C getline() function: return value vs second argument

Question

Do the return value (type ssize_t) of the C getline function and the second argument *n (type size_t) contain the same information, after invocation? Empirically, it seems that *n equals (size_t)pow(2, ceil(log2(<return value> + 1))). Is this relation true in general? Can someone explain its (in)validity, conceptually?

You state in your question that, in your cases, `*n` has generally held a value different from the return value, so no they don't contain the same information. — Thomas Jager, Aug 18 '22 at 15:15
As for the relation, it seems that your specific implementation chooses to make power-of-2 size allocations when resizing, according to what's needed. This is not a requirement, but may be a common choice. The buffer just needs to be large enough. — Thomas Jager, Aug 18 '22 at 15:17
@ThomasJager By "contain the same information" I mean each can be computed from the other. Paraphrasing your second comment: the numerical relationship is implementation dependent, but uniquely characterized by the buffer length being the minimal one that is 1) big enough and 2) a power of two. Sounds like an answer to me! — fmg, Aug 18 '22 at 15:20
@fmg "the buffer length being the minimal one" --> no it could be 0 for select reasons and 0 is not a power-of-2. — chux - Reinstate Monica, Aug 18 '22 at 15:29
@fmg Why is the value of `*n` as potentially a power-of-2 useful to know for your code? It not relying on _return_value_< `*n` enough? — chux - Reinstate Monica, Aug 18 '22 at 15:33
@ThomasJager You're right. You just get an upper bound. So the answer to my "do they contain the same information" question is "no". More subtle than I initially suspected! — fmg, Aug 18 '22 at 15:52
@chux-ReinstateMonica I don't need this in my code; just trying to understand things. Your point regarding the degenerate case is well taken. — fmg, Aug 18 '22 at 15:53

chux - Reinstate Monica · Accepted Answer · 2022-08-18T15:42:18.790

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

Do the return value (type ssize_t) of the C getline function and the second argument *n (type size_t) contain the same information, after invocation?

No. The return value is the count of characters read, not including the appended null character. *n is the size of the current allocation of *lineptr. The return value is signed and is -1 when an (allocation) error/end-of-file occurs. *n is an unsigned type.

It is expected that the return value is always less than *n.

it seems that *n equals (size_t)pow(2, ceil(log2( + 1))). Is this relation true in general?

No, *n may or may not be a power of 2.

getline() is not part of the C standard library and implementations differ on allocation details.

*n equals (size_t)pow(2, ceil(log2(<return value> + 1))) is invalid when:

return value == -1
getline() does not re-allocate and the passed in size was not a power-of-2.
getline() reallocates and is not using a power-of-2 scheme.
Pedantic: Very large return value round down in the conversion to double in the log2(_return value_ + 1) step.
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C getline() function: return value vs second argument

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