What's the “Content-Length” field in HTTP head

问题:

What does it mean?

1. Byte count of encoded content string with encoding specified in header.
2. Character count of content string.

Especially in case of "Content-Type: application/x-www-form-urlencoded".

回答1:

rfc2616

The Content-Length entity-header field indicates the size of the entity-body,
in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD
method, the size of the entity-body that would have been sent had the request
been a GET. 

It doesn't matter what the content-type is.

Extension at post below.

回答2:

It's the number of bytes of data in the body of the request or response. The body is the part that comes after the blank line below the headers.

回答3:

The Content-Length header is a number denoting an the exact byte length of the HTTP body. The HTTP body starts immediately after the first empty line that is found after the start-line and headers.

Generally the Content-Length header is used for HTTP 1.1 so that the receiving party knows when the current response^* has finished, so the connection can be reused for another request.

_{^{* ...or request, in the case of request methods that have a body, such as POST, PUT or PATCH}}

Alternatively, Content-Length header can be omitted and a chunked Transfer-Encoding header can be used.

If both Content-Length and Transfer-Encoding headers are missing, then at the end of the response the connection must be closed.

回答4:

One octet is 8 bits. Content-length is the number of octets that the message body represents.

回答5:

From here:

The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

   Content-Length    = "Content-Length" ":" 1*DIGIT

An example is

   Content-Length: 3495

Applications SHOULD use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in section 4.4.

Any Content-Length greater than or equal to zero is a valid value. Section 4.4 describes how to determine the length of a message-body if a Content-Length is not given.

Note that the meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the "message/external-body" content-type. In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being transferred, unless this is prohibited by the rules in section 4.4.

My interpretation is that this means the length "on the wire", i.e. the length of the *encoded" content

回答6:

From this page

The most common use of POST, by far, is to submit HTML form data to CGI scripts. In this case, the Content-Type: header is usually application/x-www-form-urlencoded, and the Content-Length: header gives the length of the URL-encoded form data (here's a note on URL-encoding). The CGI script receives the message body through STDIN, and decodes it. Here's a typical form submission, using POST:

POST /path/script.cgi HTTP/1.0
From: [e-mail]
User-Agent: HTTPTool/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 32

回答7:

According to the spec:

The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

Content-Length    = "Content-Length" ":" 1*DIGIT

An example is

Content-Length: 3495

Applications SHOULD use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in section 4.4.

Any Content-Length greater than or equal to zero is a valid value. Section 4.4 describes how to determine the length of a message-body if a Content-Length is not given.

Note that the meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the "message/external-body" content-type. In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being transferred, unless this is prohibited by the rules in section 4.4.

回答8:

The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

Content-Length = "Content-Length" ":" 1*DIGIT

An example is

Content-Length: 1024

Applications SHOULD use this field to indicate the transfer-length of the message-body.

In PHP you would use something like this.

header("Content-Length: ".filesize($filename));

In case of "Content-Type: application/x-www-form-urlencoded" the encoded data is sent to the processing agent designated so you can set the length or size of the data you are going to post.