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The process of parsing an integer or floating-point number from a string can produce many errors. The string might not contain a number. It might contain a number of the correct type that is out of range (such as an integer that is larger than INT_MAX). The string may also contain extra information after the number, which may or may not be useful after the conversion. These error conditions must be detected and addressed when a string-to-number conversion is performed .Use strtol() or a related function to parse an integer out of a string. Likewise, use strtof() or a related function to parse a floating-point number from a string. These functions provide more robust error handling than alternative solutions.using a C Standard Library function.

The strtol(), strtoll(),  strtoimax(), strtoul(), strtoull(), strtoumax(), strtof strtof(), strtod(), and strtold() functions convert the initial portion of a null-terminated byte string to a long int, long long int, intmax_t, unsigned long int, unsigned long long int, uintmax_t, float, double, and long double representation, respectively.

Use one of the C Standard Library strto*() functions to parse an integer or floating-point number from a string. These functions provide more robust error handling than alternative solutions. Also, use the strtol() function to convert to a smaller signed integer type such as signed int, signed short, and signed char, testing the result against the range limits for that type. Likewise, use the strtoul() function to convert to a smaller unsigned integer type such as unsigned int, unsigned short, and unsigned char, and test the result against the range limits for that type.These  These range tests do nothing if the smaller type happens to have the same size and representation on for a particular compilerimplementation.

Noncompliant Code Example (atoi())

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The atoi(), atol(), atoll(), and atof() functions convert the initial portion of a string token to int, long int, long long int, and double representation representation, respectively. Except for the behavior on error ([ISO/IEC 9899:2024], s7.24.1.2), they are equivalent to

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

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• do not need to set errno on an error;
• have have undefined behavior 16 if the value of the result cannot be represented;
• return 0 (or 0.0) if the string does not represent an integer (or decimal), which is indistinguishable from a correctly formatted, zero-denoting input string.

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Compliant Solution (strtol())

The strtol(), strtoll(), strtoimax()), strtoul(), strtoull(), strtoumax(), strtof(), strtod(), and strtold() functions convert a null-terminated byte string to long int, long long int, intmax_t, unsigned long int, unsigned long long int, uintmax_t, float float, double, and long double representation, respectively.

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#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include <stdio.h>
 
void func(const char *buff) {
  char *end;
  int si;

  errno = 0;

  const long sl = strtol(buff, &end, 10);

  if (end == buff) {
    (void) fprintf(stderr, "%s: not a decimal number\n", buff);
  } else if ('\0' != *end) {
    (void) fprintf(stderr, "%s: extra characters at end of input: %s\n", buff, end);
  } else if ((LONG_MIN == sl || LONG_MAX == sl) && ERANGE == errno) {
    (void) fprintf(stderr, "%s out of range of type long\n", buff);
  } else if (sl > INT_MAX) {
    (void) fprintf(stderr, "%ld greater than INT_MAX\n", sl);
  } else if (sl < INT_MIN) {
    (void) fprintf(stderr, "%ld less than INT_MIN\n", sl);
  } else {
    si = (int)sl;

    /* Process si */
  }
}

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It is rare for a violation of this rule to result in a security vulnerability unless it occurs in security-sensitive code. However, violations of this rule can easily result in lost or misinterpreted data. Checking for incorrect usage of standard library functions such as atoi() is automatically detectable, however checking for proper custom number-parsing functions is more difficult. 

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-20

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and ERR34-C

Intersection( ERR34-C, CWE-20) = Ø

CERT C does not define the concept of ‘input validation’. String-to-integer conversion (ERR34-C) may qualify as input validation, but this is outside the scope of the CERT rule.

CWE-391 and ERR34-C

CWE-391 = Union( ERR34-C, list) where list =

• Failure to errors outside of string-to-number conversion functions

CWE-676 and ERR34-C

• Independent( ENV33-C, CON33-C, STR31-C, EXP33-C, MSC30-C, ERR34-C)

• ERR34-C implies that string-parsing functions (eg atoi() and scanf()) are dangerous.

• CWE-676 = Union( ERR34-C, list) where list =

• Invocation of dangerous functions besides the following:

• atoi(), atol(), atoll(), atof(), The scanf()family

CWE-758 and ERR34-C

Independent( INT34-C, INT36-C, MSC37-C, FLP32-C, EXP33-C, EXP30-C, ERR34-C, ARR32-C)

Intersection( CWE-758, ERR34-C) =

• Undefined behavior arising from a non-representable numeric value being parsed by an ato*() or scanf() function

CWE-758 – ERR34-C =

• Undefined behavior arising from using a function outside of the ato*() or scanf() family

ERR34-C – CWE-758 =

• The ato*() or scanf() family receives input that is not a number when trying to parse one

Bibliography

Bibliography

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