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The only integer type conversions that are guaranteed to be safe for all data values and all possible conforming implementations are conversions of an integral value to a wider type of the same signedness. Subclause 6.3.1.3 of the C Standard [ISO/IEC 9899:2011] says,

When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged.

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.

Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.

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#include <limits.h>
 
void func(void) {
  unsigned long int u_a = ULONG_MAX;
  signed char sc;
  sc = (signed char)u_a; /* castCast eliminates warning */
  /* ... */
}

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#include <limits.h>
 
void func(void) {
  unsigned long int u_a = ULONG_MAX;
  signed char sc;
  if (u_a <= SCHAR_MAX) {
    sc = (signed char)u_a;  /* useUse cast to eliminate warning */
  } else {
    /* handleHandle error condition */
  }
}

Noncompliant Code Example (Signed to Unsigned)

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#include <limits.h>

void func(void) {
  signed int si = INT_MIN;
  unsigned int ui = (unsigned int)si;  /* castCast eliminates warning */

  /* ... */
}

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#include <limits.h>

void func(void) {
  signed int si = INT_MIN;
  unsigned int ui;
  if (si < 0) {
    /* handleHandle error condition */
  } else {
    ui = (unsigned int)si;  /* castCast eliminates warning */
  }
  /* ... */
}

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#include <limits.h>

void func(void) {
  signed long int s_a = LONG_MAX;
  signed char sc = (signed char)s_a; /* castCast eliminates warning */
  /* ... */
}

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#include <limits.h>

void func(void) {
  signed long int s_a = LONG_MAX;
  signed char sc;
  if ((s_a < SCHAR_MIN) || (s_a > SCHAR_MAX)) {
    /* handleHandle error condition */
  } else {
    sc = (signed char)s_a; /* useUse cast to eliminate warning */
  }
  /* ... */
}

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#include <limits.h>

void func(void) {
  unsigned long int u_a = ULONG_MAX;
  unsigned char uc = (unsigned char)u_a;  /* castCast eliminates warning */
  /* ... */
}

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#include <limits.h>

void func(void) {
  unsigned long int u_a = ULONG_MAX;
  unsigned char uc;
  if (u_a > UCHAR_MAX) ) {
    /* handleHandle error condition */
  } else {
    uc = (unsigned char)u_a; /* useUse cast to eliminate warning */
  }
  /* ... */
}

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Exceptions

INT31-EX1: The C standard defines Standard defines minimum ranges for standard integer types. For example, the minimum range for an object of type unsigned short int is 0 to 65,535, while whereas the minimum range for int is −32,767 to +32,767. This means that Consequently, it is not always possible to represent all possible values of an unsigned short int as an int. However, on the IA-32 architecture, for example, the actual integer range is from −2,147,483,648 to +2,147,483,647, meaning that it is quite possible to represent all the values of an unsigned short int as an int for this architecture. As a result, it is not necessary to provide a test for this conversion on IA-32. It is not possible to make assumptions about conversions without knowing the precision of the underlying types. If these tests are not provided, assumptions concerning precision must be clearly documented, as the resulting code cannot be safely ported to a system where these assumptions are invalid. A good way to document these assumptions is to use static assertions. (See DCL03-C. Use a static assertion to test the value of a constant expression.)

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