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diff --git a/include/dynstr.h b/include/dynstr.h
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+/*
+   Copyright 2020 Xavier Del Campo Romero
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+#ifndef DYNSTR_H
+#define DYNSTR_H
+
+#include <stddef.h>
+
+#ifdef __GNUC__
+
+/* Since dynstr_append() might fail (as it is based on dynamic memory
+ * allocation), these macros can be used to avoid boilerplate code.
+ * However, C99 variadic macros might expand with a trailing comma
+ * when no parameters are given after "format", a GNU extension is
+ * needed to avoid this problem. */
+#include <stdbool.h>
+
+/**
+ * Convenience macro that calls dynstr_append and returns NULL if failed.
+ */
+#define dynstr_append_or_ret_null(d, format, ...) if (dynstr_append(d, format, ## __VA_ARGS__)) return NULL;
+
+/**
+ * Convenience macro that calls dynstr_append and returns false if failed.
+ */
+#define dynstr_append_or_ret_false(d, format, ...) if (dynstr_append(d, format, ## __VA_ARGS__)) return false;
+
+/**
+ * Convenience macro that calls dynstr_append and returns one if failed.
+ */
+#define dynstr_append_or_ret_nonzero(d, format, ...) if (dynstr_append(d, format, ## __VA_ARGS__)) return 1;
+
+#endif /* __GNUC__ */
+
+/**
+ * Dynamic string type used for this library.
+ * @note If needed, members can be safely read but should not be modified
+ * outside this library.
+ */
+struct dynstr
+{
+    char *str; /**< Null-terminated string. */
+    size_t len; /**< String length, null character not included. */
+};
+
+/**
+ * Reportedly, it initializes an empty string with zero length.
+ * @attention Always call this function when creating a dynstr instance.
+ * @param d Dynamic string to be initialized.
+ */
+void dynstr_init(struct dynstr *d);
+
+/**
+ * This function takes a string literal in printf format and a variable
+ * number of arguments, calculates its size and concatenates it into the
+ * dynamic string.
+ * @param d Dynamic string where new string will be appended.
+ * @param format String literal in printf format.
+ * @return Returns 0 if successful, 1 if alloc operation failed.
+ */
+int dynstr_append(struct dynstr *d, const char *format, ...);
+
+/**
+ * This function frees memory used by the dynamic string.
+ * @param d Dynamic string to be freed.
+ * @attention Attempting to call this function on an uninitialized or empty
+ * instance is undefined behaviour.
+ * @attention Parameters inside the struct are reset once memory is freed.
+ */
+void dynstr_free(struct dynstr *d);
+
+#endif /* DYNSTR_H */