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

Invoking getc() and putc() with arguments that have side effects may cause unexpected results because 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, the file may be opened several times (see FIO31-C. Do not simultaneously open the same file multiple times).

const char *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.

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

/* Check fptr for validity */
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 the macro expansion of putc() with unintended results.

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

int c = 'a';

while (c <= 'z') {
  putc(c++, fptr);
}

Compliant Solution: putc()

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

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

int c = 'a';

while (c <= 'z') {
  putc(c, fptr);
  c++;
}

This example only illustrates the side effect issue.  The output will differ depending on the character set.

Risk Assessment

Using an expression that has side effects as the argument to getc() or putc() can result in unexpected behavior and possibly abnormal program termination.

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this rule.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

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

FIO40-C. Reset strings on fgets() failure      09. Input Output (FIO)       FIO42-C. Ensure files are properly closed when they are no longer needed