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As Invoking getc() and putc() may be implemented as macros, calling them with parameters with arguments that have side effects may cause unexpected results , since the argument code could be evaluated multiple timesbecause these functions may be implemented as macros and arguments to these macros may be evaluated more than once.

Non-Compliant Code Example: getc()

This code calls the getc() function with an expression as an argument. If getc() is implemented as a macro expansion, the file may be opened several times (see FIO31-C. Do not simultaneously open the same file multiple times).

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In this non-compliant example, putc() is called with c++ as an argument. If putc() is implemented as a macro, c++ could be evaluated several times " within " the macro expansion of putc(), causing with unintended results.

char const *filename = "test.txt";
FILE *fptr = fopen(filename, "w");

int c = 97;

while (c < 123) {
  putc(c++, fptr);
}

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Using an expression that has side effects as the argument to getc() or putc() could can result in unexpected behavior.

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\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 7.19.7.5, "The {{getc}} function"; Section 7.19.7.8, "The {{putc}} function"