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(See also undefined behavior 113  of Appendix J.)

Compliance with this rule is complicated by the fact that the char data type can, in any implementation, be signed or unsigned.

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Noncompliant Code Example

This noncompliant code example may pass invalid values to the isspace() function:On implementations where plain char is signed, this code example is noncompliant because the parameter to isspace(), *t, is defined as a const char *, and this value may not be representable as an unsigned char.

#include <ctype.h>
#include <stddef.h>
 
ptrdiffsize_t count_preceding_whitespace(const char *s) {
  const char *t = s;

  /* Possibly *t < 0 */size_t length = strlen(s) + 1;
  while (isspace(*t) && isspace(*tt - s < length)) { 
    ++t;
  }
  return t - s;
}
 

The argument to isspace() must be EOF or representable as an unsigned char; otherwise, the result is undefined.

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#include <ctype.h>
#include <stddef.h>
 
ptrdiffsize_t count_preceding_whitespace(const char *s) {
  const char *t = s;
  size_t length = strlen(s) + 1;
  while (*t && isspace((unsigned char)*t) && (t - s < length)) { 
    ++t;
  }
  return t - s;
}
 

Risk Assessment

Passing values to character handling functions that cannot be represented as an unsigned char results in undefined program behavior.

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