text: curl_mime_data_cb man page
  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 nmemb number of bytes by your function.
  Your read function must then return the actual number of bytes that.
  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..
  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)
  Referenced By
  curl_mime_addpart(3), curl_mime_data(3), libcurl-tutorial(3).