How to get file extension from string in C

Question

Given a string  filename conf   how to I verify the extension part   I need a cross platform solution

User · Answer

actually the STL can do this without much code, I advise you learn a bit about the STL because it lets you do some fancy things, anyways this is what I use.

std::string GetFileExtension(const std::string& FileName)
{
    if(FileName.find_last_of(".") != std::string::npos)
        return FileName.substr(FileName.find_last_of(".")+1);
    return "";
}

this solution will always return the extension even on strings like "this.a.b.c.d.e.s.mp3" if it cannot find the extension it will return "".

User · Answer

I d go with boost  filesystem  extension  std  filesystem  path  extension with C  17  but if you cannot use Boost and you just have to verify the extension  a simple solution is   bool ends with const std  string  amp filename  const std  string  amp ext      return ext length    lt   filename length    amp  amp           std  equal ext rbegin    ext rend    filename rbegin        if  ends with filename    conf

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For char array-type strings you can use this    include  lt ctype h gt   include  lt string h gt   int main         char filename      apples bmp       char extension       jpeg        if compare extension filename  extension     true                               else                               return 0     bool compare extension char  filename  char  extension           Sanity checks         if filename    NULL    extension    NULL          return false       if strlen filename     0    strlen extension     0          return false       if strchr filename          NULL    strchr extension          NULL          return false          Iterate backwards through respective strings and compare each char one at a time         for int i   0  i  lt  strlen filename   i                  if tolower filename strlen filename  - i - 1      tolower extension strlen extension  - i - 1                          if i    strlen extension  - 1                  return true            else             break             return false      Can handle file paths in addition to filenames  Works with both C and C    And cross-platform

User · Answer

Using std  string s find rfind solves THIS problem  but if you work a lot with paths then you should look at boost  filesystem  path since it will make your code much cleaner than fiddling with raw string indexes iterators   I suggest boost since it s a high quality  well tested   open source and commercially  free and fully portable library

User · Answer

Here s a function that takes a path filename as a string and returns the extension as a string   It is all standard c    and should work cross-platform for most platforms   Unlike several other answers here  it handles the odd cases that windows  PathFindExtension handles  based on PathFindExtensions s documentation   wstring get file extension  wstring filename         size t last dot offset   filename rfind L             This assumes your directory separators are either   or       size t last dirsep offset   max  filename rfind L       filename rfind L                no dot   no extension     if  last dot offset    wstring  npos           return L            directory separator after last dot   extension of directory  not file         for example  given C  temp old file that has no extension we should return    not  old      if   last dirsep offset    wstring  npos   amp  amp   last dirsep offset  gt  last dot offset            return L         return filename substr  last dot offset   1

User · Answer

Assuming you have access to STL   std  string filename  filename conf    std  string  size type idx   idx   filename rfind        if idx    std  string  npos        std  string extension   filename substr idx 1     else          No extension found     Edit  This is a cross platform solution since you didn t mention the platform   If you re specifically on Windows  you ll want to leverage the Windows specific functions mentioned by others in the thread

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Someone else mentioned boost but I just wanted to add the actual code to do this    include  lt boost filesystem hpp gt  using std  string  string texture           foo- gt GetTextureFilename    string file extension    boost  filesystem  extension texture   cout  lt  lt   attempting load texture named    lt  lt  texture       lt  lt       whose extensions seems to be          lt  lt  file extension  lt  lt  endl     Use JPEG or PNG loader function  or report invalid extension

User · Answer

Good answers but I see most of them has some problems  First of all I think a good answer should work for complete file names which have their path headings  also it should work for linux or windows or as mentioned it should be cross platform  For most of answers  file names with no extension but a path with a folder name including dot  the function will fail to return the correct extension  examples of some test cases could be as follow       const char filename1     C   init d  doc         gt  No extention     const char filename2         doc      relative path name   gt  No extention     const char filename3           emputy file name   gt  No extention     const char filename4     testing      only single name   gt  No extention     const char filename5     tested k doc       normal file name   gt  doc     const char filename6              parent folder   gt  No extention     const char filename7             linux root   gt  No extention     const char filename8      bin test d config lx wize str       ordinary path    gt  str    brian newman  suggestion will fail for filename1 and filename4  and most of other answers which are based on reverse find will fail for filename1  I suggest including the following method in your source  which is function returning index of first character of extension or the length of given string if not found   size t find ext idx const char  fileName        size t len   strlen fileName       size t idx   len-1      for size t i   0    fileName i   i              if    fileName i                        idx   i            else if    fileName   i              fileName   i                         idx   len - 1                      return idx 1      you could use the above code in your c   application like below   std  string get file ext const char  fileName        return std  string fileName  substr find ext idx fileName        The last point in some cases the a folder is given to file name as argument and includes a dot in the folder name the function will return folder s dot trailing so better first to user check that the given name is a filename and not folder name

User · Answer

I use these two functions to get the extension and filename without extension   std  string fileExtension std  string file        std  size t found   file find last of           return file substr found 1       std  string fileNameWithoutExtension std  string file        std  size t found   file find last of           return file substr 0 found           And these regex approaches for certain extra requirements   std  string fileExtension std  string file        std  regex re                                std  smatch result      if std  regex match file result re  return result 1       else return         std  string fileNameWithoutExtension std  string file        std  regex re                                std  smatch result      if std  regex match file result re  return result 1       else return file       Extra requirements that are met by the regex method    If filename is like  config or something like this  extension will be an empty string and filename without extension will be  config  If filename doesn t have any extension  extention will be an empty string  filename without extension will be the filename unchanged      EDIT   The extra requirements can also be met by the following   std  string fileExtension const std  string amp  file       std  string  size type pos file find last of           if pos  std  string  npos amp  amp pos  0 return file substr pos 1       else return         std  string fileNameWithoutExtension const std  string amp  file       std  string  size type pos file find last of           if pos  std  string  npos amp  amp pos  0 return file substr 0 pos       else return file      Note   Pass only the filenames  not path  in the above functions

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splitpath   wsplitpath   splitpath s   wsplitpath w   This is Windows  Platform SDK  only

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If you happen to use Poco libraries you can do    include  lt Poco Path h gt        std  string fileExt   Poco  Path   home user myFile abc   getExtension           abc

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A NET CLI version using System  String     System  String  GetFileExtension System  String  FileName              int Ext FileName- gt LastIndexOf              if  Ext    -1              return FileName- gt Substring Ext 1          return

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You have to make sure you take care of file names with more then one dot   example  c   directoryname file name with too many dots ext would not be handled correctly by strchr or find    My favorite would be the boost filesystem library that have an extension path  function

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I ve stumbled onto this question today myself  even though I already had a working code I figured out that it wouldn t work in some cases   While some people already suggested using some external libraries  I prefer to write my own code for learning purposes   Some answers included the method I was using in the first place  looking for the last       but I remembered that on linux hidden files folders start with      So if file file is hidden and has no extension  the whole file name would be taken for extension  To avoid that I wrote this piece of code   bool getFileExtension const char   dir separator  const std  string  amp  file  std  string  amp  ext        std  size t ext pos   file rfind           std  size t dir pos   file rfind dir separator        if ext pos gt dir pos 1                ext append file begin   ext pos file end             return true             return false      I haven t tested this fully  but I think that it should work

User · Answer

Actually  the easiest way is   char  ext  ext   strrchr filename         One thing to remember  if     doesn t exist in filename  ext will be NULL

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Or you can use this       char  ExtractFileExt char  FileName                std  string s   FileName          int Len   s length            while TRUE                        if FileName Len                          Len--              else                               char  Ext   new char s length  -Len 1                   for int a 0  a lt s length  -Len  a                        Ext a    FileName s length  - s length  -Len  a                   Ext s length  -Len      0                   return Ext                                  This code is cross-platform

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If you consider the extension as the last dot and the possible characters after it  but only if they don t contain the directory separator character  the following function returns the extension starting index  or -1 if no extension found  When you have that you can do what ever you want  like strip the extension  change it  check it etc   long get extension index string path  char dir separator                 Look from the end for the first             but give up if finding a dir separator char first     for long i   path length   - 1  i  gt   0  --i            if path i                        return i                    if path i     dir separator                return -1                      return -1

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You can use strrchr   to find last occurence of   dot  and get   dot  based extensions files  Check the below code for example    include lt stdio h gt   void GetFileExtension const char  file name         int ext            const char  extension   NULL      extension   strrchr file name  ext        if extension    NULL           printf  Invalid extension encountered n            return             printf  File extension is  s n   extension      int main         const char  file name    c    directoryname  file name with too many dots ext       GetFileExtension file name       return 0

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Is this too simple of a solution    include  lt iostream gt   include  lt string gt   int main       std  string fn    filename conf     if fn substr fn find last of        1      conf         std  cout  lt  lt   Yes      lt  lt  std  endl      else       std  cout  lt  lt   No      lt  lt  std  endl

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This is a solution I came up with   Then  I noticed that it is similar to what  serengeor posted   It works with std  string and find last of  but the basic idea will also work if modified to use char arrays and strrchr  It handles hidden files  and extra dots representing the current directory   It is platform independent   string PathGetExtension  string const  amp  path       string ext        Find the last dot  if any    size t dotIdx   path find last of           if   dotIdx    string  npos              Find the last directory separator  if any      size t dirSepIdx   path find last of                   If the dot is at the beginning of the file name  do not treat it as a file extension         e g   a hidden file     alpha          This test also incidentally avoids a dot that is really a current directory indicator         e g     alpha   bravo      if   dotIdx  gt  dirSepIdx   1               ext   path substr  dotIdx                 return ext      Unit test   int TestPathGetExtension  void       int errCount   0     string tests   2                 alpha bravo txt     txt             alpha  bravo                 alpha                  alpha txt     txt            alpha   bravo                alpha   bravo txt     txt              alpha                c   alpha  bravo net  charlie txt     txt             int n   sizeof  tests     sizeof  tests 0        for   int i   0  i  lt  n    i           string ext   PathGetExtension  tests i  0         if   ext    tests i  1                  errCount               return errCount

User · Answer

The best way is to not write any code that does it but call existing methods   In windows  the PathFindExtension method is probably the simplest   So why would you not write your own   Well  take the strrchr example  what happens when you use that method on the following string  c  program files AppleGate Net readme    Is   Net readme  the extension   It is easy to write something that works for a few example cases  but can be much harder to write something that works for all cases

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If you use Qt library  you can give a try to QFileInfo s suffix

User · Answer

Try to use strstr  char  lastSlash  lastSlash   strstr filename

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I used PathFindExtension   function to know whether it is a valid tif file or not    include  lt Shlwapi h gt  bool A2iAWrapperUtility  isValidImageFile string imageFile        char   pStrExtension     PathFindExtension imageFile c str          if  pStrExtension    NULL  amp  amp  strcmp pStrExtension    tif      0                return true             return false

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With C  17 and its std  filesystem  path  extension  the library is the successor to boost  filesystem  you would make your statement more expressive than using e g  std  string    include  lt iostream gt   include  lt filesystem gt     C  17 namespace fs   std  filesystem   int main         fs  path filePath    my path to myFile conf       if  filePath extension        conf      Heed the dot                std  cout  lt  lt  filePath stem    lt  lt    is a valid type       Output   myFile is a valid type             else               std  cout  lt  lt  filePath filename    lt  lt    is an invalid type       Output  e g   myFile cfg is an invalid type            See also std  filesystem  path  stem  std  filesystem  path  filename

[c++] How to get file extension from string in C++

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