Endianness

uint32_t x = 0x12345678

LITTLE ENDIAN (ARM / x86)     BIG ENDIAN (network / TCP)
addr  byte                    addr  byte
+0    0x78  ← *p  (LSB)       +0    0x12  ← *p  (MSB)
+1    0x56                    +1    0x34
+2    0x34                    +2    0x56
+3    0x12        (MSB)       +3    0x78        (LSB)

// Endianness test
uint32_t x = 0x12345678;
uint8_t *p = (uint8_t *)&x;
if (*p == 0x78) puts("little endian");
else             puts("big endian");

htonl(val);   // host → network (big endian)
ntohl(val);   // network → host