you're compiling C99 code. It's the modern equivalent.