Question

void init(void) 
{
   glClearColor ((float)0.0, (float)0.0, (float)0.0, (float)0.0);
   glShadeModel (GL_FLAT);
}

The parameter of glClearColor is float. But the gcc always give a warning:

main.c: In function ‘init’:
main.c:12: warning: passing argument 1 of ‘glClearColor’ as ‘float’ rather than ‘double’ due to prototype
main.c:12: warning: passing argument 2 of ‘glClearColor’ as ‘float’ rather than ‘double’ due to prototype
main.c:12: warning: passing argument 3 of ‘glClearColor’ as ‘float’ rather than ‘double’ due to prototype
main.c:12: warning: passing argument 4 of ‘glClearColor’ as ‘float’ rather than ‘double’ due to prototype 

But i just convert a digit to float, not double, can’t figure out why.

My os is mac os 64bit and the following is the makefile.

UNAME := $(shell uname)

ifeq ($(UNAME), Darwin)
    CFLAG := -framework GLUT -framework OpenGL -g -Wall -Wconversion
else
    CFLAG := -lm -lglut -lGL -lGLU -g -Wall
endif

PUB_SRC := util.c plane.c dot.c linked_dots.c controller.c time_data.c
SRC := main.c $(PUB_SRC)
TEST_SRC := test.c $(PUB_SRC)

.PHONY : main
main: $(SRC)
    gcc $(CFLAG) $(SRC)

.PHONY : test
test: $(TEST_SRC)
    gcc $(CFLAG) $(TEST_SRC)

Answer 1

I take it you’re using GCC 3.x? In GCC 3.x, the -Wconversion flag is documented as follows:

Warn if a prototype causes a type conversion that is different from what
would happen to the same argument in the absence of a prototype. This
includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed point argument
except when the same as the default promotion.

Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assignment
x = -1 if x is unsigned. But do not warn about explicit
casts like (unsigned) -1.

[link]

From what I understand, the original purpose of this flag was to help detect potential problems when migrating from traditional C to ANSI C, and cases where adding a prototype could cause problems. (To be honest, it seems to me like it would have been more meaningful for the prototype itself to trigger a warning, rather than a use of the function, but there’s probably some use I’m missing.)