Provided by: libcurl4-doc_7.88.1-8ubuntu1_all bug


       CURLOPT_READFUNCTION - read callback for data uploads


       #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);


       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. size is always 1.

       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

       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


       The default internal read callback is fread().


       This is used for all protocols when doing uploads.


       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);


       CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READFUNC_ABORT was  added  in


       This will return CURLE_OK.