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Do not invoke getc() or putc() or their wide-character analogues getwc() and putwc() with a stream argument that has side effects. The stream argument passed to these macros may be evaluated more than once if these functions are implemented as unsafe macros. (see See PRE31-C. Avoid side effects in arguments to unsafe macros for more information.).

This rule does not apply to the character argument in putc() or the wide-character argument in putwc(), which is guaranteed to be evaluated exactly once.

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This noncompliant code example calls the getc() function with an expression as the stream argument. If getc() is implemented as a macro, the file may be opened multiple times. (see See FIO24-C. Do not open a file that is already open.).

#include <stdio.h>
 
void func(const char *file_name) {
  FILE *fptr;

  int c = getc(fptr = fopen(file_name, "r"));
  if (feof(stdin) || ferror(stdin)) {
    /* Handle error */
  }

  if (fclose(fptr) == EOF) {
    /* Handle error */
  }
}

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NOTE: The output of this compliant solution differs depending on the character set. For example, when run on a machine using an ASCII-derived code set such as ISO-8859 or Unicode, this solution will print out the 26 lowercase letters of the English alphabet. However, if run with an EBCDIC-based code set, such as Codepage 037 or Codepage 285, punctuation marks or symbols may be output between the letters.

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