c99

This is the fast inverse square root implementation from Quake III Arena: float Q_rsqrt( float number ) { long i; float x2, y; const float threehalfs = 1.5F; x2 = number * 0.5F; y = number; i = * ( long * ) &y; // evil floating point bit level hacking i = 0x5f3759df - ( i >> 1 ); // what? y = * ( float * ) &i; y = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration // y = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed return y; } I noticed that long int i takes the dereferenced value at the address (cast to a long * ) of float y . The code then performs operations

What gcc options shall I use to enforce ANSI C (C99) warnings/errors? gcc (GCC) 3.4.2 (mingw-special) I'm using: gcc -pedantic -ansi -std=c99 is this correct? James McNellis The -ansi flag is synonymous with the -std=c89 flag. Just using -std=c99 with -pedantic should be sufficient. When in doubt, you can always refer to the GCC documentation . As of GCC 3.4.2, the chapter to read is 2 - Language Standards Supported by GCC . This is an old question but I just wanted to add some extra points. Firstly, regardless of the set of generic command-line switches you supply to GCC, currently it doesn't

Wiki says: The extern keyword means "declare without defining". In other words, it is a way to explicitly declare a variable, or to force a declaration without a definition. It is also possible to explicitly define a variable, i.e. to force a definition. It is done by assigning an initialization value to a variable . That means, an extern declaration that initializes the variable serves as a definition for that variable . So, /* Just for testing purpose only */ #include <stdio.h> extern int y = 0; int main(){ printf("%d\n", y); return 0; } should be valid ( compiled in C++11 ). But when

With printf() , I can use %hhu for unsigned char , %hi for a short int , %zu for a size_t , %tx for a ptrdiff_t , etc. What conversion format specifier do I use for a _Bool ? Does one exist in the standard? Or do I have to cast it like this: _Bool foo = 1; printf("foo: %i\n", (int)foo); There is no specific conversion length modifier for _Bool type. _Bool is an unsigned integer type large enough to store the values 0 and 1 . You can print a _Bool this way: _Bool b = 1; printf("%d\n", b); Because of the integer promotions rules, _Bool is guaranteed to promote to int . Until C99, bool was not

C99 adds several useful features to the language, yet I find it difficult to recommend any practice which depends upon C99. The reason for this is because there are few (any?) actual implementations of the C99 language. Sure, there is limited support in a few compilers, but nobody wants spend the time to write C code only to have it be unportable. This is frustrating given that the standard was written and finalized over 10 years ago now. Plus I hear discussions of a C1x from time to time, and I wonder why someone would be taking steps to revise the language given that the current version of

On most implementations of this day and age, a signed integer value that has a bit pattern of 1 for the sign bit and all 0 for the value bits tends to represent the lowest possible value for that signed integer type. However, as 6.2.6.2p2 states, that's not a requirement: Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones' complement), is a trap representation or a normal value. My first question is simple: Are there any implementations that use this bit pattern