CURLOPT_READFUNCTION(3)

💖 CURLOPT_READFUNCTION(3) 💖

Okay bestie, welcome to the Library Functions Manual — this one is CURLOPT_READFUNCTION(3), aka the read callback for data uploads. It's giving "let me feed libcurl the bytes it needs" energy, and honestly? Iconic. 🎀

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✨ SYNOPSIS ✨

#include <curl/curl.h>

size_t read_callback(char *buffer, size_t size, size_t nitems, void *userdata);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READFUNCTION, read_callback);

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🌸 DESCRIPTION 🌸

You pass a pointer to your callback function, exactly like the prototype above shows. No cap. 🫶

This callback function gets called by libcurl as soon as it needs to read data in order to send it to the peer — like if you ask it to upload or post data to the server. The data area pointed at by the pointer buffer should be filled up with at most nitems number of bytes by your function. And size is always 1. Literally always. 💅

Set the userdata argument with the CURLOPT_READDATA(3) option.

Your function must return the actual number of bytes that it stored in the data area pointed at by the pointer buffer. Returning 0 signals end-of-file to the library and causes it to stop the current transfer.

😭 Careful, bestie: if you stop the current transfer by returning 0 "pre-maturely" (i.e before the server expected it, like when you have said you would upload N bytes and you upload less than N bytes), you may experience that the server "hangs" waiting for the rest of the data that is not sent.

The read callback may return CURL_READFUNC_ABORT to stop the current operation immediately, resulting in a CURLE_ABORTED_BY_CALLBACK error code from the transfer. Abort mission, but make it official. 💀

The callback can return CURL_READFUNC_PAUSE to cause reading from this connection to pause. See curl_easy_pause(3) for further details. Pause, hydrate, comeback. 🧚

🐛 Bugs: when doing TFTP uploads, you must return the exact amount of data that the callback wants, or it is considered the final packet by the server end and the transfer ends there. Exact. Amount. Periodt.

If you set this callback pointer to NULL, or do not set it at all, the default internal read function is used. It is doing an fread() on the FILE * userdata set with CURLOPT_READDATA(3). So yes, the default is lowkey doing fread things for you. 🍓

You can set the total size of the data you are sending by using CURLOPT_INFILESIZE_LARGE(3) or CURLOPT_POSTFIELDSIZE_LARGE(3), depending on the type of transfer. For some transfer types it may be required and it allows for better error checking. Helpful and valid. 🌟

When this option is used in combination with telling libcurl to follow redirects with CURLOPT_FOLLOWLOCATION(3), the data might need to be rewound and sent again. The CURLOPT_SEEKFUNCTION(3) can then be invoked for that rewind operation. Rewind era, honestly. ⏪

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🎀 DEFAULT 🎀

fread(3) — that's the default, periodt. 💖

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🦋 PROTOCOLS 🦋

This functionality affects all supported protocols. Every single one it supports, no cap. 🌈

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🍓 EXAMPLE 🍓

Here's the example, bestie — keep the code exactly as it is, don't freestyle the tokens:

size_t read_callback(char *ptr, size_t size, size_t nmemb, void *userdata)
{
  FILE *readhere = (FILE *)userdata;
  curl_off_t nread;

  /* copy as much data as possible into the 'ptr' buffer, but no more than
     'size' * 'nmemb' bytes. */
  size_t retcode = fread(ptr, size, nmemb, readhere);

  nread = (curl_off_t)retcode;

  fprintf(stderr, "*** We read %" CURL_FORMAT_CURL_OFF_T
          " bytes from file\n", nread);
  return retcode;
}

int main(int argc, char **argv)
{
  FILE *file = fopen(argv[1], "rb");
  CURLcode result;

  CURL *curl = curl_easy_init();
  if(curl) {
    /* set callback to use */
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);

    /* pass in suitable argument to callback */
    curl_easy_setopt(curl, CURLOPT_READDATA, (void *)file);

    result = curl_easy_perform(curl);
  }
}

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💜 HISTORY 💜

The CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READFUNC_ABORT was added in 7.12.1. Ancient history, but still serving looks. 👑

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⭐ AVAILABILITY ⭐

Added in curl 7.1. Old-school cool. 🦋

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🌈 RETURN VALUE 🌈

This returns CURLE_OK. Easy. Slay. ✨

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👑 SEE ALSO 👑

• CURLOPT_POST(3)
• CURLOPT_READDATA(3)
• CURLOPT_SEEKFUNCTION(3)
• CURLOPT_UPLOAD(3)
• CURLOPT_UPLOAD_BUFFERSIZE(3)
• CURLOPT_WRITEFUNCTION(3)

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xoxo, libcurl 💖 — dated 2026-10-12, mwah 😘