Compile c code with float instead of double

floating-point, gcc

Solution

You can't safely do this without modifying your source code, but that shouldn't be terribly difficult to do.

Using the preprocessor to force the keyword `double` in your program to be treated as `float` is a bad idea; it will make your program difficult to read, and if you happen to use `long double` anywhere it would be treated as `long float`, which is a syntax error.

As stix's answer suggests, you can add a `typedef`, either at the top of your program (if it's a single source file) or in some header that's `#include`ed by all the relevant source files:

typedef double real; /* or pick a different name */

Then go through your source code and change each occurrence of `double` to `real`. (Be careful about doing a blind global search-and-replace.)

Make sure that the program still compiles, runs, and behaves the same way after this change. Then you can change the typedef to:

 typedef float real;

and recompile to use `float` rather than `double`.

It's not quite that simple, though. If you're using functions declared in `<math.h>`, you'll want to use the right function for whatever floating-point type you're using; for example, `sqrt()` is for `double`, `sqrtf()` is for `float`, and `sqrtl()` is for `long double`.

If your compiler supports it, you might use the `<tgmath.h>` header, which defines type-generic macros corresponding to the math functions from `<math.h>`. If you use `<tgmath.h>`, then `sqrt(x)` will resolve to call the correct square root function depending on the type of the argument. It was introduced in the 1999 edition of the ISO C standard, so availability shouldn't be a problem.

Another possible issue (thanks to Kevin Thibedeau for pointing it out) is that unsuffixed floating-point constants are of type `double`. If a `float` object appears along with a `double` constant in an expression, for example `f + 1.0`, there's likely to be an implicit `float`-to-`double` conversion. Appending `f` or `F` to a floating constant will specify that it's of type `float`.

Problem

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