Pack

## Pack Pack — Byte packing and unpacking functions |

## Synopsis

    #include <gnet.h>

    gint                gnet_pack                           (const gchar *format,
                                                             gchar *buffer,
                                                             const gint length,
                                                             ...);
    gint                gnet_pack_strdup                    (const gchar *format,
                                                             gchar **bufferp,
                                                             ...);
    gint                gnet_vpack                          (const gchar *format,
                                                             gchar *buffer,
                                                             const gint length,
                                                             va_list args);
    gint                gnet_calcsize                       (const gchar *format,
                                                             ...);
    gint                gnet_vcalcsize                      (const gchar *format,
                                                             va_list args);
    gint                gnet_unpack                         (const gchar *format,
                                                             const gchar *buffer,
                                                             gint length,
                                                             ...);
    gint                gnet_vunpack                        (const gchar *format,
                                                             const gchar *buffer,
                                                             gint length,
                                                             va_list args);

## Description

The pack module provides functions for converting data into bytes ("packing") and converting bytes to data ("unpacking"). For example, two integers and a string may be packed into a buffer to be transfered over the network. These functions are similar to the ones in Python and Perl.

## Details

### gnet_pack ()

    gint                gnet_pack                           (const gchar *format,
                                                             gchar *buffer,
                                                             const gint length,
                                                             ...);

Writes `Varargs` to `buffer`. `format` is a string that describes the `Varargs` and how they are to be packed. This string is a list of characters, each describing the type of an argument in `Varargs`.

An example:

     char buf[5];
     int myint = 42;
     int mybyte = 23;

     gnet_pack("!ib", buf, sizeof(buf), myint, mybyte);
     /* Now buf is { 42, 0, 0, 0, 23 }; */

As a shortcut, most types can be prefixed by an integer to specify how many times the type is repeated. For example, "4i2b" is equivalent to "iiiibb". This repeat argument is refered below to as REPEAT.

Native byte order and sizes are used by default. If the first character of `format` is < then little endian order and standard sizes are used. If the first character is > or !, then big endian (or network) order and standard size are used. Standard sizes are 1 byte for chars, 2 bytes for shorts, and 4 bytes for ints and longs.

The types:

x is a NULL character. It can be used for padding. It does not correspond to an argument in `Varargs`.

b/B is a signed/unsigned char.

h/H is a signed/unsigned short.

i/I is a signed/unsigned int.

l/L is a signed/unsigned long.

f/D is a float/double (always native order/size).

v is a void pointer (always native size).

s is a zero-terminated string.

S is a zero-padded string of maximum length REPEAT. We write up-to a NULL character or REPEAT characters, whichever comes first. We then write NULL characters up to a total of REPEAT characters. Special case: if REPEAT is not specified, we write the string as a non-NULL-terminated string (note that it can't be unpacked easily then).

r is a byte array of NEXT bytes. NEXT is the next argument passed to `gnet_pack()` and is a gint.

R is a byte array of REPEAT bytes. REPEAT must be specified.

p is a Pascal string. The string passed is a NULL-termiated string of less than 256 character. The string writen is a non-NULL-terminated string with a byte before the string storing the string length. REPEAT is repeat.

Mnemonics: (B)yte, s(H)ort, (I)nteger, (F)loat, (D)ouble, (V)oid pointer, (S)tring, (R)aw

Pack was inspired by Python's and Perl's pack. It is more like Python's than Perl's. Note that in GNet, a repeat of 0 does not align the data (as in Python).

`format` : | pack data format
`buffer` : | buffer to pack to
`length` : | length of `buffer`
`...` : | variables to pack from
Returns : | number of bytes packed; -1 on error.

### gnet_pack_strdup ()

    gint                gnet_pack_strdup                    (const gchar *format,
                                                             gchar **bufferp,
                                                             ...);

Writes `Varargs` into a buffer pointed to by `bufferp`. The buffer is allocated by the function and is caller owned. See `gnet_pack()` for more information.

`format` : | pack data format
`bufferp` : | pointer to a buffer (buffer is caller owned)
`...` : | variables to pack from
Returns : | bytes packed; -1 on error.

### gnet_vpack ()

    gint                gnet_vpack                          (const gchar *format,
                                                             gchar *buffer,
                                                             const gint length,
                                                             va_list args);

Var arg interface to `gnet_pack()`. See `gnet_pack()` for more information.

`format` : | pack data format
`buffer` : | buffer to pack to
`length` : | length of `buffer`
`args` : | var args
Returns : | bytes packed; -1 on error.

### gnet_calcsize ()

    gint                gnet_calcsize                       (const gchar *format,
                                                             ...);

Calculates the size of the buffer needed to pack `Varargs` by the given format. See `gnet_pack()` for more information.

`format` : | pack data format
`...` : | variables
Returns : | number of bytes required to pack; -1 on error.

### gnet_vcalcsize ()

    gint                gnet_vcalcsize                      (const gchar *format,
                                                             va_list args);

Var arg interface to `gnet_calcsize()`. See `gnet_calcsize()` for additional information.

`format` : | pack data format
`args` : | var args
Returns : | number of bytes required to pack; -1 on error.

### gnet_unpack ()

    gint                gnet_unpack                         (const gchar *format,
                                                             const gchar *buffer,
                                                             gint length,
                                                             ...);

Reads the data in `buffer` into `Varargs`. `format` is a string that describes the `Varargs` and how they are to be packed. This string is a list of characters, each describing the type of an argument in `Varargs`.

An example:

     char buf[5] = { 42, 0, 0, 0, 23 };
     int myint;
     int mybyte;

     gnet_unpack("!ib", buf, sizeof(buf), &myint, &mybyte);
     /* Now myint is 42 and mybyte is 23 */

In unpack, the arguments must be pointers to the appropriate type. Strings and byte arrays are allocated dynamicly (by `g_new()`). The caller is responsible for `g_free()`-ing it.

As a shortcut, most types can be prefixed by an integer to specify how many times the type is repeated. For example, "4i2b" is equivalent to "iiiibb". This repeat argument is refered below to as REPEAT.

The types:

x is a pad byte. The byte is skipped and not stored. We do not check its value.

b/B is a signed/unsigned char.

h/H is a signed/unsigned short.

i/I is a signed/unsigned int.

l/L is a signed/unsigned long.

f/D is a float/double (always native order/size). v is a void pointer (always native size).

s is a zero-terminated string.

S is a zero-padded string of length REPEAT. We read REPEAT characters or until a NULL character. Any remaining characters are filled in with 0's. REPEAT must be specified.

r is a byte array of NEXT bytes. NEXT is the next argument and is a gint. REPEAT is repeat.

R is a byte array of REPEAT bytes. REPEAT must be specified.

p is a Pascal string. The first byte read is the length of the string and the string follows. The unpacked string will be a normal, NULL-terminated string. REPEAT is repeat.

`format` : | unpack data format
`buffer` : | buffer to unpack from
`length` : | length of `buffer`
`...` : | addresses of variables to unpack to
Returns : | number of bytes unpacked; -1 on error. The bytes are unpacked to the variables pointed to by the `Varargs`.

### gnet_vunpack ()

    gint                gnet_vunpack                        (const gchar *format,
                                                             const gchar *buffer,
                                                             gint length,
                                                             va_list args);

Var arg interface to `gnet_unpack()`. See `gnet_unpack()` for more information.

`format` : | unpack data format
`buffer` : | buffer to unpack from
`length` : | length of `buffer`
`args` : | var args
Returns : | number of bytes packed; -1 on error.