I had the need to access some C library functions that weren't available via standard Tcl or [TclX], so I wrote a simple wrapper using [Critcl].

These are wrappers for some of the set/get uid functions in Unix, but you could use the same approach for most functions - [stevel] April 5 2004

Compile using 
   $ critcl -pkg uid.tcl

   package require critcl
   package provide uid 1.0

  if {![critcl::compiling]} {
    puts stderr "This extension cannot be compiled without critcl enabled"
    exit 1
  }

  critcl::ccode {
    #include <sys/types.h>
    #include <unistd.h>
    #include <pwd.h>
  }

  critcl::cproc setusergroup {char* name} int {
    struct passwd *pwd = getpwnam(name);
    if (pwd == NULL) {
            return 0;
    }
    initgroups(name,pwd->pw_gid);
    setgid(pwd->pw_gid);
    setuid(pwd->pw_uid);
    return 1;
  }
 
  critcl::cproc setuid {char* name} int {
    struct passwd *pwd = getpwnam(name);
    if (pwd == NULL) {
            return 0;
    }
    setuid(pwd->pw_uid);
    return 1;
  }
  
  critcl::cproc seteuid {char* name} int {
    struct passwd *pwd = getpwnam(name);
    if (pwd == NULL) {
    }
    seteuid(pwd->pw_uid);
    return 1;
  }

  critcl::cproc getuid {} int {
    return getuid();
  }

  critcl::cproc geteuid {} int {
    return geteuid();
  }

  critcl::cproc setsid {} int {
    return setsid();
  }