1curl_mime_data_cb(3) libcurl Manual curl_mime_data_cb(3)
2
6 curl_mime_data_cb - set a callback-based data source for a mime part's
7 body
8
10 #include <curl/curl.h>
11 size_t readfunc(char *buffer, size_t size, size_t nitems, void *arg);
13 int seekfunc(void *arg, curl_off_t offset, int origin);
15 void freefunc(void *arg);
17 CURLcode curl_mime_data_cb(curl_mimepart *part, curl_off_t datasize,
19 curl_read_callback readfunc,
20 curl_seek_callback seekfunc,
21 curl_free_callback freefunc, void *arg);
22
24 curl_mime_data_cb(3) sets the data source of a mime part's body content
25 from a data read callback function.
26 part is the part's to assign contents to.
28 readfunc is a pointer to a data read callback function, with a signa‐
30 ture as shown by the above prototype. It may not be set to NULL.
31 seekfunc is a pointer to a seek callback function, with a signature as
33 shown by the above prototype. This function will be used upon resending
34 data (i.e.: after a redirect); this pointer may be set to NULL, in
35 which case a resend is not possible.
36 freefunc is a pointer to a user resource freeing callback function,
38 with a signature as shown by the above prototype. If no resource is to
39 be freed, it may safely be set to NULL. This function will be called
40 upon mime structure freeing.
41 arg is a user defined argument to callback functions.
43 The read callback function gets called by libcurl as soon as it needs
45 to read data in order to send it to the peer - like if you ask it to
46 upload or post data to the server. The data area pointed at by the
47 pointer buffer should be filled up with at most size multiplied with
48 nitems number of bytes by your function.
49 Your read function must then return the actual number of bytes that it
51 stored in that memory area. Returning 0 will signal end-of-file to the
52 library and cause it to stop the current transfer.
53 If you stop the current transfer by returning 0 "pre-maturely" (i.e.
55 before the server expected it, like when you have said you will upload
56 N bytes and you upload less than N bytes), you may experience that the
57 server "hangs" waiting for the rest of the data that will not come.
58 The read callback may return CURL_READFUNC_ABORT to stop the current
60 operation immediately, resulting in a CURLE_ABORTED_BY_CALLBACK error
61 code from the transfer.
62 The callback can return CURL_READFUNC_PAUSE to cause reading from this
64 connection to pause. See curl_easy_pause(3) for further details.
65 The seek function gets called by libcurl to rewind input stream data or
67 to seek to a certain position. The function shall work like fseek(3) or
68 lseek(3) and it gets SEEK_SET, SEEK_CUR or SEEK_END as argument for
69 origin, although libcurl currently only passes SEEK_SET.
70 The callback function must return CURL_SEEKFUNC_OK on success,
72 CURL_SEEKFUNC_FAIL to cause the upload operation to fail or CURL_SEEK‐
73 FUNC_CANTSEEK to indicate that while the seek failed, libcurl is free
74 to work around the problem if possible. The latter can sometimes be
75 done by instead reading from the input or similar.
76 Care must be taken if the part is bound to a curl easy handle that is
78 later duplicated: the arg pointer argument is also duplicated, result‐
79 ing in the pointed item to be shared between the original and the
80 copied handle. In particular, special attention should be given to the
81 freefunc procedure code since it will be called twice with the same ar‐
82 gument.
83
85 Sending a huge data string will cause the same amount of memory to be
86 allocated: to avoid overhead resources consumption, one might want to
87 use a callback source to avoid data duplication. In this case, original
88 data must be retained until after the transfer terminates.
89 char hugedata[512000];
91 struct ctl {
93 char *buffer;
94 curl_off_t size;
95 curl_off_t position;
96 };
97 size_t read_callback(char *buffer, size_t size, size_t nitems, void *arg)
99 {
100 struct ctl *p = (struct ctl *) arg;
101 curl_off_t sz = p->size - p->position;
102 nitems *= size;
104 if(sz > nitems)
105 sz = nitems;
106 if(sz)
107 memcpy(buffer, p->buffer + p->position, sz);
108 p->position += sz;
109 return sz;
110 }
111 int seek_callback(void *arg, curl_off_t offset, int origin)
113 {
114 struct ctl *p = (struct ctl *) arg;
115 switch(origin) {
117 case SEEK_END:
118 offset += p->size;
119 break;
120 case SEEK_CUR:
121 offset += p->position;
122 break;
123 }
124 if(offset < 0)
126 return CURL_SEEKFUNC_FAIL;
127 p->position = offset;
128 return CURL_SEEKFUNC_OK;
129 }
130 CURL *easy = curl_easy_init();
132 curl_mime *mime = curl_mime_init(easy);
133 curl_mimepart *part = curl_mime_addpart(mime);
134 struct ctl hugectl;
135 hugectl.buffer = hugedata;
137 hugectl.size = sizeof hugedata;
138 hugectl.position = 0;
139 curl_mime_data_cb(part, hugectl.size, read_callback, seek_callback, NULL,
140 &hugectl);
141
144 As long as at least one of HTTP, SMTP or IMAP is enabled. Added in
145 7.56.0.
146
148 CURLE_OK or a CURL error code upon failure.
149
151 curl_mime_addpart(3), curl_mime_data(3), curl_mime_name(3),
152 curl_easy_duphandle(3)
153libcurl 7.85.0 May 17, 2022 curl_mime_data_cb(3)