Synopsis

Reset strings on fgets() or fgetws() failure.

Enabled by default

Yes

Severity/Certainty

Low/Medium

Full description

If either of the C Standard fgets() or fgetws() functions fail, the contents of the array being written is indeterminate. (See undefined behavior 170.) It is necessary to reset the string to a known value to avoid errors on subsequent string manipulation functions.

Coding standards

Code examples

The following code example fails the check and will give a warning:

#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 */
  }
  char c = buf[0];
}

The following code example passes the check and will not give a warning about this issue:

#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';
  }
}