bug-fix: snprintf prints NULL in place of the last character (#10419)

* bug-fix: snprintf prints NULL in place of the last character

We need to give snprintf enough space to print the last character and the null character, thus we allocate one extra byte and then ignore it when converting to std::string.

* add comment about extra null-term byte requirement

Author: kallewoof

examples/server/utils.hpp

@@ -333,7 +333,7 @@ static std::string llama_get_chat_template(const struct llama_model * model) {
     if (res < 2) {
         return "";
     } else {
-        std::vector<char> model_template(res, 0);
+        std::vector<char> model_template(res + 1, 0);
         llama_model_meta_val_str(model, template_key.c_str(), model_template.data(), model_template.size());
         return std::string(model_template.data(), model_template.size() - 1);
     }

include/llama.h

@@ -456,6 +456,7 @@ extern "C" {
     // Functions to access the model's GGUF metadata scalar values
     // - The functions return the length of the string on success, or -1 on failure
     // - The output string is always null-terminated and cleared on failure
+    // - When retrieving a string, an extra byte must be allocated to account for the null terminator
     // - GGUF array values are not supported by these functions
 
     // Get metadata value as a string by key name