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curl_mime_data_cb(3)		libcurl	Manual		  curl_mime_data_cb(3)

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	signa-
       ture 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_SEEK-
       FUNC_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,  result-
       ing  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 ar-
       gument.

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	7.74.0		       November	05, 2020	  curl_mime_data_cb(3)

NAME | SYNOPSIS | DESCRIPTION | AVAILABILITY | RETURN VALUE | EXAMPLE | SEE ALSO

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