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curl_mime_data_cb(3)                libcurl               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
       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 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.

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);

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_mime_addpart(3), curl_mime_data(3), curl_mime_name(3),
       curl_easy_duphandle(3)

libcurl 8.1.2                   April 26, 2023            curl_mime_data_cb(3)