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22 .TH curl_mime_data_cb 3 "October 04, 2017" "libcurl 7.56.1" "libcurl Manual"
25 curl_mime_data_cb - set a callback-based data source for a mime part's body
27 .B #include <curl/curl.h>
29 size_t readfunc(char *buffer, size_t size, size_t nitems, void *arg);
31 int seekfunc(void *arg, curl_off_t offset, int origin);
33 void freefunc(void *arg);
35 .BI "CURLcode curl_mime_data_cb(curl_mimepart * " part ", curl_off_t " datasize ,
37 .BI " curl_read_callback " readfunc ", curl_seek_callback " seekfunc ,
39 .BI " curl_free_callback " freefunc ", void * " arg ");"
42 \fIcurl_mime_data_cb(3)\fP sets the data source of a mime part's body content
43 from a data read callback function.
45 \fIpart\fP is the part's to assign contents to.
47 \fIreadfunc\fP is a pointer to a data read callback function, with a signature
48 as shown by the above prototype. It may not be set to NULL.
50 \fIseekfunc\fP is a pointer to a seek callback function, with a signature as
51 shown by the above prototype. This function will be used upon resending data
52 (i.e.: after a redirect); this pointer may be set to NULL, in which case a
53 resend is not possible.
55 \fIfreefunc\fP is a pointer to a user resource freeing callback function, with
56 a signature as shown by the above prototype. If no resource is to be freed, it
57 may safely be set to NULL. This function will be called upon mime structure
60 \fIarg\fP is a user defined argument to callback functions.
62 The read callback function gets called by libcurl as soon as it needs to
63 read data in order to send it to the peer - like if you ask it to upload or
64 post data to the server. The data area pointed at by the pointer \fIbuffer\fP
65 should be filled up with at most \fIsize\fP multiplied with \fInmemb\fP number
66 of bytes by your function.
68 Your read function must then return the actual number of bytes that it stored
69 in that memory area. Returning 0 will signal end-of-file to the library and
70 cause it to stop the current transfer.
72 If you stop the current transfer by returning 0 "pre-maturely" (i.e before the
73 server expected it, like when you've said you will upload N bytes and you
74 upload less than N bytes), you may experience that the server "hangs" waiting
75 for the rest of the data that won't come.
77 The read callback may return \fICURL_READFUNC_ABORT\fP to stop the current
78 operation immediately, resulting in a \fICURLE_ABORTED_BY_CALLBACK\fP error
79 code from the transfer.
81 The callback can return \fICURL_READFUNC_PAUSE\fP to cause reading from this
82 connection to pause. See \fIcurl_easy_pause(3)\fP for further details.
84 The seek function gets called by libcurl to rewind input stream data or to
85 seek to a certain position. The function shall work like fseek(3) or lseek(3)
86 and it gets SEEK_SET, SEEK_CUR or SEEK_END as argument for \fIorigin\fP,
87 although libcurl currently only passes SEEK_SET.
89 The callback function must return \fICURL_SEEKFUNC_OK\fP on success,
90 \fICURL_SEEKFUNC_FAIL\fP to cause the upload operation to fail or
91 \fICURL_SEEKFUNC_CANTSEEK\fP to indicate that while the seek failed, libcurl
92 is free to work around the problem if possible. The latter can sometimes be
93 done by instead reading from the input or similar.
96 As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.
98 CURLE_OK or a CURL error code upon failure.
100 Sending a huge data string will cause the same amount of memory to be
101 allocated: to avoid overhead resources consumption, one might want to use a
102 callback source to avoid data duplication. In this case, original data
103 must be retained until after the transfer terminates.
106 char hugedata[512000];
114 size_t read_callback(char *buffer, size_t size, size_t nitems, void *arg)
116 struct ctl *p = (struct ctl *) arg;
117 curl_off_t sz = p->size - p->position;
123 memcpy(buffer, p->buffer + p->position, sz);
128 int seek_callback(void *arg, curl_off_t offset, int origin)
130 struct ctl *p = (struct ctl *) arg;
137 offset += p->position;
142 return CURL_SEEKFUNC_FAIL;
143 p->position = offset;
144 return CURL_SEEKFUNC_OK;
147 CURL *easy = curl_easy_init();
148 curl_mime *mime = curl_mime_init(easy);
149 curl_mimepart *part = curl_mime_addpart(mime);
152 hugectl.buffer = hugedata;
153 hugectl.size = sizeof hugedata;
154 hugectl.position = 0;
155 curl_mime_data_cb(part, hugectl.size, read_callback, seek_callback, NULL,
159 .BR curl_mime_addpart "(3),"
160 .BR curl_mime_data "(3),"
161 .BR curl_mime_name "(3)"