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Asked: 2026-05-31T05:58:12+00:00

I noticed that zlib’s adler32 function does not always return the original seed when

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I noticed that zlib’s adler32 function does not always return the original seed when passed an empty string. For example:

adler32(0xFFFFFFFF,          // seed
        (const Bytef *) "",  // buffer
        0                    // length
       )

returns 0xE000E. I would think it should return 0xFFFFFFFF instead.

For most other values, it does return the original seed. However, for 1965855 numbers from 0 to 0xFFFFFFFF, adler32 changes the seed when the input string is empty.

Is this a bug or quirky behavior of the implementation, or is the range of the Adler-32 function in fact a subset of [0,0xFFFFFFFF] ?

Note that when the pointer is NULL, adler32 always returns 1 (the initial seed). This is documented behavior.

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1 Answer

  1. Editorial Team

    The first argument is not a “seed”. It is the previous adler32 value that is being appended to.

    Yes, the range of adler32 is not all 32-bit values. 0xffffffff is not a valid adler32. The only valid adler32 values are those in which the upper and lower 16-bit halves of the 32-bit value are, when interpreted as integers, both less than 65521. When you call adler32() with a zero length, it will return the upper and lower halves modulo 65521.

    1965855 is 65521*15 + 15*65521 + 15*15, which is the number of 32-bit values with a valid upper half and invalid lower half, plus the number with an invalid upper half and a valid lower half, plus the number with both halves invalid.

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