full file. should work fine.

logname.c

@@ -0,0 +1,21 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <errno.h>
+
+int main(int argc, char * argv[]) {  
+  char * logname = getlogin();
+
+  if(logname == NULL) {
+    switch(errno) {
+      case EMFILE: fprintf(stderr, "%s: all file descriptors open\n", argv[0]);
+                   break;
+      case ENFILE: fprintf(stderr, "%s: too many files open", argv[0]); break;
+      default: fprintf(stderr, "%s: other/unknown error", argv[0]);
+    }
+    return 1;
+  }
+  else {
+    printf("%s\n", logname);
+    return 0;
+  }
+}