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If the C fgets() function fails, the contents of the array it was writing to are indeterminate. (See also undefined behavior 170 of in Annex J of the C Standard.) Consequently, it is necessary to reset the string to a known value to avoid possible errors on subsequent string manipulation functions.

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#include <stdio.h>
 
void func(FILE *file) {
  char buf[BUFSIZ];

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

}

However, buf is not reset and has unknown contents.

...

#include <stdio.h>
 
void func(FILE *file) {
  char buf[BUFSIZ];

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

}

Exceptions

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

Related Vulnerabilities

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