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.TH CURLOPT_READFUNCTION 3 "16 Jun 2014" "libcurl 7.37.0" "curl_easy_setopt options"
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 \fIbuffer\fP should be
filled up with at most \fIsize\fP multiplied with \fInitems\fP number of bytes
by your function.
Set the \fIuserdata\fP argument with the \fICURLOPT_READDATA(3)\fP option.
Your function must return the actual number of bytes that it stored in the data
area pointed at by the pointer \fIbuffer\fP. 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 \fICURL_READFUNC_ABORT\fP to stop the current
operation immediately, resulting in a \fICURLE_ABORTED_BY_CALLBACK\fP error
code from the transfer.
The callback can return \fICURL_READFUNC_PAUSE\fP to cause reading from this
connection to pause. See \fIcurl_easy_pause(3)\fP for further details.
\fBBugs\fP: 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 don't set it at all, the default
internal read function will be used. It is doing an fread() on the FILE *
userdata set with \fICURLOPT_READDATA(3)\fP.
The default internal read callback is fread().
This is used for all protocols when doing uploads.
size_t read_callback(void *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.