curl_formadd(3)

📦
version 8.18.0 · 24 versions
, in fedora 44

💖 NAME 💖

okay bestie, this is curl_formadd(3) from the Library Functions Manual, and her whole thing is: add a section to a multipart form POST. that's the name vibe, periodt 🎀

✨ SYNOPSIS ✨

the signature is giving exactly this:

       #include <curl/curl.h>

       CURLFORMcode curl_formadd(struct curl_httppost **firstitem,
                                 struct curl_httppost **lastitem, ...);

🌸 DESCRIPTION 🌸

⚠️ Deprecated alert, bestie: This function is deprecated. Use curl_mime_init(3) instead. like, for real, that's the replacement. 💅

curl_formadd() is used to append sections when building a multipart form post. You append one section at a time until you have added all the sections you want included, and then you pass the firstitem pointer as parameter to CURLOPT_HTTPPOST(3). 🫶

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. yes, the actual arguments are right there behind it, bestie.

The pointers firstitem and lastitem should both be pointing to NULL in the first call to this function. All list-data is 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. do not ghost that cleanup 😭

Using POST with HTTP 1.1 implies the use of a "Expect: 100-continue" header. You can disable this header with CURLOPT_HTTPHEADER(3) as usual. ✨

First, there are some basics you need to understand about multipart form 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 discuss what options you use to set these properties in the parts you want to add to your post. 🦋

The options listed first are for making normal parts. The options from CURLFORM_FILE through CURLFORM_BUFFERLENGTH are for file upload parts. 💖

💅 OPTIONS 💅

  • CURLFORM_COPYNAME — followed by a string which provides the name of this part. libcurl copies the string so your application does not need to keep it around after this function call. If the name is not null-terminated, you must set its length with CURLFORM_NAMELENGTH. The name is not allowed to contain zero-valued bytes. The copied data is freed by curl_formfree(3). 🎀
  • CURLFORM_PTRNAME — followed by a string which provides the name of this part. libcurl uses 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 is not null-terminated, you must set its length with CURLFORM_NAMELENGTH. The name is not allowed to contain zero-valued bytes. 🔥
  • CURLFORM_COPYCONTENTS — followed by a pointer to the contents of this part, the actual data to send away. libcurl copies the provided data, so your application does not need to keep it around after this function call. If the data is not null-terminated, or if you would like it to contain zero bytes, you must set the length of the name with CURLFORM_CONTENTSLENGTH. The copied data is freed by curl_formfree(3). 💖
  • CURLFORM_PTRCONTENTS — followed by a pointer to the contents of this part, the actual data to send away. libcurl uses 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 is not null-terminated, or if you would like it to contain zero bytes, you must set its length with CURLFORM_CONTENTSLENGTH. 🌸
  • CURLFORM_CONTENTLEN — followed by a curl_off_t value giving the length of the contents. Note that for CURLFORM_STREAM contents, this option is mandatory. If you pass a 0 (zero) for this option, libcurl calls strlen() on the contents to figure out the size. If you really want to send a zero byte content then you must make sure strlen() on the data pointer returns zero. 👑
  • CURLFORM_CONTENTSLENGTH — (This option is deprecated. Use CURLFORM_CONTENTLEN instead.) followed by a long giving the length of the contents. Note that for CURLFORM_STREAM contents, this option is mandatory. If you pass a 0 (zero) for this option, libcurl calls strlen() on the contents to figure out the size. If you really want to send a zero byte content then you must make sure strlen() on the data pointer returns zero. 💀
  • CURLFORM_FILECONTENT — 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. The specified file needs to kept around until the associated transfer is done. 🦋
  • CURLFORM_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). The given upload file has to exist in its full in the file system already when the upload starts, as libcurl needs to read the correct file size beforehand. The specified file needs to kept around until the associated transfer is done. 🔥
  • 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. ✨
  • CURLFORM_FILENAME — 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 filename. 🎀
  • CURLFORM_BUFFER — 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. 🧚
  • CURLFORM_BUFFERPTR — 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. ⚠️
  • CURLFORM_BUFFERLENGTH — is used in combination with CURLFORM_BUFFER. The parameter is a long which gives the length of the buffer. 💜
  • CURLFORM_STREAM — Tells libcurl to use the CURLOPT_READFUNCTION(3) 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 unless the formpost is sent chunked encoded. 🌈
  • CURLFORM_ARRAY — 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. 👑
  • CURLFORM_CONTENTHEADER — 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. When you have passed the struct curl_httppost pointer to curl_easy_setopt(3) (using the CURLOPT_HTTPPOST(3) option), you must not free the list until after you have called curl_easy_cleanup(3) for the curl handle. See example below. 🎀

🌈 PROTOCOLS 🌈

This functionality affects http only. that's the whole protocol vibe check 💅

🎀 EXAMPLE 🎀

ok bestie here's the whole vibe in one go — a full working program, no cap:

#include <string.h> /* for strlen */

static const char record[]="data in a buffer";

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    struct curl_httppost *post = NULL;
    struct curl_httppost *last = NULL;
    char namebuffer[] = "name buffer";
    long namelength = strlen(namebuffer);
    char buffer[] = "test buffer";
    char htmlbuffer[] = "<HTML>test buffer</HTML>";
    long htmlbufferlength = strlen(htmlbuffer);
    struct curl_forms forms[3];
    char file1[] = "my-face.jpg";
    char file2[] = "your-face.jpg";
    /* add null character into htmlbuffer, to demonstrate that
       transfers of buffers containing null characters actually work
     */
    htmlbuffer[8] = '\0';

    /* Add simple name/content section */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "name",
                 CURLFORM_COPYCONTENTS, "content", CURLFORM_END);

    /* Add simple name/content/contenttype section */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "htmlcode",
                 CURLFORM_COPYCONTENTS, "<HTML></HTML>",
                 CURLFORM_CONTENTTYPE, "text/html", CURLFORM_END);

    /* Add name/ptrcontent section */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "name_for_ptrcontent",
                 CURLFORM_PTRCONTENTS, buffer, CURLFORM_END);

    /* Add ptrname/ptrcontent section */
    curl_formadd(&post, &last, CURLFORM_PTRNAME, namebuffer,
                 CURLFORM_PTRCONTENTS, buffer, CURLFORM_NAMELENGTH,
                 namelength, CURLFORM_END);

    /* Add name/ptrcontent/contenttype section */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "html_code_with_hole",
                 CURLFORM_PTRCONTENTS, htmlbuffer,
                 CURLFORM_CONTENTSLENGTH, htmlbufferlength,
                 CURLFORM_CONTENTTYPE, "text/html", CURLFORM_END);

    /* Add simple file section */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "picture",
                 CURLFORM_FILE, "my-face.jpg", CURLFORM_END);

    /* Add file/contenttype section */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "picture",
                 CURLFORM_FILE, "my-face.jpg",
                 CURLFORM_CONTENTTYPE, "image/jpeg", CURLFORM_END);

    /* Add two file section */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "pictures",
                 CURLFORM_FILE, "my-face.jpg",
                 CURLFORM_FILE, "your-face.jpg", CURLFORM_END);

    /* Add two file section using CURLFORM_ARRAY */
    forms[0].option = CURLFORM_FILE;
    forms[0].value  = file1;
    forms[1].option = CURLFORM_FILE;
    forms[1].value  = file2;
    forms[2].option  = CURLFORM_END;

    /* Add a buffer to upload */
    curl_formadd(&post, &last,
                 CURLFORM_COPYNAME, "name",
                 CURLFORM_BUFFER, "data",
                 CURLFORM_BUFFERPTR, record,
                 CURLFORM_BUFFERLENGTH, sizeof(record),
                 CURLFORM_END);

    /* no option needed for the end marker */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "pictures",
                 CURLFORM_ARRAY, forms, CURLFORM_END);
    /* Add the content of a file as a normal post text value */
    curl_formadd(&post, &last, CURLFORM_COPYNAME, "filecontent",
                 CURLFORM_FILECONTENT, ".bashrc", CURLFORM_END);
    /* Set the form info */
    curl_easy_setopt(curl, CURLOPT_HTTPPOST, post);

    curl_easy_perform(curl);

    curl_easy_cleanup(curl);

    curl_formfree(post);
  }
}

like, every single style of adding a part is in there 😭 study it, it's giving textbook. and yes bestie, notice how they shove a '\0' into htmlbuffer at htmlbuffer[8] — that's on purpose, to prove buffers with null characters inside actually transfer fine. iconic behavior fr.

💀 DEPRECATED 💀

so here's the tea, and it's a little sad: this got deprecated in 7.56.0. 🥲

before that release, field names were allowed to contain zero-valued bytes — no longer, bestie, that ship has sailed.

and about the pseudo-filename "-" used to read stdin? it's discouraged although still supported. why tho? because the data is not read before being actually sent — which means the effective data size can't be automatically determined, and you end up with a chunked encoding transfer. not the flex you think it is. 🙈

also, heads up: backslashes and double quotes in field names and filenames are now escaped before transmission. so your weird filenames get handled, but lowkey transformed. slay for safety, farewell to chaos.

🦋 AVAILABILITY 🦋

added in curl 7.1 — yes she's old money. 👑

✨ RETURN VALUE ✨

0 means everything was OK — we love a clean success. 💖

non-zero means an error occurred, corresponding to a CURL_FORMADD_ constant defined in <curl/curl.h>*. so like, check the constant, it'll tell you what went wrong. no guessing games.

🌈 SEE ALSO 🌈

aaaand that's the whole page bestie 🫶 go forth and build your multipart forms, but maybe peep curl_mime_init(3) since, y'know... deprecated and all. 💅

this lil masterpiece came from the libcurl manual, version dated 2026-10-12 — curl_formadd(3), mwah 💋

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