LEB128 or Little Endian Base 128 is a form of variable-length code compression used to store an arbitrarily large integer in a small number of bytes. LEB128 is used in the DWARF debug file format.[1][2]
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LEB128 format is very similar to variable-length quantity format. Both allow small numbers to be stored in a single byte, while also allowing encoding of arbitrarily long numbers. There are 2 versions of LEB128: unsigned LEB128 and signed LEB128. The decoder must know whether the encoded value is unsigned LEB128 or signed LEB128.
To encode an unsigned number using unsigned LEB128 first represent the number in binary. Then zero extend the number up to a multiple of 7 bits (such that the most significant 7 bits are not all 0). Break the number up into groups of 7 bits. Output one encoded byte for each 7 bit group, from least significant to most significant group. Each byte will have the group in its 7 least significant bits. Set the most significant bit on each byte except the last byte. The number zero is encoded as a single byte 0x00.
As an example, here is how the unsigned number 624485 gets encoded:
10011000011101100101 In raw binary
010011000011101100101 Padded to a multiple of 7 bits
0100110 0001110 1100101 Split into 7-bit groups
00100110 10001110 11100101 Add high 1 bits on all but last group to form bytes
0x26 0x8E 0xE5 In hexadecimal
0xE5 0x8E 0x26 Output stream
Unsigned LEB128, VLQ (variable-length quantity), and the M=128 Rice code all compress any given integer into not only the same number of bits, but exactly the same bits -- the three formats differ only in exactly how those bits are arranged.
A signed number is represented similarly, except that the two's complement number is sign extended up to a multiple of 7 bits (ensuring that the most significant bit is zero for a positive number and one for a negative number). Then the number is broken into groups as for the unsigned encoding.
For example the signed number -624485 (0xFFF6789b) is encoded as 0x9b 0xf1 0x59.
do {
byte = low order 7 bits of value;
value >>= 7;
if (value != 0) /* more bytes to come */
set high order bit of byte;
emit byte;
} while (value != 0);
more = 1;
negative = (value < 0);
size = no. of bits in signed integer;
while(more) {
byte = low order 7 bits of value;
value >>= 7;
/* the following is unnecessary if the * implementation of >>= uses an
arithmetic rather * than logical shift for a signed left operand */
if (negative)
value |= - (1 <<(size - 7)); /* sign extend */
/* sign bit of byte is second high order bit (0x40) */
if ((value == 0 && sign bit of byte is clear) || (value == -1 && sign bit of byte is set))
more = 0;
else
set high order bit of byte;
emit byte;
}
result = 0;
shift = 0;
while(true) {
byte = next byte in input;
result |= (low order 7 bits of byte << shift);
if (high order bit of byte == 0)
break;
shift += 7;
}
result = 0;
shift = 0;
size = number of bits in signed integer;
while(true) {
byte = next byte in input;
result |= (low order 7 bits of byte << shift);
shift += 7;
/* sign bit of byte is second high order bit (0x40) */
if (high order bit of byte == 0)
break;
}
if ((shift <size) && (sign bit of byte is set))
/* sign extend */
result |= - (1 << shift);
The DWARF file format uses both unsigned and signed LEB128 encoding for various fields.[2]
The mpatrol debugging tool uses LEB128 in its tracing file format.[3]
The Android project uses LEB128 in its Dalvik Executable Format (.dex) file format.[4]
Compressing tables in Hewlett-Packard IA-64 exception handling.[5]
It is used in the Linux kernel for its DWARF implementation.[6]