Feature Test Macros, C11 and __USE_MISC

I have a simple code in the file /tmp/prog.c (using a copied line from the open source rdma-core package):

#include <stdio.h>#include <stdlib.h>#include <stdint.h>#include <endian.h>static inline __attribute__((deprecated)) uint64_t ntohll(uint64_t x) { return be64toh(x); }int main() {        printf("Hello\n");        return 0;}

When compiling with the line gcc /tmp/prog.c -o /tmp/prog -Wall I get no errors.When compiling with the line gcc /tmp/prog.c -o /tmp/prog -Wall -std=c11 I get the error:

/tmp/prog.c: In function ‘ntohll’:/tmp/prog.c:14:80: error: implicit declaration of function ‘be64toh’ [-Wimplicit-function-declaration]14 | static inline attribute((deprecated)) uint64_t ntohll(uint64_t x) { return be64toh(x); }

When compiling with the line gcc /tmp/prog.c -o /tmp/prog -Wall -std=c11 -D_DEFAULT_SOURCE I once again get no errors.

I understand that this has to do with the following snippet in endian.h:

#if defined __USE_MISC && !defined __ASSEMBLER__ /* Conversion interfaces.  */# include <bits/byteswap.h># include <bits/uintn-identity.h># if __BYTE_ORDER == __LITTLE_ENDIAN#  define htobe16(x) __bswap_16 (x)#  define htole16(x) __uint16_identity (x)#  define be16toh(x) __bswap_16 (x)#  define le16toh(x) __uint16_identity (x)#  define htobe32(x) __bswap_32 (x)#  define htole32(x) __uint32_identity (x)#  define be32toh(x) __bswap_32 (x)#  define le32toh(x) __uint32_identity (x)#  define htobe64(x) __bswap_64 (x)#  define htole64(x) __uint64_identity (x)#  define be64toh(x) __bswap_64 (x)#  define le64toh(x) __uint64_identity (x)# else#  define htobe16(x) __uint16_identity (x)#  define htole16(x) __bswap_16 (x)#  define be16toh(x) __uint16_identity (x)#  define le16toh(x) __bswap_16 (x)#  define htobe32(x) __uint32_identity (x)#  define htole32(x) __bswap_32 (x)#  define be32toh(x) __uint32_identity (x)#  define le32toh(x) __bswap_32 (x)#  define htobe64(x) __uint64_identity (x)#  define htole64(x) __bswap_64 (x)#  define be64toh(x) __uint64_identity (x)#  define le64toh(x) __bswap_64 (x)# endif#endif

because when I comment out if defined __USE_MISC I get no errors again with the initial compilation script.

I need to understand what the purpose of __USE_MISC is and why _DEFAULT_SOURCE includes it and why does c11 not.Also what are the implications of defining _DEFAULT_SOURCE versus not defining anything?

Any help is much appreciated!!

P.S. I understand the line above is deprecated but it's in an opensource package that I use. Thanks!