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Few programmers consider the issues around formatted I/O and type definitions. A programmer-defined integer type might be any type supported by the implementation, even a type larger than unsigned long long. For example, given an implementation that supports 128-bit unsigned integers and provides a uint_fast128_t type, a programmer may define the following type:

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Furthermore, the definition of programmer-defined types may change, which creates a problem when these types are used with formatted output functions, such as printf(), and formatted input functions, such as scanf(). (See FIO00FIO47-C. Take care when creating Use valid format strings.)

The C intmax_t and uintmax_t types can represent any value representable by any other integer types of the same signedness. (See INT00-C. Understand the data model used by your implementation(s).) This capability allows conversion between programmer-defined integer types (of the same signedness) and intmax_t and uintmax_t:

mytypedef_t x;
uintmax_t temp;

temp = x; /* Always secure if mytypedef_t is unsigned*/

/* ... Change the value of temp ... */

if (temp <= MYTYPEDEF_MAX) {
  x = temp;
}

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In addition to programmer-defined types, there is no requirement that an implementation provide format-length modifiers for implementation-defined integer types. For example, a machine with an implementation-defined 48-bit integer type may not provide format-length modifiers for the type. Such a machine still must have a 64-bit long long, with intmax_t being at least that large.

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#include <stdio.h>
#include <inttypes.h>

mytypedef_t x;

/* ... */

#ifdef _MSC_VER
  printf("%llu", (uintmax_t) x);
#else
  printf("%ju", (uintmax_t) x);
#endif  

Microsoft A feature request has been submitted a feature request to Microsoft to add support for the j length modifier to a future release of Microsoft Visual Studio.

Noncompliant Code Example (scanf())

The following This noncompliant code example reads an unsigned long long value from standard input and stores the result in x, which is of a programmer-defined integer type:

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This noncompliant code example can result in a buffer overflow if the size of mytypedef_t is smaller than unsigned long long, or it might result in an incorrect value if the size of mytypedef_t is larger than unsigned long long.  Moreover, scanf() lacks the error checking capabilities of alternative conversion routines, such as strtol(). For more information, see INT06-C. Use strtol() or a related function to convert a string token to an integer.

Compliant Solution (

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strtoumax())

This compliant solution guarantees that a correct value in the range of mytypedef_t is read, or an error condition is detected, assuming the value of MYTYPEDEF_MAX is correct as the largest value representable by mytypedef_t:  The strtoumax() function is used instead of scanf() as it provides enhanced error checking functionality.  The fgets() function is used to read input from stdin.

#include <stdio.h>
#include <inttypes.h>
#include <errno.h> 

mytypedef_t x;
uintmax_t temp;

/* ... */
if (fgets(buff, sizeof(buff), stdin) == NULL) {
  if (scanfputs("%ju", &temp) != 1EOF or read error\n") == EOF) {
    /* Handle error */
  }
} else {
  /* Check for errors in the conversion */
  errno = 0;
  temp = strtoumax(buff, &end_ptr, 10);
  if (ERANGE == errno) {
    if (temp > MYTYPEDEF_MAXputs("number out of range\n") == EOF) {
      /* Handle error */
    } 
  } else if (end_ptr == buff) {
  x = temp;
} 

Compliant Solution (Microsoft scanf())

Visual Studio 2012 and earlier versions do not support the standard j length modifier and do not have a nonstandard analog. Consequently, the programmer must hard code the knowledge that intmax_t is int64_t and uintmax_t is uint64_t for Microsoft Visual Studio versions.

#include <stdio.h>
#include <inttypes.h>

mytypedef_t x;
uintmax_t temp;

/* ... */
#ifdef _MSC_VER
#  define UINTMAX_CS "%llu"
#else
#  define UINTMAX_CS "%ju"
#endif

if (scanf(UINTMAX_CS, &temp) != 1) {
 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 */
    }
  }
  
  /* No conversion errors, attempt to store the converted value into x */
  if (temp > MYTYPEDEF_MAX) {
    /* Handle error */
  }
 else {
    x = temp;
} 

...

  }
}

Risk Assessment

Failure to use an appropriate conversion specifier when inputting or outputting programmer-defined integer types can result in buffer overflow and 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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