SEI CERT C Coding Standard

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Section 7.19.7.11 of C99 defines {{ungetc()}} as follows: \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\]:

The ungetc function pushes the character specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. Pushed-back characters will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards any pushed-back characters for the stream. The external storage corresponding to the stream is unchanged.

One character of pushback is guaranteed.

Consequently, multiple calls to ungetc() on the same stream must be separated by a call to a read function or a file-positioning function (which will discard any data pushed by ungetc()).

Likewise, for ungetwc(), C99 only guarantees one wide character of pushback (section Section 7.24.3.10). Consequently, multiple calls to ungetwc() on the same stream must be separated by a call to a read function or a file-positioning function (which will discard any data pushed by ungetwc()).

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Remember to always call fgetpos() before fsetpos(). (see See rule FIO44-C. Only use values for fsetpos() that are returned from fgetpos().).

Risk Assessment

If used improperly, ungetc() and ungetwc() can cause data to be truncated or lost.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

FIO13-C

medium

probable

high

P4

L3

Automated Detection

Tool

Version

Checker

Description

Section

Compass/ROSE

 

 

Section

can detect simple violations of this recommendation. In particular, it warns when two calls to ungetc() on the same stream are not interspersed with a file positioning or file read function. It is unable to handle cases where ungetc() is called from inside a loop

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

Related Guidelines

Wiki Markup\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 7.19.7.11, "The {{ungetc}} function"

Bibliography

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FIO12-C. Prefer setvbuf() to setbuf()      09. Input Output (FIO)