CURLOPT_READFUNCTION(3) Library Functions Manual CURLOPT_READFUNCTION(3)
NAME
CURLOPT_READFUNCTION - read callback for data uploads
SYNOPSIS
#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);
DESCRIPTION
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 num-
ber 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 signals
end-of-file to the library and causes it to stop the current transfer.
If you stop the current transfer by returning 0 "pre-maturely" (i.e be-
fore the server expected it, like when you have said you would 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 is not sent.
The read callback may return CURL_READFUNC_ABORT to stop the current op-
eration 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 is considered the final packet by the
server end and the transfer ends there.
If you set this callback pointer to NULL, or do not set it at all, the
default internal read function is 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 CUR-
LOPT_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.
When this option is used in combination with telling libcurl to follow
redirects with CURLOPT_FOLLOWLOCATION(3), the data might need to be re-
wound and sent again. The CURLOPT_SEEKFUNCTION(3) can then be invoked
for that rewind operation.
DEFAULT
fread(3)
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
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;
}
int main(int argc, char **argv)
{
FILE *file = fopen(argv[1], "rb");
CURLcode result;
CURL *curl = curl_easy_init();
if(curl) {
/* 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);
}
}
HISTORY
CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READ-
FUNC_ABORT was added in 7.12.1.
AVAILABILITY
Added in curl 7.1
RETURN VALUE
This returns CURLE_OK.
SEE ALSO
CURLOPT_POST(3), CURLOPT_READDATA(3), CURLOPT_SEEKFUNCTION(3), CUR-
LOPT_UPLOAD(3), CURLOPT_UPLOAD_BUFFERSIZE(3), CURLOPT_WRITEFUNCTION(3)
libcurl 2026-07-12 CURLOPT_READFUNCTION(3)
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