103 lines
2.8 KiB
Markdown
103 lines
2.8 KiB
Markdown
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---
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c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
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SPDX-License-Identifier: curl
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Title: CURLOPT_SEEKFUNCTION
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Section: 3
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Source: libcurl
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See-also:
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- CURLOPT_DEBUGFUNCTION (3)
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- CURLOPT_IOCTLFUNCTION (3)
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- CURLOPT_SEEKDATA (3)
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- CURLOPT_STDERR (3)
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---
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# NAME
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CURLOPT_SEEKFUNCTION - user callback for seeking in input stream
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# SYNOPSIS
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~~~c
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#include <curl/curl.h>
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/* These are the return codes for the seek callbacks */
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#define CURL_SEEKFUNC_OK 0
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#define CURL_SEEKFUNC_FAIL 1 /* fail the entire transfer */
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#define CURL_SEEKFUNC_CANTSEEK 2 /* tell libcurl seeking cannot be done, so
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libcurl might try other means instead */
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int seek_callback(void *clientp, curl_off_t offset, int origin);
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CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_callback);
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~~~
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# DESCRIPTION
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Pass a pointer to your callback function, which should match the prototype
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shown above.
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This function gets called by libcurl to seek to a certain position in the
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input stream and can be used to fast forward a file in a resumed upload
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(instead of reading all uploaded bytes with the normal read
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function/callback). It is also called to rewind a stream when data has already
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been sent to the server and needs to be sent again. This may happen when doing
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an HTTP PUT or POST with a multi-pass authentication method, or when an
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existing HTTP connection is reused too late and the server closes the
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connection. The function shall work like fseek(3) or lseek(3) and it gets
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SEEK_SET, SEEK_CUR or SEEK_END as argument for *origin*, although libcurl
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currently only passes SEEK_SET.
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*clientp* is the pointer you set with CURLOPT_SEEKDATA(3).
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The callback function must return *CURL_SEEKFUNC_OK* on success,
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*CURL_SEEKFUNC_FAIL* to cause the upload operation to fail or
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*CURL_SEEKFUNC_CANTSEEK* to indicate that while the seek failed, libcurl
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is free to work around the problem if possible. The latter can sometimes be
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done by instead reading from the input or similar.
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If you forward the input arguments directly to fseek(3) or lseek(3), note that
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the data type for *offset* is not the same as defined for curl_off_t on
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many systems!
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# DEFAULT
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By default, this is NULL and unused.
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# PROTOCOLS
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HTTP, FTP, SFTP
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# EXAMPLE
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~~~c
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#include <unistd.h> /* for lseek */
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struct data {
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int our_fd;
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};
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static int seek_cb(void *clientp, curl_off_t offset, int origin)
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{
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struct data *d = (struct data *)clientp;
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lseek(d->our_fd, offset, origin);
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return CURL_SEEKFUNC_OK;
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}
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int main(void)
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{
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struct data seek_data;
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CURL *curl = curl_easy_init();
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if(curl) {
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curl_easy_setopt(curl, CURLOPT_SEEKFUNCTION, seek_cb);
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curl_easy_setopt(curl, CURLOPT_SEEKDATA, &seek_data);
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}
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}
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~~~
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# AVAILABILITY
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Added in 7.18.0
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# RETURN VALUE
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Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
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