NAME

       curl_mime_data_cb - set a callback-based data source for a mime part's body

SYNOPSIS

       #include <curl/curl.h>

       size_t readfunc(char *buffer, size_t size, size_t nitems, void *arg);

       int seekfunc(void *arg, curl_off_t offset, int origin);

       void freefunc(void *arg);

       CURLcode curl_mime_data_cb(curl_mimepart *part, curl_off_t datasize,
                                  curl_read_callback readfunc,
                                  curl_seek_callback seekfunc,
                                  curl_free_callback freefunc, void *arg);

DESCRIPTION

       curl_mime_data_cb(3)  sets  the  data  source  of  a  mime  part's body content from a data read callback
       function.

       part is the part's to assign contents to.

       readfunc is a pointer to a data read callback function, with a signature as shown by the above prototype.
       It may not be set to NULL.

       seekfunc is a pointer to a seek callback function, with a signature as shown by the above prototype. This
       function is used when resending data (i.e.: after a redirect); this pointer may be set to NULL, in  which
       case a resend might not be not possible.

       freefunc  is  a  pointer  to  a user resource freeing callback function, with a signature as shown by the
       above prototype. If no resource is to be freed, it may safely be set to NULL.  This  function  is  called
       upon mime structure freeing.

       arg is a user defined argument to callback functions.

       The read 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.

       Your read function must then return the actual number of bytes  that  it  stored  in  that  memory  area.
       Returning 0 signals 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 intend to upload N bytes and yet you upload less than N bytes), you may experience
       that the server "hangs" waiting for the rest of the data that does 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.

       The  seek  function  gets called by libcurl to rewind input stream data or to seek to a certain position.
       The function shall work like fseek(3) or lseek(3) and it gets SEEK_SET, SEEK_CUR or SEEK_END as  argument
       for origin, although libcurl currently only passes SEEK_SET.

       The  callback  function  must  return CURL_SEEKFUNC_OK on success, CURL_SEEKFUNC_FAIL to cause the upload
       operation to fail or CURL_SEEKFUNC_CANTSEEK to indicate that while the seek failed, libcurl  is  free  to
       work  around  the problem if possible. The latter can sometimes be done by instead reading from the input
       or similar.

       Care must be taken if the part is bound to a curl easy handle that is later duplicated: the  arg  pointer
       argument  is  also  duplicated,  resulting  in the pointed item to be shared between the original and the
       copied handle. In particular, special attention should be given to the freefunc procedure code  since  it
       then gets called twice with the same argument.

EXAMPLE

       Sending  a huge data string causes the same amount of memory to be allocated: to avoid overhead resources
       consumption, one might want to use a callback source to avoid data duplication. In  this  case,  original
       data must be retained until after the transfer terminates.
       #include <string.h> /* for memcpy */
       char hugedata[512000];

       struct ctl {
         char *buffer;
         curl_off_t size;
         curl_off_t position;
       };

       size_t read_callback(char *buffer, size_t size, size_t nitems, void *arg)
       {
         struct ctl *p = (struct ctl *) arg;
         curl_off_t sz = p->size - p->position;

         nitems *= size;
         if(sz > nitems)
           sz = nitems;
         if(sz)
           memcpy(buffer, p->buffer + p->position, sz);
         p->position += sz;
         return sz;
       }

       int seek_callback(void *arg, curl_off_t offset, int origin)
       {
         struct ctl *p = (struct ctl *) arg;

         switch(origin) {
         case SEEK_END:
           offset += p->size;
           break;
         case SEEK_CUR:
           offset += p->position;
           break;
         }

         if(offset < 0)
           return CURL_SEEKFUNC_FAIL;
         p->position = offset;
         return CURL_SEEKFUNC_OK;
       }

       int main(void)
       {
         CURL *curl = curl_easy_init();
         if(curl) {
           curl_mime *mime = curl_mime_init(curl);
           curl_mimepart *part = curl_mime_addpart(mime);
           struct ctl hugectl;

           hugectl.buffer = hugedata;
           hugectl.size = sizeof(hugedata);
           hugectl.position = 0;
           curl_mime_data_cb(part, hugectl.size, read_callback, seek_callback, NULL,
                             &hugectl);
         }
       }

AVAILABILITY

       As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.

RETURN VALUE

       CURLE_OK or a CURL error code upon failure.

SEE ALSO

       curl_easy_duphandle(3), curl_mime_addpart(3), curl_mime_data(3), curl_mime_name(3)