libcurl delays for 1 second before uploading data, command-line curl does not

I am using libcurl to send an API command to a local service (i.e. on 127.0.0.1).

The program is intended to replace a shell script (that uses the curl program.)

Everything is working, except that there is a 1-second delay somewhere, i.e 1 second elapses from the time I call curl_easy_perform() to when my read callback function is first called.

The C program is using these options (error checking & callback code omitted):

curl_easy_setopt(curl, CURLOPT_URL, "http://127.0.0.1:12345/x");
curl_easy_setopt(curl, CURLOPT_UPLOAD, 1);
curl_easy_setopt(curl, CURLOPT_INFILESIZE, (long)getLengthOfCommandObject());
curl_easy_setopt(curl, CURLOPT_READFUNCTION, &myReadFunction);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &myWriteFunction);

But if I run curl from the shell like this:

$ curl --data-binary '<command>' http://127.0.0.1:12345/x

it sends the request immediately, without suffering from the 1-second delay.

What might be causing the delay, and is there an option I can set to prevent it?

---

Edit The server is based on mongoose

The reason for the delay was:

• libcurl was sending an Expect: 100-Continue header
• The server (which is based on mongoose) is not configured to send 100 Continue responses automatically.
• libcurl waits up to 1 second for this response. If it does not receive it after this time then it proceeds to send the request body anyway.

A solution on the client side is to disable the Expect header like so:

headers = curl_slist_append(NULL, "Expect:");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
// ...
result = curl_easy_perform(curl);
curl_slist_free_all(headers);

Equivalent fix for PHP client and related PHP question