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Netstring for C

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A netstring is a way of encoding a sequence of bytes for transmission over a network, or for serialization. They're very easy to work with. They encode the data's length, and can be concatenated trivially. The format was defined by D. J. Bernstein and is used in various software. Examples of netstrings:

"12:hello, world!,"
"3:foo,"
"0:,"

To specify a list of strings, just concatenate the netstrings. The list ["hey", "everyone"] can be encoded as

"3:hey,8:everyone,"

This is some code in C for netstring serialization and deserialization. It checks netstrings for validity when parsing them, and guards against malicious input. It's also fast.

Basic API

All the code is in netstring.c and netstring.h, and these have no external dependencies. To use them, just include them in your application. Include netstring.h and link with the C code.

Creating netstrings

You can create your netstrings manually like in this example:

sprintf(buf, "%lu:%s,", strlen(str), str);

This code provides a convenience function for creating netstrings:

size_t netstring_add(char **netstring, char *data);

Here is how to use it:

char *netstring=0;  /* we must initialize it to zero */

netstring_add(&netstring, "first");
netstring_add(&netstring, "second");
netstring_add(&netstring, "third");

do_something(netstring);

free(netstring);    /* we must free after using it */

The extended version netstring_add_ex accepts a string length as the last argument:

size_t netstring_add_ex(char **netstring, char *data, size_t len);

This allocates and creates a netstring containing the first len bytes of data. If len is 0 then no data will be read from data, and it may be null.

Parsing netstrings

To parse a netstring use netstring_read():

int netstring_read(char **buffer_start, size_t *buffer_length,
                   char **netstring_start, size_t *netstring_length);

It reads a netstring from buffer_start of initial buffer_length and writes to netstring_start a pointer to the beginning of the string in the buffer and to netstring_length the length of the string. It also updates the buffer_start to the start of the next netstring item and buffer_length to the number of remaining bytes not processed in the buffer.

It does not allocate any memory.

Return Value

If it reads successfully then it returns 0. If there is an error then the return value will be negative. The error values are:

NETSTRING_ERROR_TOO_LONG      More than 999999999 bytes in a field
NETSTRING_ERROR_NO_COLON      No colon was found after the number
NETSTRING_ERROR_TOO_SHORT     Number of bytes greater than buffer length
NETSTRING_ERROR_NO_COMMA      No comma was found at the end
NETSTRING_ERROR_LEADING_ZERO  Leading zeros are not allowed
NETSTRING_ERROR_NO_LENGTH     Length not given at start of netstring

Usage Example:

char  *str, *base = buffer;
size_t len,  size = bytes_read;

while(netstring_read(&base, &size, &str, &len) == 0) {
  do_something(str, len);
}

We can replace the comma with a null terminator when reading (zero copy):

while(netstring_read(&base, &size, &str, &len) == 0) {
  str[len] = 0;
  puts(str);
  str[len] = ',';   /* and optionally restore it */
}

If you're sending messages with more than 999999999 bytes (about 2 GB) then you probably should not be doing so in the form of a single netstring. This restriction is in place partially to protect from malicious or erroneous input, and partly to be compatible with D. J. Bernstein's reference implementation.

Message Framing on stream-based connections (sockets, pipes...)

On stream-based connections the messages can arrive coalesced or fragmented.

Here is an example of reading those messages using netstring for message framing:

char buffer[1024], *buffer_base, *str;
int bytes_read, buffer_used = 0, len;

while(1) {
  /* read data from socket */
  bytes_read = recv(sock, &buffer[buffer_used], sizeof(buffer) - buffer_used);
  if (bytes_read < 0) break; if (bytes_read == 0) continue;
  buffer_used += bytes_read;

  /* parse the strings from the read buffer */
  buffer_base = buffer;
  while(netstring_read(&buffer_base, &buffer_used, &str, &len) == 0) {
    do_something(str, len);
  }

  /* if there are remaining bytes, move to the beginning of buffer */
  if (buffer_base > buffer && buffer_used > 0)
    memmove(buffer, buffer_base, buffer_used);
}

Note: this example is lacking error checking from netstring_read function and it does not allocate memory for bigger messages.

Additional Functions

netstring_list_size

Retrieves the size of the netstring list (concatenated netstrings) discarding trailing spaces.

int netstring_list_size(char *buffer, size_t size, size_t *ptotal);

netstring_list_count

Retrieves the number of items in a netstring list.

int netstring_list_count(char *buffer, size_t size, int *pcount);

Contributing

All this code is Public Domain. If you want to contribute, you can send bug reports, or fork the project on GitHub. Contributions are welcomed with open arms.

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