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Updated: Wednesday, July 27, 2022
 
 

CURLOPT_READFUNCTION (3)

Name

CURLOPT_READFUNCTION - read callback for data uploads

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);

Description

CURLOPT_READFUNCTION(3)    curl_easy_setopt options    CURLOPT_READFUNCTION(3)



NAME
       CURLOPT_READFUNCTION - read callback for data uploads

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);

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 buffer should be filled up with at most  size  multiplied  with
       nitems number of bytes by your function.

       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 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 CURL_READFUNC_ABORT to  stop  the  current
       operation  immediately,  resulting in a CURLE_ABORTED_BY_CALLBACK error
       code from the transfer.

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

       Bugs: 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 CURLOPT_READDATA(3).

       You  can  set  the total size of the data you are sending by using CUR-
       LOPT_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.

DEFAULT
       The default internal read callback is fread().

PROTOCOLS
       This is used for all protocols when doing uploads.

EXAMPLE
       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);
       }

AVAILABILITY
       CURL_READFUNC_PAUSE return code was  added  in  7.18.0  and  CURL_READ-
       FUNC_ABORT was added in 7.12.1.

RETURN VALUE
       This will return CURLE_OK.


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | web/curl         |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+

SEE ALSO
       CURLOPT_READDATA(3), CURLOPT_WRITEFUNCTION(3), CURLOPT_SEEKFUNCTION(3),
       CURLOPT_UPLOAD(3), CURLOPT_POST(3), CURLOPT_UPLOAD_BUFFERSIZE(3),



NOTES
       Source code for open source software components in Oracle  Solaris  can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-
       code-downloads.html.

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source      was      downloaded       from        https://curl.se/down-
       load/curl-7.83.1.tar.bz2.

       Further information about this software can be found on the open source
       community website at http://curl.haxx.se/.



libcurl 7.83.1                 November 26, 2021       CURLOPT_READFUNCTION(3)