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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 INT06See ERR34-A. Use strtol() or a related function to convert a string token to an integer).

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C. Detect errors when converting a string to a number.)

Noncompliant Code Example

This non-compliant noncompliant 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;

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 sl 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 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 rule recommendation to result in a security vulnerability, it can easily result in loss lost or misinterpreted data.

Automated Detection

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Related Vulnerabilities

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

References

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

Related Guidelines

Bibliography

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Image Added Image Added Image AddedINT04-A. Enforce limits on integer values originating from untrusted sources      04. Integers (INT)       INT06-A. Use strtol() or a related function to convert a string token to an integer