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From: Doug Hardie <bc979@lafn.org>
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Subject: vsnprintf and friends modify the va_list argument
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Date: Sun, 16 Nov 2014 14:15:10 -0800
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I suspect this needs to be in a WARNING section of the printf(3) man =
page.  The print functions that use variable arguments modify the =
argument list so that only one can be called.  For example, to print the =
arguments and also write them to a file using the following code will =
cause a segmentation violation:

vprintf (fd, fmt, args);
vsnprintf (buf, sizeof(buf), fmt, args);

Switching the order of the calls doesn=E2=80=99t eliminate the =
segmentation violation.  You have to do something on the order of:

vsnprintf (buf, sizeof(buf), fmt, args);
fprintf (fd, =E2=80=9C%s=E2=80=9D, buf);=