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If either of the C Standard fgets() or fgetws() functions fail, the contents of the array being written is indeterminate. (see See undefined behavior 170175.).  It is necessary to reset the string to a known value to avoid errors on subsequent string manipulation functions.

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#include <stdio.h>
 
enum { BUFFER_SIZE = 1024 };

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

  if (fgets(buf, sizeof(buf), file) == NULL) {
    /* Set 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.

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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.

Automated Detection

Related Vulnerabilities

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

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