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Alternatively, input character data as a null-terminated byte string and convert to an integer value using strtol() or a related function. (See recommendation INT06ERR34-C. Use strtol() or a related function to convert a string token to an integerDetect errors when converting a string to a number.)

Noncompliant Code Example

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long slnum_long;

if (scanf("%ld", &slnum_long) != 1) {
  /* handleHandle error */
}

In general, do not use scanf() to parse integers or floating-point numbers from input strings because the input could contain numbers not representable by the argument type.

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This compliant example uses the Linux scanf() implementation's built-in error handling to validate input. On Linux platforms, scanf() sets errno to ERANGE if the result of integer conversion cannot be represented within the size specified by the format string [Linux 2008]. Note that this solution is a platform dependent solution. Therefore, this so it should only be used only where portability is not a concern.

long slnum_long;
errno = 0;

if (scanf("%ld", &slnum_long) != 1) {
  /* handleHandle error */
}
else if (ERANGE == errno) {
  if (puts("number out of range\n") == EOF) {
      /* Handle error */
  }
}

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This compliant example uses fgets() to input a string and strtol() to convert the string to an integer. Error checking is provided to make sure that the value is a valid integer in the range of long.

char buff[25];
char *end_ptr;
long slnum_long;

if (fgets(buff, sizeof(buff), stdin) == NULL) {
  if (puts("EOF or read error\n") == EOF) {
    /* Handle error */
  }
} else {
  errno = 0;

  slnum_long = strtol(buff, &end_ptr, 10);

  if (ERANGE == errno) {
    if (puts("number out of range\n") == EOF) {
      /* Handle error */
    }
  }
  else if (end_ptr == buff) {
    if (puts("not valid numeric input\n") == EOF) {
      /* Handle error */
    }
  }
  else if ('\n' != *end_ptr && '\0' != *end_ptr) {
    if (puts("extra characters on input line\n") == EOF) {
      /* Handle error */
    }
  }
}

Note that this solution treats any trailing characters, including white-space whitespace characters, as an error condition.

Risk Assessment

While it Although it is relatively rare for a violation of this recommendation to result in a security vulnerability, it can easily result in lost or misinterpreted data.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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Bibliography

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