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


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


       #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);


       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 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 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.

       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

       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.


       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

       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;
           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;
         case SEEK_CUR:
           offset += p->position;

         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,


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


       CURLE_OK or a CURL error code upon failure.


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