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 will be used upon resending data (i.e.: after a redirect); this pointer may be set to  NULL,  in
       which case a resend is 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  will  be
       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  nmemb  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 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've  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 won't 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
       will be called twice with the same argument.

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.

EXAMPLE

       Sending a huge data string will cause 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.

       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;
       }

        CURL *easy = curl_easy_init();
        curl_mime *mime = curl_mime_init(easy);
        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);

SEE ALSO

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

libcurl 7.68.0                                   April 17, 2018                             curl_mime_data_cb(3)