CURLOPT_POSTFIELDSIZE(3)

💖 CURLOPT_POSTFIELDSIZE(3) 💖

okay so this is a page from the Library Functions Manual, and it's literally about telling curl exactly how big your POST data is so it doesn't have to go measure it itself. bestie, it's giving "let me do the math once so nobody else has to" ✨

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🎀 NAME 🎀

CURLOPT_POSTFIELDSIZE — the size of the POST data you're pointing at, like a little name tag for your payload 🫶

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🌸 SYNOPSIS 🌸

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_POSTFIELDSIZE, long size);

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🦋 DESCRIPTION 🦋

okay so picture this: you wanna post some static data to the server, and you do not want libcurl out here doing a whole strlen() to go measure it, because why would you make it do that when you already know the number? that's literally what this option is for 💅

when you set this option, you can post fully binary data, which otherwise is likely to fail — no cap, binary data and strlen() are not besties 😭

if this size is set to -1, libcurl just uses strlen() to get the size, or it relies on the CURLOPT_READFUNCTION(3) (if you're using it) to signal the end of the data. so like, it's got a fallback plan, she's resourceful 🌈

and if you're posting more than 2GB? girl. use CURLOPT_POSTFIELDSIZE_LARGE(3) instead. this option does not have the range for that, periodt 👑

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⭐ DEFAULT ⭐

-1

(the classic "figure it out yourself" setting, iconic honestly)

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💜 PROTOCOLS 💜

this functionality affects http, mqtt and rtsp 🧚

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🔥 EXAMPLE 🔥

#include <string.h> /* for strlen */

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 */
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long)strlen(data));

    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);

    curl_easy_perform(curl);
  }
}

see how she set the size before setting CURLOPT_POSTFIELDS? yeah. that's the move 🙈

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✨ AVAILABILITY ✨

added in curl 7.2 — an elder, but a legend 👑

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💀 RETURN VALUE 💀

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

CURLE_OK (0) means everything was OK 🎉 — and non-zero means an error occurred, so go check libcurl-errors(3) to find out what went wrong, iykyk 🍓

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🌈 SEE ALSO 🌈

• CURLOPT_POSTFIELDS(3)
• CURLOPT_POSTFIELDSIZE_LARGE(3)

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and that's the whole page, bestie!! fr fr the original was written by the libcurl girlies on 2026-10-12, and it's filed under CURLOPT_POSTFIELDSIZE(3) — mwah, go post some data responsibly 💖🎀🦋