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
_mime_data_cb§ion=3">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 _easy_pause§ion=3">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 7.77.0 November 5, 2020 curl_mime_data_cb(3)
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