OK, I understand your idea. However, the patch is not really complete as
a) it doesn't compile for various configurations and
b) you can't just assume to have the function getETagHeader() defined without having documented a prototype anywhere or without any ifdef for it.

Aditionally, right now I'm not sure what happens if these header pointers are left at NULL. It seems like we would call strlen() on them. The standard does not say that "strlen(NULL)==0".

However, this bug could alreay exist in the rest of the httpd code...

Really, I'm not sure I understand this post. Could you supply a patch clarifying your changes?

You have httpd.c file to control GET/POST requests. It works fine. I run my web-server on stm32 microcontroller using it.

But I had to change it with my workarounds to get extra functions as follows:

- ETag and Cache-Control headers for files transmit
- Cookie: Session ID read (and probably other parameter-value pairs)

ETag header allow me to open pages faster. To get this headers work I need following lines in code:
httpd.c:
"""
....
/* The number of individual strings that comprise the headers sent before each

• requested file.

 */
#define NUM_FILE_HDR_STRINGS 7
#define HDR_STRINGS_IDX_HTTP_STATUS          0 /* e.g. "HTTP/1.0 200 OK\r\n" */
#define HDR_STRINGS_IDX_SERVER_NAME          1 /* e.g. "Server: "HTTPD_SERVER_AGENT"\r\n" */
#define HDR_STRINGS_IDX_CONTENT_LEN_KEPALIVE 2 /* e.g. "Content-Length: xy\r\n" and/or "Connection: keep-alive\r\n" */
#define HDR_STRINGS_IDX_CONTENT_LEN_NR       3 /* the byte count, when content-length is used */
#define HDR_STRINGS_IDX_ETAG                 4 /* e.g. "ETag: "2073041041"" */
#define HDR_STRINGS_IDX_CACHE_CONTROL_MAX_AGE 5 /* e.g. "Cache-Control: max-age=31536000\r\nExpires: Tue, 01 Jan 2030 00:00:00 GMT\r\n" */
#define HDR_STRINGS_IDX_CONTENT_TYPE         6 /* the content type (or default answer content type including default document) */
.....


/**

• Generate the relevant HTTP headers for the given filename and write
• them into the supplied buffer.

 */
static void
get_http_headers(struct http_state *hs, const char *uri)
....
  /* We are dealing with a particular filename. Look for one other
      special case.  We assume that any filename with "404" in it must be
      indicative of a 404 server error whereas all other files require
      the 200 OK header. */
  if (strstr(uri, "404")) {
    hs->hdrs[HDR_STRINGS_IDX_HTTP_STATUS] =
g_psHTTPHeaderStrings[HTTP_HDR_NOT_FOUND];
  } else if (strstr(uri, "400")) {
    hs->hdrs[HDR_STRINGS_IDX_HTTP_STATUS] =
g_psHTTPHeaderStrings[HTTP_HDR_BAD_REQUEST];
  } else if (strstr(uri, "501")) {
    hs->hdrs[HDR_STRINGS_IDX_HTTP_STATUS] =
g_psHTTPHeaderStrings[HTTP_HDR_NOT_IMPL];
  } else if (strstr(uri, "304")) {
    hs->hdrs[HDR_STRINGS_IDX_HTTP_STATUS] =
g_psHTTPHeaderStrings[HTTP_HDR_NOT_MODIFIED];
    hs->hdrs[HDR_STRINGS_IDX_CACHE_CONTROL_MAX_AGE] =
g_psHTTPHeaderStrings[CACHE_CONTROL_MAX_AGE];
    hs->hdr_index = 0;
    hs->hdr_pos = 0;
    return;
  } else {
    hs->hdrs[HDR_STRINGS_IDX_HTTP_STATUS] = g_psHTTPHeaderStrings[HTTP_HDR_OK];
  }
....

  /* Reinstate the parameter marker if there was one in the original URI. */
  if (vars) {
    *vars = '?';
  }

  if (hs->handle && hs->handle->is_custom_file){
    char * const eTagHeader = getETagHeader(hs->handle->pextension);
    if (eTagHeader != NULL) {
      hs->hdrs[HDR_STRINGS_IDX_ETAG] = eTagHeader;
      hs->hdrs[HDR_STRINGS_IDX_CACHE_CONTROL_MAX_AGE] =
g_psHTTPHeaderStrings[CACHE_CONTROL_MAX_AGE];
    }
  }
.....


"""

httpd_struct.h:
"""
...

/** A list of strings used in HTTP headers (see RFC 1945 HTTP/1.0 and

• RFC 2616 HTTP/1.1 for header field definitions) */

static const char * const g_psHTTPHeaderStrings[] =
{
 "HTTP/1.0 200 OK\r\n",
 "HTTP/1.0 404 File not found\r\n",
 "HTTP/1.0 400 Bad Request\r\n",
 "HTTP/1.0 501 Not Implemented\r\n",
 "HTTP/1.1 200 OK\r\n",
 "HTTP/1.1 404 File not found\r\n",
 "HTTP/1.1 400 Bad Request\r\n",
 "HTTP/1.1 501 Not Implemented\r\n",
 "Content-Length: ",
 "Connection: Close\r\n",
 "Connection: keep-alive\r\n",
 "Connection: keep-alive\r\nContent-Length: ",
 "Server: "HTTPD_SERVER_AGENT"\r\n",
 "\r\n<html><body><h2>404: The requested file cannot be found.</h2></body></html>\r\n"
#if LWIP_HTTPD_SUPPORT_11_KEEPALIVE
 ,"Connection: keep-alive\r\nContent-Length: 77\r\n\r\n<html><body><h2>404: The requested file cannot be found.</h2></body></html>\r\n"
#endif
 ,"Cache-Control: max-age=31536000\r\nExpires: Tue, 01 Jan 2030 00:00:00 GMT\r\n",
 "HTTP/1.0 304 Not Modified\r\n",
 "HTTP/1.0 302 Found\r\n",
};
...

#if LWIP_HTTPD_SUPPORT_11_KEEPALIVE
#define DEFAULT_404_HTML_PERSISTENT 14 /* default 404 body, but including Connection: keep-alive */
#endif
#if LWIP_HTTPD_SUPPORT_11_KEEPALIVE
#define CACHE_CONTROL_MAX_AGE   15 /* file expires in 1 year */
#define HTTP_HDR_NOT_MODIFIED   16 /* file not modified */
#define HTTP_HDR_FOUND 17 /* file not modified */
#else
#define CACHE_CONTROL_MAX_AGE   14 /* file expires in 1 year */
#define HTTP_HDR_NOT_MODIFIED   15 /* file not modified */
#define HTTP_HDR_FOUND          16 /* file not modified */
#endif
...

"""

HDR_STRINGS_IDX_CONTENT_TYPE I have to put in the end because this header MUST be at the end, that is probably not very universal.