Using relative filepaths on a portable C++ application

Question

I am developing a portable C++ application. Development environment is Linux. I have a code that loads data from Xml file and create a object model out of it. Currently path to file is provided as /home/myuser/projectdir/xmlfilename.xml. This is problematic when I use from a different computer where the home directory name will be different. I tried something like ~/myuserprojectdir/xmlfilename.xml but it didn't worked.

So is there a standard method in defining file names that will work on variety of platforms without any issues? Or any standard method that will work on Linux machines?

Any thoughts?

Answer 1

You need to locate the user's home directory. To do this, Use getpwent to get the user record and from there the home directory. Then add the rest of the path to your xml file /myuserprojectdir/xmlfilename.xml to the value you get.

This will work even if the users's home directory is not /home/$USER. It works on linux and OSX, and will probably work on windows with cygwin installed.

Here's a working example with error checking omitted for clarity:

#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>

main()
{
  char* user = getlogin();
  struct passwd* userrecord;
  while((userrecord = getpwent()) != 0)
    if (0 == strcmp(user, userrecord->pw_name))
      printf("save file is %s/myuserprojectdir/xmlfilename.xml\n", userrecord->pw_dir);
}

output:

save file is /Users/alex/myuserprojectdir/xmlfilename.xml

This is how it works (from man getpwent):

struct passwd * getpwent(void);
  // The getpwent() function sequentially reads the password database and is intended for programs that wish to
 process the complete list of users.

   struct passwd {
               char    *pw_name;       /* user name */ // <<----- check this one
               char    *pw_passwd;     /* encrypted password */
               uid_t   pw_uid;         /* user uid */
               gid_t   pw_gid;         /* user gid */
               time_t  pw_change;      /* password change time */
               char    *pw_class;      /* user access class */
               char    *pw_gecos;      /* Honeywell login info */
               char    *pw_dir;        /* home directory */ // <<----- read this one
               char    *pw_shell;      /* default shell */
               time_t  pw_expire;      /* account expiration */
               int     pw_fields;      /* internal: fields filled in */
       };

To get the username, use getlogin... here's a snippet from man getlogin.

char * getlogin(void);
  // The getlogin() routine returns the login name of the user associated with the current session ...

Answer 2

Or any standard method that will work on Linux machines?

On Linux, take a look at XDG Base Directory Specification from freedesktop.org. It specifies locations for different kinds of files e.g. data, config, etc.

Answer 3

For portable paths you might want to use boosts Filesystem library.

http://www.boost.org/doc/libs/1_41_0/libs/filesystem/doc/index.htm