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Conversions can occur explicitly as the result of a cast or implicitly as required by an operation. Although conversions are generally required for the correct execution of a program, they can also lead to lost or misinterpreted data. Conversion of an operand value to a compatible type causes no change to the value or the representation.

The C integer conversion rules define how C compilers handle conversions. These rules include integer promotions, integer conversion rank, and the usual arithmetic conversions. The intent of the rules is to ensure that the conversions result in the same numerical values and that these values minimize surprises in the rest of the computation. Prestandard C usually preferred to preserve signedness of the type.

Integer Promotions

Integer types smaller than int are promoted when an operation is performed on them. If all values of the original type can be represented as an int, the value of the smaller type is converted to an int; otherwise, it is converted to an unsigned int. Integer promotions are applied as part of the usual arithmetic conversions to certain argument expressions; operands of the unary +, -, and ~ operators; and operands of the shift operators. The following code fragment shows the application of integer promotions:

Integer promotions require the promotion of each variable (c1 and c2) to int size. The two int values are added, and the sum is truncated to fit into the char type. Integer promotions are performed to avoid arithmetic errors resulting from the overflow of intermediate values:

In this example, the value of c1 is multiplied by c2. The product of these values is then divided by the value of c3 (according to operator precedence rules). Assuming that signed char is represented as an 8-bit value, the product of c1 and c2 (300) cannot be represented. Because of integer promotions, however, c1, c2, and c3 are each converted to int, and the overall expression is successfully evaluated. The resulting value is truncated and stored in cresult. Because the final result (75) is in the range of the signed char type, the conversion from int back to signed char does not result in lost data.

Integer Conversion Rank

Every integer type has an integer conversion rank that determines how conversions are performed. The ranking is based on the concept that each integer type contains at least as many bits as the types ranked below it. The following rules for determining integer conversion rank are defined in the C Standard, subclause 6.3.1.1 [ISO/IEC 9899:2011]:

  • No two signed integer types shall have the same rank, even if they have the same representation.
  • The rank of a signed integer type shall be greater than the rank of any signed integer type with less precision.
  • The rank of long long int shall be greater than the rank of long int, which shall be greater than the rank of int, which shall be greater than the rank of short int, which shall be greater than the rank of signed char.
  • The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type, if any.
  • The rank of any standard integer type shall be greater than the rank of any extended integer type with the same width.
  • The rank of char shall equal the rank of signed char and unsigned char.
  • The rank of _Bool shall be less than the rank of all other standard integer types.
  • The rank of any enumerated type shall equal the rank of the compatible integer type.
  • The rank of any extended signed integer type relative to another extended signed integer type with the same precision is implementation-defined but still subject to the other rules for determining the integer conversion rank.
  • For all integer types T1, T2, and T3, if T1 has greater rank than T2 and T2 has greater rank than T3, then T1 has greater rank than T3.

The integer conversion rank is used in the usual arithmetic conversions to determine what conversions need to take place to support an operation on mixed integer types.

Usual Arithmetic Conversions

The usual arithmetic conversions are rules that provide a mechanism to yield a common type when both operands of a binary operator are balanced to a common type or the second and third operands of the conditional operator ( ? : ) are balanced to a common type. Conversions involve two operands of different types, and one or both operands may be converted. Many operators that accept arithmetic operands perform conversions using the usual arithmetic conversions. After integer promotions are performed on both operands, the following rules are applied to the promoted operands:

  1. If both operands have the same type, no further conversion is needed.
  2. If both operands are of the same integer type (signed or unsigned), the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank.
  3. If the operand that has unsigned integer type has rank greater than or equal to the rank of the type of the other operand, the operand with signed integer type is converted to the type of the operand with unsigned integer type.
  4. If the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, the operand with unsigned integer type is converted to the type of the operand with signed integer type.
  5. Otherwise, both operands are converted to the unsigned integer type corresponding to the type of the operand with signed integer type.

Example

In the following example, assume the code is compiled using an implementation with 8-bit char, 32-bit int, and 64-bit long long:

Both the signed char sc and the unsigned char uc are subject to integer promotions in this example. Because all values of the original types can be represented as int, both values are automatically converted to int as part of the integer promotions. Further conversions are possible if the types of these variables are not equivalent as a result of the usual arithmetic conversions. The actual addition operation, in this case, takes place between the two 32-bit int values. This operation is not influenced by the resulting value being stored in a signed long long integer. The 32-bit value resulting from the addition is simply sign-extended to 64 bits after the addition operation has concluded.

Assuming that the precision of signed char is 7 bits, and the precision of unsigned char is 8 bits, this operation is perfectly safe. However, if the compiler represents the signed char and unsigned char types using 31- and 32-bit precision (respectively), the variable uc would need to be converted to unsigned int instead of signed int. As a result of the usual arithmetic conversions, the signed int is converted to unsigned, and the addition takes place between the two unsigned int values. Also, because uc is equal to UCHAR_MAX, which is equal to UINT_MAX, the addition results in an overflow in this example. The resulting value is then zero-extended to fit into the 64-bit storage allocated by sll.

Noncompliant Code Example (Comparison)

The programmer must be careful when performing operations on mixed types. This noncompliant code example shows an idiosyncrasy of integer promotions:

In this example, the comparison operator operates on a signed int and an unsigned int. By the conversion rules, si is converted to an unsigned int. Because −1 cannot be represented as an unsigned int value, the −1 is converted to UINT_MAX in accordance with the C Standard, subclause 6.3.1.3, paragraph 2 [ISO/IEC 9899:2011]:

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

Consequently, the program prints 0 because UINT_MAX is not less than 1.

Compliant Solution

The noncompliant code example can be modified to produce the intuitive result by forcing the comparison to be performed using signed int values:

This program prints 1 as expected. Note that (int)ui is correct in this case only because the value of ui is known to be representable as an int. If it were not known, the compliant solution would need to be written as

Noncompliant Code Example

This noncompliant code example demonstrates how performing bitwise operations on integer types smaller than int may have unexpected results:

In this example, a bitwise complement of port is first computed and then shifted 4 bits to the right. If both of these operations are performed on an 8-bit unsigned integer, then result_8 will have the value 0x0a. However, port is first promoted to a signed int, with the following results (on a typical architecture where type int is 32 bits wide):

Expression

Type

Value

Notes

port

uint8_t

0x5a

 

~port

int

0xffffffa5

 

~port >> 4

int

0x0ffffffa

Whether or not value is negative is implementation-defined

result_8

uint8_t

0xfa

 

Compliant Solution

In this compliant solution, the bitwise complement of port is converted back to 8 bits. Consequently, result_8 is assigned the expected value of 0x0aU.

Risk Assessment

Misunderstanding integer conversion rules can lead to errors, which in turn can lead to exploitable vulnerabilities. The major risks occur when narrowing the type (which requires a specific cast or assignment), converting from unsigned to signed, or converting from negative to unsigned.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

INT02-C

Medium

Probable

Medium

P8

L2

Automated Detection

Tool

Version

Checker

Description

CodeSonar4.4

ALLOC.SIZE.TRUNC

LANG.CAST.COERCE

LANG.CAST.VALUE

MISC.MEM.SIZE.TRUNC

Truncation of Allocation Size

Coercion Alters Value

Cast Alters Value

Truncation of Size

ECLAIR

1.2

CC2.INT02

Fully implemented

Klocwork2017MISRA.CAST.INT
MISRA.CAST.UNSIGNED_BITS
MISRA.CONV.INT.SIGN
MISRA.CVALUE.IMPL.CAST
MISRA.UMINUS.UNSIGNED
PRECISION.LOSS
 
LDRA tool suite9.5.6


52 S, 93 S, 96 S, 101 S, 107 S, 332 S, 334 S, 433 S, 434 S, 446 S, 452 S, 457 S, 458 S

Fully implemented

Parasoft C/C++test9.5MISRA-043{b}Partially implemented
Polyspace Bug FinderR2016a

Integer conversion overflow

Integer overflow

Tainted sign change conversion

Overflow when converting between integer types

Overflow from operation between integers

Value from an unsecure source changes sign

PRQA QA-C9.31256, 1257, 1266, 1290, 1291, 1292, 1293, 1294, 1295, 1296, 1297, 1298, 1299, 4401, 4402, 4403,
4404, 4405, 4410, 4412, 4413, 4414, 4415, 4420, 4421, 4422, 4423, 4424, 4425, 4430, 4431, 4432,
4434, 4435, 4436, 4437, 4440, 4441, 4442, 4443, 4445, 4446, 4447, 4460, 4461, 4463, 4464, 4470,
4471, 4480, 4481, 1250, 1251, 1252, 1253, 1260, 1263, 1274, 1800, 1802, 1803, 1804, 1810, 1811,
1812, 1813, 1820, 1821, 1822, 1823, 1824, 1830, 1831, 1832, 1833, 1834, 1840, 1841, 1842, 1843,
1844, 1850, 1851, 1852, 1853, 1854, 1860, 1861, 1862, 1863, 1864, 1880, 1881, 1882, 2100, 2101,
2102, 2103, 2104, 2105, 2106, 2107, 2109, 2110, 2111, 2112, 2113, 2114, 2115, 2116, 2117, 2118,
2119, 2120, 2122, 2124, 2130, 2132, 2134
Fully implemented

Related Vulnerabilities

This vulnerability in Adobe Flash arises because Flash passes a signed integer to calloc(). An attacker has control over this integer and can send negative numbers. Because calloc() takes size_t, which is unsigned, the negative number is converted to a very large number, which is generally too big to allocate, and as a result, calloc() returns NULL, causing the vulnerability to exist.

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

Related Guidelines

SEI CERT C++ Coding StandardINT02-CPP. Understand integer conversion rules
ISO/IEC TR 24772:2013Numeric Conversion Errors [FLC]
MISRA C:2012Rule 10.1 (required)
Rule 10.3 (required)
Rule 10.4 (required)
Rule 10.6 (required)
Rule 10.7 (required)
Rule 10.8 (required)
MITRE CWECWE-192, Integer coercion error
CWE-197, Numeric truncation error

Bibliography

[Dowd 2006]Chapter 6, "C Language Issues" ("Type Conversions," pp. 223–270)
[Seacord 2013]

Chapter 5, "Integer Security"

 


11 Comments

  1. In NCCE, why do you need to say "unsigned int is treated modularly" since unsigend in ui = 1 and there's no possibility of wraparound?

    1. Well, because there IS possibility of wraparound; it's just not mandated by the standard. On many platforms, including x86, signed ints do wrap. But the standard leaves this as undefined behavior, allowing other platforms to treat signed int overflow differently.

      I changed the NCCE language you cite to reference the standard, and added the relevant quote from C99. It boils down to modular arithmetic, but is not defined in those terms, and we try to prefer the language used by the standard. HTH.

  2. The explicit cast in the last line of the first compliant solution seems to be redundant:

    The automatic conversion in such a case is mentioned above and confirmed by C99, Section 6.3.1.8, paragraph 1.

    1. Yes it is technically redundant. It's still worthwhile IMHO as the conversion is not obvious to most programmers.

  3. Because −1 cannot be represented as an unsigned int value, the −1 is converted to UINT_MAX in accordance with the C Standard, subclause 6.3.1.3, paragraph 2 [ISO/IEC 9899:2011]:

    This doesn't seem to be correct. IIUC the -1 is converted to an unsigned integer by adding UINT_MAX+1.

    Outputs 4294967286, which is not the same as UINT_MAX.

    1. Did you mean to use -10 instead of -1?

      A quick test of:

      Yields:

      4294967295
      4294967295

      Which is what I would expect given the section quoted in the standard. Of special note, the footnote in that paragraph is not replicated. It reads:

      The rules describe arithmetic on the mathematical value, not the value of a given type of expression.

      which helps explain how this conversion process works.

      1. I understood the part I quoted to mean that every signed value that can't be represented as an unsigned value is converted to UINT_MAX. I used -10 to demonstrate that that is not correct.

        For -1 the conversion will actually yield UINT_MAX, because -1+UINT_MAX+1 equals UINT_MAX. For every other value that cannot be represented by an unsigned this is not the case.

        Am I misunderstanding the quoted part, or is it incorrect? (smile)

        1. I think you're misunderstanding the quoted part. It's saying that the signed value -10 cannot be represented as an unsigned int, so the value is converted to an unsigned int as described by the standard. The way the standard phrases that is rather obtuse. In this example, the new type is unsigned int, and the maximum value for that is UINT_MAX. The way the conversion works is to take the original value and add/subtract UINT_MAX + 1 until you get a value that is in the range allowed by unsigned int. Eg) unsigned int converted_value = -10 + UINT_MAX + 1; which is how you arrive at the value 4294967286.

  4. there is a strange sentence at the end of the usual arithmetic conversion rules:

     

    Specific operations can add to or modify the semantics of the usual arithmetic operations.

     

    (added since revision 43)

    I think it should be removed.  Or, any meaning for this sentence I didn't notice?

    C11 and C++11 both have nothing like this around the usual arithmetic conversion rules...

     

    1. I don't think that text was meant to be part of a bullet point, but was meant to be a new paragraph after. I think the intent was to call out operations where the usual arithmetic conversions normally apply, but with slightly different semantics (the wording for ?: used to have something for that, IIRC). However, in a quick look through the C11 spec, I could not find any cases where the usual arithmetic conversions apply with different semantics, and so I've removed that text entirely.