cURL libcurl API curl_formadd
curl_formadd.3 -- man page
CURLFORMcode curl_formadd(struct curl_httppost ** firstitem, struct curl_httppost ** lastitem, ...);
curl_formadd() is used to append sections when building a multipart/formdata HTTP POST (sometimes referred to as RFC 2388-style posts). Append one section at a time until you've added all the sections you want included and then you pass the firstitem pointer as parameter to CURLOPT_HTTPPOST. lastitem is set after each curl_formadd(3) call and on repeated invokes it should be left as set to allow repeated invokes to find the end of the list faster.
After the lastitem pointer follow the real arguments.
The pointers firstitem and lastitem should both be pointing to NULL in the first call to this function. All list-data will be allocated by the function itself. You must call curl_formfree(3) on the firstitem after the form post has been done to free the resources.
Using POST with HTTP 1.1 implies the use of a "Expect: 100-continue" header. You can disable this header with CURLOPT_HTTPHEADER as usual.
First, there are some basics you need to understand about multipart/formdata posts. Each part consists of at least a NAME and a CONTENTS part. If the part is made for file upload, there are also a stored CONTENT-TYPE and a FILENAME. Below, we'll discuss what options you use to set these properties in the parts you want to add to your post.
followed by a string which provides the name of this part. libcurl copies the string so your application doesn't need to keep it around after this function call. If the name isn't NUL-terminated, or if you'd like it to contain zero bytes, you must set its length with CURLFORM_NAMELENGTH. The copied data will be freed by curl_formfree(3).
followed by a string which provides the name of this part. libcurl will use the pointer and refer to the data in your application, so you must make sure it remains until curl no longer needs it. If the name isn't NUL-terminated, or if you'd like it to contain zero bytes, you must set its length with CURLFORM_NAMELENGTH.
followed by a pointer to the contents of this part, the actual data to send away. libcurl copies the provided data, so your application doesn't need to keep it around after this function call. If the data isn't null terminated, or if you'd like it to contain zero bytes, you must set the length of the name with CURLFORM_CONTENTSLENGTH. The copied data will be freed by curl_formfree(3).
followed by a pointer to the contents of this part, the actual data to send away. libcurl will use the pointer and refer to the data in your application, so you must make sure it remains until curl no longer needs it. If the data isn't NUL-terminated, or if you'd like it to contain zero bytes, you must set its length with CURLFORM_CONTENTSLENGTH.
followed by a long giving the length of the contents. Note that for CURLFORM_STREAM contents, this option is mandatory.
followed by a filename, causes that file to be read and its contents used as data in this part. This part does not automatically become a file upload part simply because its data was read from a file.
followed by a filename, makes this part a file upload part. It sets the filename field to the basename of the provided filename, it reads the contents of the file and passes them as data and sets the content-type if the given file match one of the internally known file extensions. For CURLFORM_FILE the user may send one or more files in one part by providing multiple CURLFORM_FILE arguments each followed by the filename (and each CURLFORM_FILE is allowed to have a CURLFORM_CONTENTTYPE).
is used in combination with CURLFORM_FILE. Followed by a pointer to a string which provides the content-type for this part, possibly instead of an internally chosen one.
is used in combination with CURLFORM_FILE. Followed by a pointer to a string, it tells libcurl to use the given string as the filename in the file upload part instead of the actual file name.
is used for custom file upload parts without use of CURLFORM_FILE. It tells libcurl that the file contents are already present in a buffer. The parameter is a string which provides the filename field in the content header.
is used in combination with CURLFORM_BUFFER. The parameter is a pointer to the buffer to be uploaded. This buffer must not be freed until after curl_easy_cleanup(3) is called. You must also use CURLFORM_BUFFERLENGTH to set the number of bytes in the buffer.
is used in combination with CURLFORM_BUFFER. The parameter is a long which gives the length of the buffer.
Tells libcurl to use the CURLOPT_READFUNCTION callback to get data. The parameter you pass to CURLFORM_STREAM is the pointer passed on to the read callback's fourth argument. If you want the part to look like a file upload one, set the CURLFORM_FILENAME parameter as well. Note that when using CURLFORM_STREAM, CURLFORM_CONTENTSLENGTH must also be set with the total expected length of the part. (Option added in libcurl 7.18.2)
Another possibility to send options to curl_formadd() is the CURLFORM_ARRAY option, that passes a struct curl_forms array pointer as its value. Each curl_forms structure element has a CURLformoption and a char pointer. The final element in the array must be a CURLFORM_END. All available options can be used in an array, except the CURLFORM_ARRAY option itself! The last argument in such an array must always be CURLFORM_END.
specifies extra headers for the form POST section. This takes a curl_slist prepared in the usual way using curl_slist_append and appends the list of headers to those libcurl automatically generates. The list must exist while the POST occurs, if you free it before the post completes you may experience problems.
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Page updated January 20, 2014.
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