try to discover if there is input waiting on stdin

otherwise give the user a hint
resolves #12

tool/util.c

@@ -57,6 +57,23 @@ FILE *open_file(const char *file_name, int mode) {
   return file;
 }
 
+bool input_ready(FILE *file) {
+  int fd = fileno(file);
+  fd_set rfds;
+  struct timeval tv;
+
+  FD_ZERO(&rfds);
+  FD_SET(fd, &rfds);
+
+  tv.tv_sec = 0;
+  tv.tv_usec = 0;
+
+  if(select(fd + 1, &rfds, NULL, NULL, &tv) == 1) {
+    return true;
+  }
+  return false;
+}
+
 unsigned char get_algorithm(EVP_PKEY *key) {
   int type = EVP_PKEY_type(key->type);
   switch(type) {

tool/util.h

@@ -45,5 +45,6 @@ unsigned char get_algorithm(EVP_PKEY*);
 FILE *open_file(const char*, int);
 int get_object_id(enum enum_slot slot);
 bool set_component_with_len(unsigned char**, const BIGNUM*, int);
+bool input_ready(FILE*);
 
 #endif

tool/yubico-piv-tool.c

@@ -290,6 +290,10 @@ static bool import_key(ykpiv_state *state, enum enum_key_format key_format,
     return false;
   }
 
+  if(input_file == stdin && !input_ready(input_file)) {
+    fprintf(stderr, "Please paste the private key...\n");
+  }
+
   if(key_format == key_format_arg_PEM) {
     private_key = PEM_read_PrivateKey(input_file, NULL, NULL, password);
     if(!private_key) {
@@ -412,6 +416,10 @@ static bool import_cert(ykpiv_state *state, enum enum_key_format cert_format,
     return false;
   }
 
+  if(input_file == stdin && !input_ready(input_file)) {
+    fprintf(stderr, "Please paste the certificate...\n");
+  }
+
   if(cert_format == key_format_arg_PEM) {
     cert = PEM_read_X509(input_file, NULL, NULL, password);
     if(!cert) {
@@ -555,6 +563,10 @@ static bool request_certificate(ykpiv_state *state, enum enum_key_format key_for
     goto request_out;
   }
 
+  if(input_file == stdin && !input_ready(input_file)) {
+    fprintf(stderr, "Please paste the public key...\n");
+  }
+
   if(key_format == key_format_arg_PEM) {
     public_key = PEM_read_PUBKEY(input_file, NULL, NULL, NULL);
     if(!public_key) {
@@ -734,6 +746,10 @@ static bool selfsign_certificate(ykpiv_state *state, enum enum_key_format key_fo
     goto selfsign_out;
   }
 
+  if(input_file == stdin && !input_ready(input_file)) {
+    fprintf(stderr, "Please paste the public key...\n");
+  }
+
   if(key_format == key_format_arg_PEM) {
     public_key = PEM_read_PUBKEY(input_file, NULL, NULL, NULL);
     if(!public_key) {
@@ -1007,6 +1023,10 @@ static bool sign_file(ykpiv_state *state, const char *input, const char *output,
     return false;
   }
 
+  if(input_file == stdin && !input_ready(input_file)) {
+    fprintf(stderr, "Please paste the input...\n");
+  }
+
   output_file = open_file(output, OUTPUT);
   if(!output_file) {
     return false;