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Do not use functions that input characters and convert them to integers if the functions cannot handle all possible inputs. For example, formatted input functions such as scanf(), fscanf(), vscanf(), and vfscanf() can be used to read string data from stdin or (in the cases of fscanf() and vfscanf()) other input streams. These functions work fine for valid integer values but lack robust error handling for invalid values.

Wiki MarkupAlternatively, input character data as a null-terminated byte string and covert convert to an integer value using {{strtol()}} or a related function \[[INT06-A. Use strtol() or a related function to convert a string token to an integer]\].

Non-Compliant Code Example

function. (See ERR34-C. Detect errors when converting a string to a number.)

Noncompliant Code Example

This noncompliant This non-compliant code example uses the scanf() function to read a string from stdin and convert it to a long. The scanf() and fscanf() functions have undefined behavior if the value of the result of this operation cannot be represented as an integer.

long num_long;

long sl;

if (scanf("%ld", &num_long) != 1) {
  /* Handle 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.

Compliant Solution (Linux)

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 platform dependent, so it should be used only where portability is not a concern.

long num_long;
errno = 0;

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

Compliant Solution

This compliant example uses fgets() to input a string and strtol() to convert the string to a 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;

sl  num_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 rule recommendation to result in a security vulnerability, it could more can easily result in loss 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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References

Wiki Markup\[[Klein 02|AA. C References#Klein 02]\] \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.1.4, "The strtol, strtoll, strtoul, and strtoull functions," and Section 7.19.6, "Formatted input/output functions"