curl_mime_data_cb.3 6.2 KB

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  22. .TH curl_mime_data_cb 3 "22 August 2017" "libcurl 7.56.0" "libcurl Manual"
  23. .SH NAME
  24. curl_mime_data_cb - set a callback-based data source for a mime part's body
  25. .SH SYNOPSIS
  26. .nf
  27. #include <curl/curl.h>
  28. size_t readfunc(char *buffer, size_t size, size_t nitems, void *arg);
  29. int seekfunc(void *arg, curl_off_t offset, int origin);
  30. void freefunc(void *arg);
  31. CURLcode curl_mime_data_cb(curl_mimepart *part, curl_off_t datasize,
  32. curl_read_callback readfunc,
  33. curl_seek_callback seekfunc,
  34. curl_free_callback freefunc, void *arg);
  35. .fi
  36. .SH DESCRIPTION
  37. \fIcurl_mime_data_cb(3)\fP sets the data source of a mime part's body content
  38. from a data read callback function.
  39. \fIpart\fP is the part's to assign contents to.
  40. \fIreadfunc\fP is a pointer to a data read callback function, with a signature
  41. as shown by the above prototype. It may not be set to NULL.
  42. \fIseekfunc\fP is a pointer to a seek callback function, with a signature as
  43. shown by the above prototype. This function will be used upon resending data
  44. (i.e.: after a redirect); this pointer may be set to NULL, in which case a
  45. resend is not possible.
  46. \fIfreefunc\fP is a pointer to a user resource freeing callback function, with
  47. a signature as shown by the above prototype. If no resource is to be freed, it
  48. may safely be set to NULL. This function will be called upon mime structure
  49. freeing.
  50. \fIarg\fP is a user defined argument to callback functions.
  51. The read callback function gets called by libcurl as soon as it needs to
  52. read data in order to send it to the peer - like if you ask it to upload or
  53. post data to the server. The data area pointed at by the pointer \fIbuffer\fP
  54. should be filled up with at most \fIsize\fP multiplied with \fInitems\fP number
  55. of bytes by your function.
  56. Your read function must then return the actual number of bytes that it stored
  57. in that memory area. Returning 0 will signal end-of-file to the library and
  58. cause it to stop the current transfer.
  59. If you stop the current transfer by returning 0 "pre-maturely" (i.e. before the
  60. server expected it, like when you have said you will upload N bytes and you
  61. upload less than N bytes), you may experience that the server "hangs" waiting
  62. for the rest of the data that will not come.
  63. The read callback may return \fICURL_READFUNC_ABORT\fP to stop the current
  64. operation immediately, resulting in a \fICURLE_ABORTED_BY_CALLBACK\fP error
  65. code from the transfer.
  66. The callback can return \fICURL_READFUNC_PAUSE\fP to cause reading from this
  67. connection to pause. See \fIcurl_easy_pause(3)\fP for further details.
  68. The seek function gets called by libcurl to rewind input stream data or to
  69. seek to a certain position. The function shall work like fseek(3) or lseek(3)
  70. and it gets SEEK_SET, SEEK_CUR or SEEK_END as argument for \fIorigin\fP,
  71. although libcurl currently only passes SEEK_SET.
  72. The callback function must return \fICURL_SEEKFUNC_OK\fP on success,
  73. \fICURL_SEEKFUNC_FAIL\fP to cause the upload operation to fail or
  74. \fICURL_SEEKFUNC_CANTSEEK\fP to indicate that while the seek failed, libcurl
  75. is free to work around the problem if possible. The latter can sometimes be
  76. done by instead reading from the input or similar.
  77. Care must be taken if the part is bound to a curl easy handle that is later
  78. duplicated: the \fIarg\fP pointer argument is also duplicated, resulting in
  79. the pointed item to be shared between the original and the copied handle.
  80. In particular, special attention should be given to the \fIfreefunc\fP
  81. procedure code since it will be called twice with the same argument.
  82. .SH EXAMPLE
  83. Sending a huge data string will cause the same amount of memory to be
  84. allocated: to avoid overhead resources consumption, one might want to use a
  85. callback source to avoid data duplication. In this case, original data
  86. must be retained until after the transfer terminates.
  87. .nf
  88. char hugedata[512000];
  89. struct ctl {
  90. char *buffer;
  91. curl_off_t size;
  92. curl_off_t position;
  93. };
  94. size_t read_callback(char *buffer, size_t size, size_t nitems, void *arg)
  95. {
  96. struct ctl *p = (struct ctl *) arg;
  97. curl_off_t sz = p->size - p->position;
  98. nitems *= size;
  99. if(sz > nitems)
  100. sz = nitems;
  101. if(sz)
  102. memcpy(buffer, p->buffer + p->position, sz);
  103. p->position += sz;
  104. return sz;
  105. }
  106. int seek_callback(void *arg, curl_off_t offset, int origin)
  107. {
  108. struct ctl *p = (struct ctl *) arg;
  109. switch(origin) {
  110. case SEEK_END:
  111. offset += p->size;
  112. break;
  113. case SEEK_CUR:
  114. offset += p->position;
  115. break;
  116. }
  117. if(offset < 0)
  118. return CURL_SEEKFUNC_FAIL;
  119. p->position = offset;
  120. return CURL_SEEKFUNC_OK;
  121. }
  122. CURL *easy = curl_easy_init();
  123. curl_mime *mime = curl_mime_init(easy);
  124. curl_mimepart *part = curl_mime_addpart(mime);
  125. struct ctl hugectl;
  126. hugectl.buffer = hugedata;
  127. hugectl.size = sizeof hugedata;
  128. hugectl.position = 0;
  129. curl_mime_data_cb(part, hugectl.size, read_callback, seek_callback, NULL,
  130. &hugectl);
  131. .SH AVAILABILITY
  132. As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.
  133. .SH RETURN VALUE
  134. CURLE_OK or a CURL error code upon failure.
  135. .SH "SEE ALSO"
  136. .BR curl_mime_addpart "(3),"
  137. .BR curl_mime_data "(3),"
  138. .BR curl_mime_name "(3),"
  139. .BR curl_easy_duphandle "(3)"