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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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23 .TH CURLOPT_WRITEFUNCTION 3 "16 Jun 2014" "libcurl 7.37.0" "curl_easy_setopt options"
24 .SH NAME
25 CURLOPT_WRITEFUNCTION \- set callback for writing received data
26 .SH SYNOPSIS
27 .nf
28 #include <curl/curl.h>
29
30 size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata);
31
32 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_WRITEFUNCTION, write_callback);
33 .SH DESCRIPTION
34 Pass a pointer to your callback function, which should match the prototype
35 shown above.
36
37 This callback function gets called by libcurl as soon as there is data
38 received that needs to be saved. For most transfers, this callback gets called
39 many times and each invoke delivers another chunk of data. \fIptr\fP points to
40 the delivered data, and the size of that data is \fInmemb\fP; \fIsize\fP is
41 always 1.
42
43 The callback function will be passed as much data as possible in all invokes,
44 but you must not make any assumptions. It may be one byte, it may be
45 thousands. The maximum amount of body data that will be passed to the write
46 callback is defined in the curl.h header file: \fICURL_MAX_WRITE_SIZE\fP (the
47 usual default is 16K). If \fICURLOPT_HEADER(3)\fP is enabled, which makes
48 header data get passed to the write callback, you can get up to
49 \fICURL_MAX_HTTP_HEADER\fP bytes of header data passed into it. This usually
50 means 100K.
51
52 This function may be called with zero bytes data if the transferred file is
53 empty.
54
55 The data passed to this function will not be zero terminated!
56
57 Set the \fIuserdata\fP argument with the \fICURLOPT_WRITEDATA(3)\fP option.
58
59 Your callback should return the number of bytes actually taken care of. If
60 that amount differs from the amount passed to your callback function, it'll
61 signal an error condition to the library. This will cause the transfer to get
62 aborted and the libcurl function used will return \fICURLE_WRITE_ERROR\fP.
63
64 If your callback function returns CURL_WRITEFUNC_PAUSE it will cause this
65 transfer to become paused. See \fIcurl_easy_pause(3)\fP for further details.
66
67 Set this option to NULL to get the internal default function used instead of
68 your callback. The internal default function will write the data to the FILE *
69 given with \fICURLOPT_WRITEDATA(3)\fP.
70 .SH DEFAULT
71 libcurl will use 'fwrite' as a callback by default.
72 .SH PROTOCOLS
73 For all protocols
74 .SH AVAILABILITY
75 Support for the CURL_WRITEFUNC_PAUSE return code was added in version 7.18.0.
76 .SH RETURN VALUE
77 This will return CURLE_OK.
78 .SH EXAMPLE
79 A common technique is to use this callback to store the incoming data into a
80 dynamically growing allocated buffer. Like in the getinmemory example:
81 https://curl.haxx.se/libcurl/c/getinmemory.html
82 .SH "SEE ALSO"
83 .BR CURLOPT_WRITEDATA "(3), " CURLOPT_READFUNCTION "(3), "