Integer to IP Address - C

Question

I m preparing for a quiz  and I have a strong suspicion I may be tasked with implementing such a function  Basically  given an IP address in network notation  how can we get that from a 32 bit integer into a string in it s dotted decimal notation  something like 155 247 182 83      Obviously we can t be using any type of inet functions either   I m stumped

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Here s a simple method to do it  The  ip  gt  gt  8    ip  gt  gt  16  and  ip  gt  gt  24  moves the 2nd  3rd and 4th bytes into the lower order byte  while the  amp  0xFF isolates the least significant byte at each step   void print ip unsigned int ip        unsigned char bytes 4       bytes 0    ip  amp  0xFF      bytes 1     ip  gt  gt  8   amp  0xFF      bytes 2     ip  gt  gt  16   amp  0xFF      bytes 3     ip  gt  gt  24   amp  0xFF         printf   d  d  d  d n   bytes 3   bytes 2   bytes 1   bytes 0                There is an implied bytes 0     ip  gt  gt  0   amp  0xFF  at the first step   Use snprintf   to print it to a string

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My alternative solution with subtraction     void convert  unsigned int addr         unsigned int num OCTET                next addr OCTET    int bits   8  unsigned int shift bits  int i   next addr 0    addr   shift bits -  bits    num 0    next addr 0   gt  gt  shift bits   for   i   0  i  lt  OCTET-1  i                   next addr i   1    next addr i  -   num i   lt  lt  shift bits       next subaddr     shift bits -  bits     next shift     num i   1    next addr i   1   gt  gt  shift bits     octet    printf    d  d  d  d n   num 0   num 1   num 2   num 3

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Another approach   union IP       unsigned int ip      struct         unsigned char d        unsigned char c        unsigned char b        unsigned char a        ip2          char  ips 20   IP ip  ip ip   0xAABBCCDD   sprintf ips    x  x  x  x   ip ip2 a  ip ip2 b  ip ip2 c  ip ip2 d   printf   s n   ips

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Hint  break up the 32-bit integer to 4 8-bit integers  and print them out   Something along the lines of this  not compiled  YMMV    int i   0xDEADBEEF     some 32-bit integer printf   i  i  i  i              i  gt  gt  24   amp  0xFF             i  gt  gt  16   amp  0xFF             i  gt  gt  8   amp  0xFF            i  amp  0xFF

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This is what I would do if passed a string buffer to fill and I knew the buffer was big enough  ie at least 16 characters long    sprintf buffer    d  d  d  d      ip  gt  gt  24   amp  0xFF     ip  gt  gt  16   amp  0xFF     ip  gt  gt   8   amp  0xFF     ip         amp  0xFF     This would be slightly faster than creating a byte array first  and I think it is more readable   I would normally use snprintf  but IP addresses can t be more than 16 characters long including the terminating null   Alternatively if I was asked for a function returning a char    char  IPAddressToString int ip      char   result   new char 16      sprintf result    d  d  d  d        ip  gt  gt  24   amp  0xFF       ip  gt  gt  16   amp  0xFF       ip  gt  gt   8   amp  0xFF       ip         amp  0xFF      return result

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void ul2chardec char pcIP  unsigned long ulIPN   int i  int k 0  char c0  c1  for  i   0  i lt 4  i         c0       ulIPN  amp   0xff  lt  lt    3 - i    8     gt  gt    3 - i    8      100    0x30      if  c0     0      pcIP   k    c0  k          c1        ulIPN  amp   0xff  lt  lt    3 - i    8     gt  gt    3 - i    8      100    10    0x30      if    c1    0   amp  amp  c0   0       pcIP   k    c1  k            pcIP  k         ulIPN  amp   0xff  lt  lt    3 - i    8      gt  gt    3 - i    8      10    0x30      k        if  i lt 3     pcIP   k         k         pcIP   k    0     pcIP should be x10 bytes

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You actually can use an inet function  Observe   main c    include  lt arpa inet h gt   main         uint32 t ip   2110443574      struct in addr ip addr      ip addr s addr   ip      printf  The IP address is  s n   inet ntoa ip addr        The results of gcc main c -ansi    a out is      The IP address is 54 208 202 125   Note that a commenter said this does not work on Windows

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include  stdio h   void print ip int ip       unsigned char bytes 4      int i     for i 0  i lt 4  i            bytes i     ip  gt  gt  i 8   amp  0xFF          printf   d  d  d  d n   bytes 3   bytes 2   bytes 1   bytes 0       int main        int ip   0xDEADBEEF     print ip ip

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From string to int and back  const char   s ip    192 168 0 5   unsigned int ip  unsigned char   c ip    unsigned char    amp ip  sscanf s ip    hhu  hhu  hhu  hhu    amp c ip 3    amp c ip 2    amp c ip 1    amp c ip 0    printf   u  u  u  u     ip  amp  0xff000000   gt  gt  24     ip  amp  0x00ff0000   gt  gt  16     ip  amp  0x0000ff00   gt  gt  8    ip  amp  0x000000ff       hhu instructs sscanf to read into unsigned char pointer   Reading small int with scanf     inet ntoa from glibc  char   inet ntoa  struct in addr in    unsigned char  bytes    unsigned char     amp in    snprintf  buffer  sizeof  buffer     d  d  d  d   bytes 0   bytes 1   bytes 2   bytes 3    return buffer

[c] Integer to IP Address - C

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