INT06-A. Use strtol() or a related function to convert a string token to an integer

Use strtol() or a related function to convert a string token to an integer. These functions provide more robust error handling than alternative solutions.

The strtol(), strtoll(), strtoul(), and strtoull() functions convert the initial portion of a null-terminated byte string to long int, long long int, unsigned long int, and unsigned long long int representation, respectively.

Use the strtol() function to convert to smaller, signed integer types such as signed int, signed short or signed char), testing the result against the range limits for that type.

Use the strtoul() function to convert to a smaller unsigned integer type such as unsigned int, unsigned short, or unsigned char, testing the result against the range limits for that type.

These range tests do nothing if the smaller type happens to have the same size and representation on a particular compiler.

Non-Compliant Code Example

This non-compliant code example converts the string token stored in the static array buff to a signed integer value using the atoi() function.

int si;

if (argc > 1) {
  si = atoi(argv[1]);
}

The atoi(), atol(), and atoll() functions convert the initial portion of a string token to int, long int, and long long int representation, respectively. Except for the behavior on error, they are equivalent to

atoi: (int)strtol(nptr, (char **)NULL, 10)
atol: strtol(nptr, (char **)NULL, 10)
atoll: strtoll(nptr, (char **)NULL, 10)

Unfortunately, atoi() and related functions lack a mechanism for reporting errors for invalid values. Specifically, the atoi(), atol(), and atoll() functions:

• do not need to set errno on an error
• have undefined behavior if the value of the result cannot be represented

Non-Compliant Example

This non-compliant example uses the sscanf() function to convert a string token to an integer. The sscanf() function has the same limitations as atoi().

int si;

if (argc > 1) {
  sscanf(argv[1], "%d", &si);
}

Compliant Solution

The strtol(), strtoll(), strtoul(), and strtoull() functions convert a null-terminated byte string to long int, long long int, unsigned long int, and unsigned long long int representation, respectively.

This compliant solution uses strtol() to convert a string token to an integer and ensures that the value is in the range of int.

long sl;
int si;
char *end_ptr;

if (argc > 1) {

  errno = 0;

  sl = strtol(argv[1], &end_ptr, 10);

  if (ERANGE == errno) {
    puts("number out of range\n");
  }
  else if (sl > INT_MAX) {
    printf("%ld too large!\n", sl);
  }
  else if (sl < INT_MIN) {
    printf("%ld too small!\n", sl);
  }
  else if (end_ptr == argv[1]) {
    puts("invalid numeric input\n");
  }
  else if ('\0' != *end_ptr) {
    puts("extra characters on input line\n");
  }
  else {
    si = (int)sl;
  }
}

Risk Assessment

While it is relatively rare for a violation of this rule to result in a security vulnerability, it could more easily result in loss or misinterpreted data.

Related Vulnerabilities

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

References

[[Klein 02]]
[[ISO/IEC 9899-1999]] Section 7.20.1.4, "The strtol, strtoll, strtoul, and strtoull functions," Section 7.20.1.2, "The atoi, atol, and atoll functions," and Section 7.19.6.7, "The sscanf function"