Can any one explain these c operators

What is the function of these operators ( =& , ~ ), i found this code posted as a joke and titled

"Found in early Unix sources:"

if(rp->p_flag & SSW开发者_StackOverflow中文版AP) {
    rp->p_flag =& ~SSWAP;
    aretu(u.u_ssav);
}

EDIT: Source: http://www.bsdlover.cn/study/UnixTree/V6/usr/sys/ken/slp.c.html

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~ is unary bitwise complement, which flips the bits in an integer.

However, after reading a comment and realizing I read your code wrong, I realized that the code that you have presented won't even compile in a modern compiler.

Thanks to @Avi: the operator =& means what &= means today, but this syntax really predates current C standards so it is truly ancient UNIX code.

What is would really mean today

The & here should act as the address-of operator, and not the bitwise AND operator.

int main()
{
    int x = 5;
    int y = 2;
    x =& ~y;
}

Compiling this code would produce:

error: lvalue required as unary ‘&’ operand

I really think there was a problem in transcription, as logically, it should be &= and not the other way.

In the case that it is actually &=, then it is doing a bitwise and.

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In early C, the assignment operators had the operator on the right. So it was =& instead of &=, and =+ instead of +=.

So this code just checks if a certain bit is set, and if so, takes action and turns off the bit.

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You young whippersnappers were born too late :)

This code comes from a very early version of Unix (pre V7). The joke is that the comment just above it is "You are not supposed to understand this", which has become a bit of a legend.

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=& is actually two separate operators: = and &, so the line is equivalent to rp->p_flag = & ~SSWAP;.

~ is the bitwise NOT operator, so ~SSWAP will be result of flipping the bits of SSWAP. & ~SSWAP will result in the reference of the result of ~SSWAP, which is a compile error. So the thing won't compile using a modern compiler (since C89), but =& is equivalent to &= on modern compilers.

&= applies the bitwise AND on rp->p_flag and ~SSWAP and places the result in rp->p_flag. The end result is that all 0 bits in SSWAP will turn off the respective bits in rp->p_flag if they are set. This will only be executed if at least one 1 bit will be turned off, so as to only call aretu(u.u_ssav) if the value of rp->p_flag will change as a result of the operation.

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From the horse's mouth:

B introduced generalized assignment operators, using x=+y to add y to x. The notation came from Algol 68 [Wijngaarden 75] via McIlroy, who had incorporated it into his version of TMG. (In B and early C, the operator was spelled =+ instead of += ; this mistake, repaired in 1976, was induced by a seductively easy way of handling the first form in B's lexical analyzer.)