curl/docs/libcurl/opts/CURLOPT_READFUNCTION.3

.\" **************************************************************************
.\" *                                  _   _ ____  _
.\" *  Project                     ___| | | |  _ \| |
.\" *                             / __| | | | |_) | |
.\" *                            | (__| |_| |  _ <| |___
.\" *                             \___|\___/|_| \_\_____|
.\" *
.\" * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
.\" *
.\" * This software is licensed as described in the file COPYING, which
.\" * you should have received as part of this distribution. The terms
.\" * are also available at https://curl.se/docs/copyright.html.
.\" *
.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
.\" * copies of the Software, and permit persons to whom the Software is
.\" * furnished to do so, under the terms of the COPYING file.
.\" *
.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
.\" * KIND, either express or implied.
.\" *
.\" **************************************************************************
.\"
.TH CURLOPT_READFUNCTION 3 "25 Jun 2020" "libcurl 7.71.0" "curl_easy_setopt options"
.SH NAME
CURLOPT_READFUNCTION \- read callback for data uploads
.SH SYNOPSIS
.nf
#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);
.fi
.SH DESCRIPTION
Pass a pointer to your callback function, as the prototype shows above.

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 \fIbuffer\fP should be
filled up with at most \fIsize\fP multiplied with \fInitems\fP number of bytes
by your function.

Set the \fIuserdata\fP argument with the \fICURLOPT_READDATA(3)\fP option.

Your function must return the actual number of bytes that it stored in the data
area pointed at by the pointer \fIbuffer\fP. Returning 0 will signal
end-of-file to the library and cause it to stop the current transfer.

If you stop the current transfer by returning 0 "pre-maturely" (i.e before the
server expected it, like when you have said you will 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 will not come.

The read callback may return \fICURL_READFUNC_ABORT\fP to stop the current
operation immediately, resulting in a \fICURLE_ABORTED_BY_CALLBACK\fP error
code from the transfer.

The callback can return \fICURL_READFUNC_PAUSE\fP to cause reading from this
connection to pause. See \fIcurl_easy_pause(3)\fP for further details.

\fBBugs\fP: when doing TFTP uploads, you must return the exact amount of data
that the callback wants, or it will be considered the final packet by the
server end and the transfer will end there.

If you set this callback pointer to NULL, or do not set it at all, the default
internal read function will be used. It is doing an fread() on the FILE *
userdata set with \fICURLOPT_READDATA(3)\fP.

You can set the total size of the data you are sending by using
\fICURLOPT_INFILESIZE_LARGE(3)\fP or \fICURLOPT_POSTFIELDSIZE_LARGE(3)\fP,
depending on the type of transfer. For some transfer types it may be required
and it allows for better error checking.
.SH DEFAULT
The default internal read callback is fread().
.SH PROTOCOLS
This is used for all protocols when doing uploads.
.SH EXAMPLE
.nf
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;
}

void setup(char *uploadthis)
{
  FILE *file = fopen(uploadthis, "rb");
  CURLcode result;

  /* 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);
}
.fi
.SH AVAILABILITY
CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READFUNC_ABORT
was added in 7.12.1.
.SH RETURN VALUE
This will return CURLE_OK.
.SH "SEE ALSO"
.BR CURLOPT_READDATA "(3), " CURLOPT_WRITEFUNCTION "(3), "
.BR CURLOPT_SEEKFUNCTION "(3), " CURLOPT_UPLOAD "(3), " CURLOPT_POST "(3), "
.BR CURLOPT_UPLOAD_BUFFERSIZE "(3), "