CURLOPT_POSTFIELDS(3)

💅 CURLOPT_POSTFIELDS 💅

okay bestie, this page is from the Library Functions Manual, and it's all about CURLOPT_POSTFIELDS(3) — the option that hands libcurl the actual data you want to POST. 🫶 No cap, the vibe is "you bring the bytes, libcurl sends the bytes."

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✨ NAME ✨

CURLOPT_POSTFIELDS - data to POST to server. 💌

🌸 SYNOPSIS 🌸

#include <curl/curl.h>

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

🦋 DESCRIPTION 🦋

Pass a char pointer as the parameter — it points to the data buffer to use in an HTTP POST operation or an MQTT subscribe. The data must be formatted and encoded the way you want the server to receive it. libcurl does not convert or encode it in any way, periodt. For example, a web server may assume that this data is URL encoded. 🙈

⚠️ Important, bestie: the data pointed to is NOT copied by the library. As a consequence, it must be preserved by the calling application until the associated transfer finishes. You can change this behavior (so libcurl does copy the data) by instead using the CURLOPT_COPYPOSTFIELDS(3) option. 🫶

This POST is a normal application/x-www-form-urlencoded kind (and libcurl sets that Content-Type by default when this option is used), which is commonly used by HTML forms. Change Content-Type with CURLOPT_HTTPHEADER(3). 💅

You can use curl_easy_escape(3) to URL encode your data, if necessary. It returns a pointer to an encoded string that can be passed as postdata. ✨

Using CURLOPT_POSTFIELDS(3) implies setting CURLOPT_POST(3) to 1. slay.

If CURLOPT_POSTFIELDS(3) is explicitly set to NULL then libcurl gets the POST data from the read callback. To send a zero-length (empty) POST, set CURLOPT_POSTFIELDS(3) to an empty string, or set CURLOPT_POST(3) to 1 and CURLOPT_POSTFIELDSIZE(3) to 0. 🎀

🌸 Null-byte tea: libcurl assumes this option points to a null-terminated string unless you also set CURLOPT_POSTFIELDSIZE(3) to specify the length of the provided data, which then is strictly required if you want to send off null bytes included in the data.

Using POST with HTTP 1.1 implies the use of a Expect: 100-continue header, and libcurl adds that header automatically if the POST is either known to be larger than 1MB or if the expected size is unknown. You can disable this header with CURLOPT_HTTPHEADER(3) as usual. 👑

To make multipart/formdata posts, check out the CURLOPT_MIMEPOST(3) option combined with curl_mime_init(3). 🧚

Using this option multiple times makes the last set pointer override the previous ones. Set it to NULL to disable its use again. 💀

🍓 DEFAULT 🍓

NULL

🌈 PROTOCOLS 🌈

This functionality affects http and mqtt.

🔥 EXAMPLE 🔥

/* send an application/x-www-form-urlencoded POST */
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    const char *data = "data to send";

    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

    /* size of the POST data if strlen() is not good enough */
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, 12L);

    /* pass in a pointer to the data - libcurl does not copy */
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);

    curl_easy_perform(curl);
  }

  /* send an application/json POST */
  curl = curl_easy_init();
  if(curl) {
    const char *json = "{\"name\": \"daniel\"}";
    struct curl_slist *slist1 = NULL;
    slist1 = curl_slist_append(slist1, "Content-Type: application/json");
    slist1 = curl_slist_append(slist1, "Accept: application/json");

    /* set custom headers */
    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, slist1);

    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

    /* pass in a pointer to the data - libcurl does not copy */
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, json);

    curl_easy_perform(curl);
  }
}

⭐ AVAILABILITY ⭐

Added in curl 7.1.

💖 RETURN VALUE 💖

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3). 🫶

👑 SEE ALSO 👑

• CURLOPT_COPYPOSTFIELDS(3)
• CURLOPT_MIMEPOST(3)
• CURLOPT_POSTFIELDSIZE(3)
• CURLOPT_READFUNCTION(3)
• CURLOPT_UPLOAD(3)

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🎀 okay that's the whole page, bestie — source/version libcurl, dated 2026-10-12, for CURLOPT_POSTFIELDS(3). mwah 💋