Is there a libcurl curl_easy_setopt() version of curl --data-urlencode

Question

I'm tying to connect to the latest Royal Mail API V4 to get a token Based on existing working code in C and libcurl to connect to V3. I have a working curl example script which gets a token using --data-urlencode which returns a token.

curl -k --location 'https://authentication.proshipping.net/connect/token' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'client_id=AAAAAAA' \
--data-urlencode 'client_secret=BBBBBBBBB' \
--data-urlencode 'grant_type=client_credentials'

I tried the following in C and libcurl

curl_easy_setopt(hnd, CURLOPT_SSL_VERIFYPEER, FALSE);

curl_easy_setopt(hnd, CURLOPT_URL, "https://authentication.proshipping.net/connect/token");
curl_easy_setopt(hnd, CURLOPT_POST, TRUE);

headers = curl_slist_append(headers, "Content-Type: application/x-www-form-urlencoded");
curl_easy_setopt(hnd, CURLOPT_HTTPHEADER, headers);

postfields = curl_slist_append(postfields, "client_id=AAAAAAA");
postfields = curl_slist_append(postfields, "client_secret=BBBBBBBBB");
postfields = curl_slist_append(postfields, "grant_type=client_credentials");
curl_easy_setopt(hnd, CURLOPT_POSTFIELDS, postfields);

curl_easy_setopt(hnd, CURLOPT_WRITEFUNCTION, writefunc);
curl_easy_setopt(hnd, CURLOPT_WRITEDATA, &s);

res = curl_easy_perform(hnd);

But the libcurl sample returns the output

{"error":"invalid_client"}

Can anyone help with --data-urlencode or equivalent command in libcurl? Thanks

Answer 1

There does not appear to be a single easy option that mimics the full functionality of --data-urlencode, which includes features for loading and encoding files with a special syntax, and also concatenates its arguments to form a single blob.

The library function used to URL-encode null-terminated byte strings is curl_easy_escape. The resulting string must be passed to curl_free when the user is done with it.

char *curl_easy_escape(CURL *curl, const char *string, int length);
void curl_free(void *ptr);

(The internal data_urlencode (implementing --data-urlencode) uses curl_easy_escape.)

---

As for CURLOPT_POSTFIELDS,

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_POSTFIELDS, char *postdata);

the third argument is expected to be a null-terminated byte string (by default; can manually size with CURLOPT_POSTFIELDSIZE) containing the full data to send.

From your example, --libcurl generates:

curl_easy_setopt(hnd, CURLOPT_POSTFIELDS, "client_id=AAAAAAA&client_secret=BBBBBBBBB&grant_type=client_credentials");

See also: http-post.c.

---

A vague mimicry of --data-urlencode's most basic functionality (the name=content syntax) would be approximately:

#include <curl/curl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static int data_append(char **to, const char *name, const char *field)
{
    /* 1 if initial segment, 0 if concat, justifies & below */
    unsigned init = (NULL == *to);

    /* first argument ignored in 7.82.0+, otherwise modify `data_append` */
    char *escaped_field = curl_easy_escape(NULL, field, 0);

    if (!escaped_field)
        return 0;

    /* previous */
    size_t offset = !init ? strlen(*to) : 0;
    /* name=escaped */
    size_t length = strlen(name) + 1 + strlen(escaped_field);

    /* previous&name=escaped\0 */
    char *result = realloc(*to, offset + !init + length + 1);

    if (result) {
        sprintf(result + offset, "&%s=%s" + init, name, escaped_field);
        *to = result;
    }

    curl_free(escaped_field);

    return NULL != result;
}

int main(void)
{
    CURL *curl = curl_easy_init();

    char *data = NULL;
    data_append(&data, "name", "field");
    data_append(&data, "foo", "hello world!");
    fprintf(stderr, "DEBUG: Sending: <%s>\n", data);

    curl_easy_setopt(curl, CURLOPT_URL, "https://mockbin.org/request");
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);

    curl_easy_perform(curl);
    curl_easy_cleanup(curl);

    free(data);
}

Results:

./a.out 
DEBUG: Sending: <name=field&foo=hello%20world%21>
...
...
  "postData": {
    "mimeType": "application/x-www-form-urlencoded",
    "text": "name=field&foo=hello%20world%21",
    "params": {
      "name": "field",
      "foo": "hello world!"
    }
  }
...
...

This is roughly

$ curl -L 'https://mockbin.org/request' --data-urlencode 'name=field' --data-urlencode 'foo=hello world!'

(--data-urlencode implements the ' ' => '+' conversion described in RFC1866.)

---

Aside: there is also CURLOPT_MIMEPOST which can be used for multipart/form-data.