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The C Standard, subclause 7.19.5.3, paragraph 66 [ISO/IEC 9899:1999], places the following restrictions on FILE objects opened for both reading and writing [ISO/IEC 9899:1999]:
When a file is opened with update mode . . ., both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the
fflushfunction or to a file positioning function (fseek,fsetpos, orrewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.
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Alternately inputting and outputting from a stream without an intervening flush or positioning call is undefined behavior.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
FIO50-CPP | Low | Likely | Medium | P6 | L2 |
Automated Detection
Tool | Version | Checker | Description |
|---|
| Axivion Bauhaus Suite |
| CertC++-FIO50 | |||||||
| CodeSonar |
| IO.IOWOP IO.OIWOP | Input After Output Without Positioning Output After Input Without Positioning | ||||||
| Helix QAC |
| DF4711, DF4712, DF4713 | |||||||
| CERT_CPP-FIO50-a | Do not alternately input and output from a stream without an intervening flush or positioning call | |||||||
| Polyspace Bug Finder |
| CERT C++: FIO50-CPP | Checks for alternating input and output from a stream without flush or positioning call (rule fully covered) |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
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This rule supplements FIO39-C. Do not alternately input and output from a stream without an intervening flush or positioning call.
Bibliography
| [ISO/IEC 9899:1999] | Subclause 7.19.5.3, "The fopen Function" |
| [ISO/IEC 14882-2014] | Clause 27, "Input/Output Library" |
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