FIO41-C. Do not call getc() or putc() with parameters that have side effects

As getc() and putc() may be implemented as macros, calling them with parameters that have side effects may cause unexpected results, since the argument code could be evaluated multiple times.

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 a file multiple times).

char const *filename = "test.txt"; 
FILE *fptr; 
 
int c = getc(fptr = fopen(filename, "r")); 

Compliant Solution: getc()

In this compliant solution, getc() is no longer called with an expression as its argument.

char const *filename = "test.txt"; 
FILE *fptr = fopen(filename, "r");
 
int c = getc(fptr); 

Non-Compliant Code Example: putc()

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" putc(), causing unintended results.

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

int c = 97;

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

Compliant Solution: putc()

In the compliant solution, c++ is no longer an argument to putc().

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

int c = 97;

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

Risk Assessment

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

References

[[ISO/IEC 9899-1999:TC2]] Section 7.19.7.5, "The getc function"; 7.19.7.8, "The putc function"