Encode Decode URLs in C

Question

Does anyone know of any good C   code that does this

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Adding a follow-up to Bill s recommendation for using libcurl  great suggestion  and to be updated  after 3 years  the curl escape function is deprecated  so for future use it s better to use curl easy escape

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This version is pure C and can optionally normalize the resource path  Using it with C   is trivial    include  lt string gt   include  lt iostream gt   int main int argc  char   argv        const std  string src   some url foo    bar  2e         std  cout  lt  lt   src      lt  lt  src  lt  lt        lt  lt  std  endl          either do it the C   conformant way      char  dst buf   new char src size     1       urldecode dst buf  src c str    1       std  string dst1 dst buf       delete   dst buf      std  cout  lt  lt   dst1      lt  lt  dst1  lt  lt        lt  lt  std  endl          or in-place with the  amp  0  trick to skip the new delete     std  string dst2      dst2 resize src size     1       dst2 resize urldecode  amp dst2 0   src c str    1        std  cout  lt  lt   dst2      lt  lt  dst2  lt  lt        lt  lt  std  endl      Outputs   src   some url foo    bar  2e   dst1   some url bar   dst2   some url bar     And the actual function    include  lt stddef h gt   include  lt ctype h gt          decode a percent-encoded C string with optional path normalization       The buffer pointed to by  dst must be at least strlen  src  bytes     Decoding stops at the first character from  src that decodes to null     Path normalization will remove redundant slashes and slash dot sequences     as well as removing path components when slash dot dot is found  It will    keep the root slash  if one was present  and will stop normalization    at the first questionmark found  so query parameters won t be normalized          param dst       destination buffer     param src       source buffer     param normalize perform path normalization if nonzero     return          number of valid characters in  dst     author          Johan Lindh  lt johan linkdata se gt      legalese        BSD licensed  http   opensource org licenses BSD-2-Clause      ptrdiff t urldecode char  dst  const char  src  int normalize        char  org dst   dst      int slash dot dot   0      char ch  a  b      do           ch    src            if  ch         amp  amp  isxdigit a   src 0    amp  amp  isxdigit b   src 1                  if  a  lt   A   a -   0               else if a  lt   a   a -   A  - 10              else a -   a  - 10              if  b  lt   A   b -   0               else if b  lt   a   b -   A  - 10              else b -   a  - 10              ch   16   a   b              src    2                    if  normalize                switch  ch                case                      if  slash dot dot  lt  3                           compress consecutive slashes and remove slash-dot                        dst -  slash dot dot                      slash dot dot   1                      break                                       fall-through                case                         at start of query  stop normalizing                    if  ch                             normalize   0                     fall-through                case   0                   if  slash dot dot  gt  1                           remove trailing slash-dot- dot                         dst -  slash dot dot                         remove parent directory if it was two dots                        if  slash dot dot    3                          while  dst  gt  org dst  amp  amp   --dst                                        empty body                         slash dot dot    ch           1   0                         keep the root slash if any                        if   slash dot dot  amp  amp  dst    org dst  amp  amp   dst                                   dst                                    break              case                      if  slash dot dot    1    slash dot dot    2                          slash dot dot                      break                                       fall-through                default                  slash dot dot   0                                   dst     ch        while ch       return  dst - org dst  - 1

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I ended up on this question when searching for an api to decode url in a win32 c   app  Since the question doesn t quite specify platform assuming windows isn t a bad thing   InternetCanonicalizeUrl is the API for windows programs  More info here          LPTSTR lpOutputBuffer   new TCHAR 1           DWORD dwSize   1          BOOL fRes     InternetCanonicalizeUrl strUrl  lpOutputBuffer   amp dwSize  ICU DECODE   ICU NO ENCODE           DWORD dwError     GetLastError            if   fRes  amp  amp  dwError    ERROR INSUFFICIENT BUFFER                        delete lpOutputBuffer              lpOutputBuffer   new TCHAR dwSize               fRes     InternetCanonicalizeUrl strUrl  lpOutputBuffer   amp dwSize  ICU DECODE   ICU NO ENCODE               if  fRes                                  lpOutputBuffer has decoded url                           else                                 failed to decode                           if  lpOutputBuffer   NULL                                delete    lpOutputBuffer                  lpOutputBuffer   NULL                                  else                         some other error OR the input string url is just 1 char and was successfully decoded             InternetCrackUrl  here  also seems to have flags to specify whether to decode url

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cpp-netlib has functions  namespace boost     namespace network       namespace uri             inline std  string decoded const std  string  amp input         inline std  string encoded const std  string  amp input                 they allow to encode and decode URL strings very easy

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CGICC includes methods to do url encode and decode  form urlencode and form urldecode

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Had to do it in a project without Boost  So  ended up writing my own  I will just put it on GitHub  https   github com corporateshark LUrlParser  clParseURL URL   clParseURL  ParseURL   https   name pwd github com 80 path res      if   URL IsValid           cout  lt  lt   Scheme         lt  lt  URL m Scheme  lt  lt  endl      cout  lt  lt   Host           lt  lt  URL m Host  lt  lt  endl      cout  lt  lt   Port           lt  lt  URL m Port  lt  lt  endl      cout  lt  lt   Path           lt  lt  URL m Path  lt  lt  endl      cout  lt  lt   Query          lt  lt  URL m Query  lt  lt  endl      cout  lt  lt   Fragment       lt  lt  URL m Fragment  lt  lt  endl      cout  lt  lt   User name      lt  lt  URL m UserName  lt  lt  endl      cout  lt  lt   Password       lt  lt  URL m Password  lt  lt  endl

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the juicy bits   include  lt ctype h gt     isdigit  tolower  from hex char ch      return isdigit ch    ch -  0    tolower ch  -  a    10     char to hex char code      static char hex      0123456789abcdef     return hex code  amp  15       noting that   char d   from hex hex 0    lt  lt  4   from hex hex 1      as in      7B        char d   from hex  7    lt  lt  4   from hex  B

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Inspired by xperroni I wrote a decoder  Thank you for the pointer    include  lt iostream gt   include  lt sstream gt   include  lt string gt   using namespace std   char from hex char ch        return isdigit ch    ch -  0    tolower ch  -  a    10     string url decode string text        char h      ostringstream escaped      escaped fill  0         for  auto i   text begin    n   text end    i    n    i            string  value type c     i            if  c                       if  i 1   amp  amp  i 2                     h   from hex i 1    lt  lt  4   from hex i 2                    escaped  lt  lt  h                  i    2                          else if  c                       escaped  lt  lt                 else               escaped  lt  lt  c                       return escaped str       int main int argc  char   argv        string msg    J C3 B8rn        cout  lt  lt  msg  lt  lt  endl      string decodemsg   url decode msg       cout  lt  lt  decodemsg  lt  lt  endl       return 0      edit  Removed unneeded cctype and iomainip includes

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Answering my own question     libcurl has curl easy escape for encoding   For decoding  curl easy unescape

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The Windows API has the functions UrlEscape UrlUnescape  exported by shlwapi dll  for this task

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you can simply use function AtlEscapeUrl   from atlutil h  just go through its documentation on how to use it

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Necromancer mode on  Stumbled upon this question when was looking for fast  modern  platform independent and elegant solution  Didnt like any of above  cpp-netlib would be the winner but it has horrific memory vulnerability in  decoded  function  So I came up with boost s spirit qi karma solution   namespace bsq   boost  spirit  qi  namespace bk   boost  spirit  karma  bsq  int parser lt unsigned char  16  2  2 gt  hex byte  template  lt typename InputIterator gt  struct unescaped string       bsq  grammar lt InputIterator  std  string char const    gt      unescaped string     unescaped string  base type unesc str        unesc char add                 unesc str     unesc char        gt  gt  hex byte   bsq  char           bsq  rule lt InputIterator  std  string char const    gt  unesc str    bsq  symbols lt char const  char const gt  unesc char      template  lt typename OutputIterator gt  struct escaped string   bk  grammar lt OutputIterator  std  string char const    gt      escaped string     escaped string  base type esc str         esc str     bk  char   a-zA-Z0-9   -          lt  lt  bk  right align 2 0  bk  hex          bk  rule lt OutputIterator  std  string char const    gt  esc str       The usage of above as following   std  string unescape const std  string  amp input      std  string retVal    retVal reserve input size       typedef std  string  const iterator iterator type     char const  start         iterator type beg   input begin      iterator type end   input end      unescaped string lt iterator type gt  p     if   bsq  parse beg  end  p start   retVal       retVal   input    return retVal     std  string escape const std  string  amp input      typedef std  back insert iterator lt std  string gt  sink type    std  string retVal    retVal reserve input size     3     sink type sink retVal     char const  start          escaped string lt sink type gt  g    if   bk  generate sink  g start   input       retVal   input    return retVal       Necromancer mode off   EDIT01  fixed the zero padding stuff - special thanks to Hartmut KaiserEDIT02  Live on CoLiRu

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Another solution is available using Facebook s folly library   folly  uriEscape and folly  uriUnescape

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I faced the encoding half of this problem the other day  Unhappy with the available options  and after taking a look at this C sample code  i decided to roll my own C   url-encode function    include  lt cctype gt   include  lt iomanip gt   include  lt sstream gt   include  lt string gt   using namespace std   string url encode const string  amp value        ostringstream escaped      escaped fill  0        escaped  lt  lt  hex       for  string  const iterator i   value begin    n   value end    i    n    i            string  value type c     i               Keep alphanumeric and other accepted characters intact         if  isalnum c     c     -     c           c           c                       escaped  lt  lt  c              continue                        Any other characters are percent-encoded         escaped  lt  lt  uppercase          escaped  lt  lt       lt  lt  setw 2   lt  lt  int  unsigned char  c           escaped  lt  lt  nouppercase             return escaped str        The implementation of the decode function is left as an exercise to the reader   P

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string urlDecode string  amp SRC        string ret      char ch      int i  ii      for  i 0  i lt SRC length    i              if  int SRC i    37                sscanf SRC substr i 1 2  c str      x    amp ii               ch static cast lt char gt  ii               ret  ch              i i 2            else               ret  SRC i                       return  ret       not the best  but working fine  -

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Ordinarily adding     to the int value of a char will not work when encoding  the value is supposed to the the hex equivalent   e g     is   2F  not   47    I think this is the best and concise solutions for both url encoding and decoding  No much header dependencies    string urlEncode string str       string new str           char c      int ic      const char  chars   str c str        char bufHex 10       int len   strlen chars        for int i 0 i lt len i             c   chars i           ic   c             uncomment this if you want to encode spaces with             if  c       new str                    else   if  isalnum c     c     -     c           c           c         new str    c          else               sprintf bufHex   X  c               if ic  lt  16                   new str      0                else                 new str                     new str    bufHex                      return new str      string urlDecode string str       string ret      char ch      int i  ii  len   str length         for  i 0  i  lt  len  i             if str i                       if str i                          ret                     else                 ret    str i            else              sscanf str substr i   1  2  c str      x    amp ii               ch   static cast lt char gt  ii               ret    ch              i   i   2                      return ret

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I know the question asks for a C   method  but for those who might need it  I came up with a very short function in plain C to encode a string  It doesn t create a new string  rather it alters the existing one  meaning that it must have enough size to hold the new string  Very easy to keep up   void urlEncode char  string        char charToEncode      int posToEncode      while    posToEncode strspn string  1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-        0   amp  amp  posToEncode lt strlen string                  charToEncode string posToEncode           memmove string posToEncode 3 string posToEncode 1 strlen string posToEncode            string posToEncode               string posToEncode 1   0123456789ABCDEF  charToEncode gt  gt 4           string posToEncode 2   0123456789ABCDEF  charToEncode amp 0xf           string  posToEncode 3

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I couldn t find a URI decode unescape here that also decodes 2 and 3 byte sequences  Contributing my own high performance version  that on-the-fly converts the c sting input to a wstring    include  lt string gt   const char HEX2DEC 55           0  1  2  3   4  5  6  7   8  9 -1 -1  -1 -1 -1 -1      -1 10 11 12  13 14 15 -1  -1 -1 -1 -1  -1 -1 -1 -1      -1 -1 -1 -1  -1 -1 -1 -1  -1 -1 -1 -1  -1 -1 -1 -1      -1 10 11 12  13 14 15      define   x2d   s  HEX2DEC   s -48   define   x2d2   s    x2d   s   lt  lt  4     x2d   s 1   std  wstring decodeURI const char   s        unsigned char b      std  wstring ws      while   s            if   s                     if   b     x2d2   s   1    gt   0x80                    if  b  gt   0xE0       three byte codepoint                     ws      b  amp  0b00001111   lt  lt  12        x2d2   s   4   amp  0b00111111   lt  lt  6       x2d2   s   7   amp  0b00111111                       s    9                                    else      two byte codepoint                     ws       x2d2   s   4   amp  0b00111111     b  amp  0b00000011   lt  lt  6                      s    6                                              else      one byte codepoints                 ws    b                  s    3                        else      no               ws     s              s                        return ws

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You can use  g uri escape string    function provided glib h  https   developer gnome org glib stable glib-URI-Functions html   include  lt stdio h gt   include  lt stdlib h gt   include  lt glib h gt  int main         char  uri    http   www example com hello world       char  encoded uri   NULL        as per wiki  https   en wikipedia org wiki Percent-encoding      char  escape char str             amp                  encoded uri   g uri escape string uri  escape char str  TRUE       printf    s  n   encoded uri       free encoded uri        return 0      compile it with   gcc encoding URI c  pkg-config --cflags --libs glib-2 0

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