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diff --git a/sternenblog/cgiutil.h b/sternenblog/cgiutil.h
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+/*!
+ * @file cgiutil.h
+ * @brief Simple CGI/HTTP helper functions used by sternenblog.
+ */
+
+#include <stdbool.h>
+
+/*!
+ * @brief Print a HTTP header
+ *
+ * Prints a HTTP Header to `stdout` like CGI requires.
+ *
+ * @param key Name of the HTTP Header
+ * @param val Contents of the header to send
+ */
+void send_header(char key[], char val[]);
+
+/*!
+ * @brief Print end of HTTP header section
+ *
+ * Terminates the header section of a CGI/HTTP Response by printing `\r\n` to `stdout`.
+ */
+void terminate_headers(void);
+
+/*!
+ * @brief Value of a HTTP status header for a given status code.
+ *
+ * Helper function that returns the status code plus its
+ * accompanying reason phrase as a string.
+ *
+ * The value is statically allocated so do not attempt
+ * to free it.
+ *
+ * Example usage:
+ *
+ * ```
+ * send_header("Status", http_status_line(404);
+ * // Prints: Status: 404 Not Found
+ * ```
+ *
+ * @param status HTTP status code
+ * @return status code and reason phrase as a string.
+ */
+char *http_status_line(int status);
+
+/*!
+ * @brief Return HTTP error code for given errno
+ *
+ * Incomplete mapping of `errno`s to HTTP error codes.
+ * Defaults to 500.
+ *
+ * @param err POSIX errno
+ * @return HTTP error code
+ */
+int http_errno(int err);
+
+/*!
+ * @brief Urlencode a given dynamically allocated string
+ *
+ * urlencode_realloc() receives a pointer to a pointer to
+ * a dynamically allocated string to encode plus its size
+ * including the null byte at the end.
+ *
+ * It then replaces every reserved character in the string
+ * except `/` with the appropriate percent encoding. If
+ * the size of the buffer is not enough, it uses `realloc()`
+ * to increase it.
+ *
+ * Note that the implementation of url encoding is not 100%
+ * correct, but should be good enough in the context of
+ * sternenblog. `/` is not encoded since on unix
+ * a slash should always a path delimiter and never part of
+ * a filename. Another limitation of the url encoding is
+ * that it only checks for a list of characters to encode
+ * instead of checking if the characters are unreserved
+ * and don't need to be encoded which would be more correct.
+ * The approach taken has the big advantage that we don't
+ * need to worry about UTF-8, which makes the implementation
+ * considerably simpler. As a consequence however it will
+ * be not aggressive enough in terms of encoding in some
+ * cases.
+ *
+ * On error -1 is returned. In such a case the original
+ * pointer remains intact, so you can either `free()` it
+ * or continue with the unencoded string.
+ *
+ * Otherwise it returns new size of the buffer.
+ *
+ * @param **input pointer to input string
+ * @param size size of input string including null byte
+ * @return -1 on error, else size of buffer
+ */
+int urlencode_realloc(char **input, int size);
+
+/*!
+ * @brief Returns URL of server addressed by the current CGI request
+ *
+ * server_url() uses the CGI 1.1 environment variables `SERVER_NAME`
+ * and `SERVER_PORT` to construct an URL to the server the current
+ * request is addressed to. Since CGI only reveals the HTTP version
+ * used and not wether an encrypted version of HTTP is used,
+ * server_url() will use the parameter `https` to decide which protocol
+ * identifier to prefix.
+ *
+ * The returned `char *` is dynamically allocated and must be cleaned
+ * up using `free()` before it goes out of scope.
+ *
+ * @param https if true, prefix `https://` else `http://`
+ * @return Pointer to dynamically allocated char buffer containing the URL.
+ */
+char *server_url(bool https);