[metadata]
id = "FIO39-C"
type = "recommendation"
category = "FIO"
number = 39
title = "Do not alternately input and output from a stream without an intervening flush or positioning call"
description = """
The C Standard, 7.23.5.3, paragraph 7 [ISO/IEC 9899:2024], places the following
restrictions on update streams: The following scenarios can result inundefined
behavior. (Seeundefined behavior 156.) Consequently, a call tofseek(),fflush(),
orfsetpos()is necessary between input and output to the same stream. SeeERR07-C.
Prefer functions that support error checking over equivalent functions that
don'tfor more information on whyfseek()is preferred overrewind().
"""
severity = "Low"
likelihood = "Likely"
priority = "P6"
level = "L2"
cert_version = "2016 Edition (Wiki)"
last_modified = "Aug 06, 2025"
[rules.cert_c.FIO39-C]
enabled = true
[references]
wiki = "https://wiki.sei.cmu.edu/confluence/display/c/FIO39-C.+Do+not+alternately+input+and+output+from+a+stream+without+an+intervening+flush+or+positioning+call"
cwe = ["CWE-664"]