CURLOPT_READFUNCTION - read callback for data uploads
#include <curl/curl.h>
size_t read_callback(char *buffer, size_t size, size_t nitems, void *userdata);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READFUNCTION, read_callback);
Pass a pointer to your callback function, as the prototype shows above.
This 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.
size is always 1.
Set the
userdata argument with the
CURLOPT_READDATA(3) option.
Your function must return the actual number of bytes that it stored in the data
area pointed at by the pointer
buffer. 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.
Bugs: when doing TFTP uploads, you must return the exact amount of data
that the callback wants, or it will be considered the final packet by the
server end and the transfer will end there.
If you set this callback pointer to NULL, or do not set it at all, the default
internal read function will be used. It is doing an fread() on the FILE *
userdata set with
CURLOPT_READDATA(3).
You can set the total size of the data you are sending by using
CURLOPT_INFILESIZE_LARGE(3) or
CURLOPT_POSTFIELDSIZE_LARGE(3),
depending on the type of transfer. For some transfer types it may be required
and it allows for better error checking.
The default internal read callback is fread().
This is used for all protocols when doing uploads.
size_t read_callback(char *ptr, size_t size, size_t nmemb, void *userdata)
{
FILE *readhere = (FILE *)userdata;
curl_off_t nread;
/* copy as much data as possible into the 'ptr' buffer, but no more than
'size' * 'nmemb' bytes! */
size_t retcode = fread(ptr, size, nmemb, readhere);
nread = (curl_off_t)retcode;
fprintf(stderr, "*** We read %" CURL_FORMAT_CURL_OFF_T
" bytes from file\n", nread);
return retcode;
}
void setup(char *uploadthis)
{
FILE *file = fopen(uploadthis, "rb");
CURLcode result;
/* set callback to use */
curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);
/* pass in suitable argument to callback */
curl_easy_setopt(curl, CURLOPT_READDATA, (void *)file);
result = curl_easy_perform(curl);
}
CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READFUNC_ABORT was
added in 7.12.1.
This will return CURLE_OK.
CURLOPT_READDATA(3),
CURLOPT_WRITEFUNCTION(3),
CURLOPT_SEEKFUNCTION(3),
CURLOPT_UPLOAD(3),
CURLOPT_POST(3),
CURLOPT_UPLOAD_BUFFERSIZE(3),