Why does errno contain ENOTTY after a call to printf?
Many implementations of the stdio package adjust their behavior slightly if stdout is a terminal. To make the determination, these implementations perform some operation which happens to fail (with ENOTTY) if stdout is not a terminal. Although the output operation goes on to complete successfully, errno still contains ENOTTY. (Note that it is only meaningful for a program to inspect the contents of errno after an error has been reported.)
References:
ANSI Sec. 4.1.3, Sec. 4.9.10.3
ISO Sec. 7.1.4, Sec. 7.9.10.3
CT&P Sec. 5.4 p. 73
PCS Sec. 14 p. 254
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This page by Steve Summit // Copyright 1995 // mail feedback