Shortest code that raises a SIGSEGV

C, 5 characters

main;

It's a variable declaration - int type is implied (feature copied from B language) and 0 is default value. When executed this tries to execute a number (numbers aren't executable), and causes SIGSEGV.

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Bash, 11      

kill -11 $$

Assembly (Linux, x86-64), 1 byte

RET

This code segfaults.