CURLOPT_COPYPOSTFIELDS(3)

💅 CURLOPT_COPYPOSTFIELDS(3) — the libcurl manual, but make it make sense

okay bestie, so you know libcurl? iconic library, lives in the Library Functions Manual, and today we're talking about one specific little function: CURLOPT_COPYPOSTFIELDS. it's giving "POST but make it safe" and honestly? we love her 🫶

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✨ NAME ✨

CURLOPT_COPYPOSTFIELDS — the option that has libcurl copy your data for a POST 💖

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🌸 SYNOPSIS 🌸

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_COPYPOSTFIELDS, char *data);

that's it. that's the whole vibe check. ✨

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💫 DESCRIPTION 💫

so you pass a char pointer as the parameter, and that pointer should be the full data to post in an HTTP POST operation. easy.

it behaves exactly like CURLOPT_POSTFIELDS(3), except — and this is the main character moment — the original data gets copied by the library. which means your app is free to overwrite the original data after setting this option. no cap, that's huge. 🔥

now, because the data is copied, you gotta be careful when you're using this option together with CURLOPT_POSTFIELDSIZE(3) or CURLOPT_POSTFIELDSIZE_LARGE(3). here's the tea:

• if the size has not been set prior to CURLOPT_COPYPOSTFIELDS(3), the data is assumed to be a null-terminated string 🎀
• otherwise, the stored size tells the library the byte count to copy 📏

and in any case, the size must not be changed after CURLOPT_COPYPOSTFIELDS(3) — unless another CURLOPT_POSTFIELDS(3) or CURLOPT_COPYPOSTFIELDS(3) option is set. like, don't touch it. periodt. 💀

the best part? your application does not have to keep the string around after setting this option. it's copied. it's safe. it's cozy. 🧚

and if you use this option multiple times? the last set string overrides the previous ones. set it to NULL to disable its use again. slay. 👑

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🎀 DEFAULT 🎀

NULL

(as in, nothing. empty. no vibes until you give her something 💅)

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🦋 PROTOCOLS 🦋

this functionality affects http, mqtt and rtsp 💖

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⭐ EXAMPLE ⭐

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    char local_buffer[1024]="data to send";
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

    /* size of the data to copy from the buffer and send in the request */
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, 12L);

    /* send data from the local stack */
    curl_easy_setopt(curl, CURLOPT_COPYPOSTFIELDS, local_buffer);

    curl_easy_perform(curl);
  }
}

look at her go 😭 stack buffer, copied out, request sent. chef's kiss. 👑

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🌈 AVAILABILITY 🌈

added in curl 7.17.1 — vintage but still iconic ✨

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💜 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 😭 — go check libcurl-errors(3)

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👑 SEE ALSO 👑

• CURLOPT_MIMEPOST(3)
• CURLOPT_POSTFIELDS(3)
• CURLOPT_POSTFIELDSIZE(3)
• CURLOPT_UPLOAD(3)

go read them, iykyk 💅

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okay that's the whole page bestie 🫶 this manual page came from the mad lads over at libcurl, dated 2026-10-12 — and the page itself is CURLOPT_COPYPOSTFIELDS(3). literally iconic of them to document this so we could all understand it. mwah. 🍓✨