core.stdc.stdio.snprintf returns wrong value if the buffer is not sufficiently large

[mdparker]

ogion.art reported this on 2021-09-02T12:09:26Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=22266

Description

According to C99 specification, if the buffer is not sufficiently large, snprintf should still return the number of characters that would have been written. However, the implementation that DMD uses by default on Windows returns -1 instead.

void main() {
    import core.stdc.stdio;
    auto n = snprintf(null, 0, "test");
    assert(n == 4); //fails 
}

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