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CURLOPT_READFUNCTION(3) libcurl CURLOPT_READFUNCTION(3)
CURLOPT_READFUNCTION - read callback for data uploads
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;
/* 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);
void setup(char *uploadthis)
FILE *file = fopen(uploadthis, "rb");
/* 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_SEEKFUNCTION(3), CURLOPT_UPLOAD(3), CURLOPT_POST(3),
ibcurl 8.1.2 April 26, 2023 CURLOPT_READFUNCTION(3)