Netstrings D. J. Bernstein, [email protected] 19970201 1. Introduction A netstring is a self-delimiting encoding of a string. Netstrings are very easy to generate and to parse. Any string may be encoded as a netstring; there are no restrictions on length or on allowed bytes. Another virtue of a netstring is that it declares the string size up front. Thus an application can check in advance whether it has enough space to store the entire string. Netstrings may be used as a basic building block for reliable network protocols. Most high-level protocols, in effect, transmit a sequence of strings; those strings may be encoded as netstrings and then concatenated into a sequence of characters, which in turn may be transmitted over a reliable stream protocol such as TCP. Note that netstrings can be used recursively. The result of encoding a sequence of strings is a single string. A series of those encoded strings may in turn be encoded into a single string. And so on. In this document, a string of 8-bit bytes may be written in two different forms: as a series of hexadecimal numbers between angle brackets, or as a sequence of ASCII characters between double quotes. For example, is a string of length 12; it is the same as the string "hello world!". Although this document restricts attention to strings of 8-bit bytes, netstrings could be used with any 6-bit-or-larger character set. 2. Definition Any string of 8-bit bytes may be encoded as [len]":"[string]",". Here [string] is the string and [len] is a nonempty sequence of ASCII digits giving the length of [string] in decimal. The ASCII digits are for 0, for 1, and so on up through for 9. Extra zeros at the front of [len] are prohibited: [len] begins with exactly when [string] is empty. For example, the string "hello world!" is encoded as , i.e., "12:hello world!,". The empty string is encoded as "0:,". [len]":"[string]"," is called a netstring. [string] is called the interpretation of the netstring. 3. Sample code The following C code starts with a buffer buf of length len and prints it as a netstring. if (printf("%lu:",len) 999999999 bytes is bad */ if (getchar() != ':') barf(); buf = malloc(len + 1); /* malloc(0) is not portable */ if (!buf) barf(); if (fread(buf,1,len,stdin)