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If the C99 {{fgets()}} function fails, the contents of the array it was writing to are [indeterminate| BB. Definitions#indeterminate value] \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] (see also [undefined behavior 161| CC. Undefined Behavior#ub_161] of Annex J). Consequently, it is necessary to reset the string to a known value to avoid possible errors on subsequent string manipulation functions.

The fgetws() function is similarly affectedIf either of the C Standard fgets() or fgetws() functions fail, the contents of the array being written is indeterminate. (See undefined behavior 175.)  It is necessary to reset the string to a known value to avoid errors on subsequent string manipulation functions.

Noncompliant Code Example

In this noncompliant code example, an error flag is set upon if fgets() failure. fails. However, buf is not reset and has indeterminate contents:

char buf[BUFSIZ];
#include <stdio.h>
 
enum { BUFFER_SIZE = 1024 };
void func(FILE *file;) {
/* Initialize file */char buf[BUFFER_SIZE];

  if (fgets(buf, sizeof(buf), file) == NULL) {
    /* setSet error flag and continue */
  }

...

}

Compliant Solution

In this compliant solution, buf is set to an empty string if fgets() fails. The equivalent solution for fgetws() would set buf to an empty wide string.

char buf[BUFSIZ];
#include <stdio.h>
 
enum { BUFFER_SIZE = 1024 };

void func(FILE *file;) {
/* Initialize file */char buf[BUFFER_SIZE];

  if (fgets(buf, sizeof(buf), file) == NULL) {
    /* setSet error flag and continue */
    *buf = '\0';
  }
}

Exceptions

FIO40-C-EX1: If the string goes out of scope immediately following the call to fgets() or fgetws(), or is not referenced in the case of a failure, it need not be reset.

Risk Assessment

Making invalid assumptions about the contents of an array modified by fgets() or fgetws() can result in undefined behavior and 175 and abnormal program termination.

Related Vulnerabilities

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

Other Languages

This rule appears in the C++ Secure Coding Standard as FIO40-CPP. Reset strings on fgets() failure.

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.7.2, "The {{fgets}} function" and Section 7.24.3.2, "the {{fgetws}} function"

Automated Detection

Related Vulnerabilities

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

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Image Added Image Added Image AddedImage Removed      09. Input Output (FIO)      FIO41-C. Do not call getc() or putc() with stream arguments that have side effects