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Subclause 7.23.7.2 of the C Standard paragraph 3 [ISO/IEC 9899:2024] says,

The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the members of the array have unspecified values and a null pointer is returned.

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Rule	Severity	Likelihood	Remediation Cost	Priority	Level
FIO37-C	High	Probable	Medium	P12	L1

Automated Detection

Tool

Version

Checker

Description

Astrée

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	Astrée_V
	Astrée_V

Supported: Astrée reports defects due to returned (empty) strings.

Axivion Bauhaus Suite

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	Axivion Bauhaus Suite_V
	Axivion Bauhaus Suite_V

CertC-FIO37

CodeSonar

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	CodeSonar_V
	CodeSonar_V

(general)

Considers the possibility that fgets() and fgetws() may return empty strings (Warnings of various classes may be triggered depending on subsequent operations on those strings. For example, the noncompliant code example cited above would trigger a buffer underrun warning.)

Compass/ROSE

Could detect some violations of this rule (In particular, it could detect the noncompliant code example by searching for fgets(), followed by strlen() - 1, which could be −1. The crux of this rule is that a string returned by fgets() could still be empty, because the first char is '\0'. There are probably other code examples that violate this guideline; they would need to be enumerated before ROSE could detect them.)

Helix QAC

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