Add a fully implementation of endian.h

[thisisaname1928]

Add a fully implementation of endian.h

Overview

• Specs: https://pubs.opengroup.org/onlinepubs/9799919799.2024edition/basedefs/endian.h.html, implement all functions, macros listed in the specs.

Functions & Macros

#define LITTLE_ENDIAN
#define BIG_ENDIAN 
#define __LIBC_BYTE_ORDER 

uint16_t  be16toh(uint16_t);
uint32_t  be32toh(uint32_t);
uint64_t  be64toh(uint64_t);
uint16_t  htobe16(uint16_t);
uint32_t  htobe32(uint32_t);
uint64_t  htobe64(uint64_t);
uint16_t  htole16(uint16_t);
uint32_t  htole32(uint32_t);
uint64_t  htole64(uint64_t);
uint16_t  le16toh(uint16_t);
uint32_t  le32toh(uint32_t);
uint64_t  le64toh(uint64_t);

[BoredDevNL]

Nice work overall - the three-tier compiler detection and manual fallback bswap are solid. A few things are worth addressing before i merge:

---

[bug] Macro argument not consistently parenthesized in manual bswap fallback

In the #else bswap falback, x isn't wrapped in (x) on every expansion site. This causes operator-precedence bugs when callers pass an expression like a | b or *ptr++:

// e.g. __bswap32(a | b) expands to:
(uint32_t)((((uint32_t)(a | b) >> 24) & 0xff) | ...)
//                      ^^^^^^^ fine here, but not guaranteed safe everywhere

Every occurrence of x inside the macro body should be (x).

---

[bug] LITTLE_ENDIAN / BIG_ENDIAN defined unconditionally

Defining these without an #ifndef guard will produce redefinition confliicts if any included header (e.g. a compiler runtime header) already defines them. Either guard them:

#ifndef LITTLE_ENDIAN
#define LITTLE_ENDIAN 1234
#endif
#ifndef BIG_ENDIAN
#define BIG_ENDIAN 4321
#endif

or use a private namespace alias (e.g. __LIBC_LITTLE_ENDIAN) and reference that throughout.

---

[robustness] No fallback #error if __LIBC_BYTE_ORDER is an unrecognised value

If a compiler defines __BYTE_ORDER__ to something unexpected, the header silently generates broken macros with no diagnostic. Add a catch-all:

#if (__LIBC_BYTE_ORDER == LITTLE_ENDIAN)
  // ...
#elif (__LIBC_BYTE_ORDER == BIG_ENDIAN)
  // ...
#else
#error "Unrecognised byte order — please add support for this target"
#endif

[thisisaname1928]

Thanks for ur reply, btw I have university entrance exam and graduation exam so Ima work on the fix a bit late ❤️