How to concatenate a name of file using fopen function?

I want to create a file which I create this code:

void create_file(char name[],char user[],char date[]){

FILE* fichier=NULL;
fichier=fopen(strcat(name,".txt"),"a+");
fprintf(fichier,"user: %s connected at : %s \n",user,date);
fclose(fichier);

}

But it doesn't work.

via Chebli Mohamed