Limits of esp_http_client's set_header

[zliudr]

How many headers can I add to the esp_http_client? Is it limited only by memory, with some sort of linked list structure? The doc is very bare. No example snippets at all on headers. I figured out my answer with the source code. In case you wonder or you have comments.

Here is what I found from http_header.c so I guess it is a linked list.

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/**
 * dictionary item struct, with key-value pair
 */
typedef struct http_header_item {
    char *key;                          /*!< key */
    char *value;                        /*!< value */
    STAILQ_ENTRY(http_header_item) next;   /*!< Point to next entry */
} http_header_item_t;

Here you can see adding an item involves duplicating the value with strdup() so you can free the strings you use to call set_header:

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esp_err_t http_header_set(http_header_handle_t header, const char *key, const char *value)
{
    http_header_item_handle_t item;

    if (value == NULL) {
        return http_header_delete(header, key);
    }

    item = http_header_get_item(header, key);

    if (item) {
        free(item->value);
        item->value = strdup(value);
        http_utils_trim_whitespace(&item->value);
        return ESP_OK;
    }
    return http_header_new_item(header, key, value);
}

The esp_http_client_set_header is just a wrapper over this function:

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esp_err_t esp_http_client_set_header(esp_http_client_handle_t client, const char *key, const char *value)
{
    return http_header_set(client->request->headers, key, value);
}

All these http utils and header calls are convoluted with some calling realloc and others calling strdup. Maybe they were written by different teams.

[zliudr]

What about default headers? What headers and values are included?
I tested:

Host:xxx is included by init()
Content-length:xxx is added by perform()
Content-Type: application/x-www-form-urlencoded is also added by perform() when using POST method
Connection: is not included
User-Agent: is not included
X-Amzn-Trace-Id: Root=1-5e8016c0-b21bbdbb018e6d570295aabf is sent back from httpbin.org so it's gotta be included by perform(), what for?