CURLOPT_POSTFIELDS(3) Library Functions Manual CURLOPT_POSTFIELDS(3)
NAME
CURLOPT_POSTFIELDS - data to POST to server
SYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_POSTFIELDS, char *postdata);
DESCRIPTION
Pass a char pointer as parameter, pointing to the data buffer to use in
an HTTP POST operation or an MQTT subscribe. The data must be formatted
and encoded the way you want the server to receive it. libcurl does not
convert or encode it in any way. For example, a web server may assume
that this data is URL encoded.
The data pointed to is NOT copied by the library: as a consequence, it
must be preserved by the calling application until the associated trans-
fer finishes. This behavior can be changed (so libcurl does copy the
data) by instead using the CURLOPT_COPYPOSTFIELDS(3) option.
This POST is a normal application/x-www-form-urlencoded kind (and
libcurl sets that Content-Type by default when this option is used),
which is commonly used by HTML forms. Change Content-Type with CUR-
LOPT_HTTPHEADER(3).
You can use curl_easy_escape(3) to URL encode your data, if necessary.
It returns a pointer to an encoded string that can be passed as post-
data.
Using CURLOPT_POSTFIELDS(3) implies setting CURLOPT_POST(3) to 1.
If CURLOPT_POSTFIELDS(3) is explicitly set to NULL then libcurl gets the
POST data from the read callback. To send a zero-length (empty) POST,
set CURLOPT_POSTFIELDS(3) to an empty string, or set CURLOPT_POST(3) to
1 and CURLOPT_POSTFIELDSIZE(3) to 0.
libcurl assumes this option points to a null-terminated string unless
you also set CURLOPT_POSTFIELDSIZE(3) to specify the length of the pro-
vided data, which then is strictly required if you want to send off null
bytes included in the data.
Using POST with HTTP 1.1 implies the use of a "Expect: 100-continue"
header, and libcurl adds that header automatically if the POST is either
known to be larger than 1MB or if the expected size is unknown. You can
disable this header with CURLOPT_HTTPHEADER(3) as usual.
To make multipart/formdata posts, check out the CURLOPT_MIMEPOST(3) op-
tion combined with curl_mime_init(3).
Using this option multiple times makes the last set pointer override the
previous ones. Set it to NULL to disable its use again.
DEFAULT
NULL
PROTOCOLS
This functionality affects http and mqtt
EXAMPLE
/* send an application/x-www-form-urlencoded POST */
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
const char *data = "data to send";
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
/* size of the POST data if strlen() is not good enough */
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, 12L);
/* pass in a pointer to the data - libcurl does not copy */
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);
curl_easy_perform(curl);
}
/* send an application/json POST */
curl = curl_easy_init();
if(curl) {
const char *json = "{\"name\": \"daniel\"}";
struct curl_slist *slist1 = NULL;
slist1 = curl_slist_append(slist1, "Content-Type: application/json");
slist1 = curl_slist_append(slist1, "Accept: application/json");
/* set custom headers */
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, slist1);
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
/* pass in a pointer to the data - libcurl does not copy */
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, json);
curl_easy_perform(curl);
}
}
AVAILABILITY
Added in curl 7.1
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_COPYPOSTFIELDS(3), CURLOPT_MIMEPOST(3), CURLOPT_POSTFIELD-
SIZE(3), CURLOPT_READFUNCTION(3), CURLOPT_UPLOAD(3)
libcurl 2026-07-12 CURLOPT_POSTFIELDS(3)
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