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23 .TH CURLOPT_READFUNCTION 3 "December 13, 2017" "libcurl 7.59.0" "curl_easy_setopt options"
26 CURLOPT_READFUNCTION \- read callback for data uploads
28 #include <curl/curl.h>
30 size_t read_callback(char *buffer, size_t size, size_t nitems, void *instream);
32 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READFUNCTION, read_callback);
35 Pass a pointer to your callback function, as the prototype shows above.
37 This callback function gets called by libcurl as soon as it needs to read data
38 in order to send it to the peer - like if you ask it to upload or post data to
39 the server. The data area pointed at by the pointer \fIbuffer\fP should be
40 filled up with at most \fIsize\fP multiplied with \fInitems\fP number of bytes
43 Your function must then return the actual number of bytes that it stored in
44 that memory area. Returning 0 will signal end-of-file to the library and cause
45 it to stop the current transfer.
47 If you stop the current transfer by returning 0 "pre-maturely" (i.e before the
48 server expected it, like when you've said you will upload N bytes and you
49 upload less than N bytes), you may experience that the server "hangs" waiting
50 for the rest of the data that won't come.
52 The read callback may return \fICURL_READFUNC_ABORT\fP to stop the current
53 operation immediately, resulting in a \fICURLE_ABORTED_BY_CALLBACK\fP error
54 code from the transfer.
56 The callback can return \fICURL_READFUNC_PAUSE\fP to cause reading from this
57 connection to pause. See \fIcurl_easy_pause(3)\fP for further details.
59 \fBBugs\fP: when doing TFTP uploads, you must return the exact amount of data
60 that the callback wants, or it will be considered the final packet by the
61 server end and the transfer will end there.
63 If you set this callback pointer to NULL, or don't set it at all, the default
64 internal read function will be used. It is doing an fread() on the FILE *
65 userdata set with \fICURLOPT_READDATA(3)\fP.
67 The default internal read callback is fread().
69 This is used for all protocols when doing uploads.
71 Here's an example setting a read callback for reading that to upload to an FTP
72 site: https://curl.haxx.se/libcurl/c/ftpupload.html
74 CURL_READFUNC_PAUSE return code was added in 7.18.0 and CURL_READFUNC_ABORT
77 This will return CURLE_OK.
79 .BR CURLOPT_READDATA "(3), " CURLOPT_WRITEFUNCTION "(3), "
80 .BR CURLOPT_SEEKFUNCTION "(3), " CURLOPT_UPLOAD "(3), " CURLOPT_POST "(3), "