CERT C++: FIO37-C

Do not assume that fgets() or fgetws() returns a nonempty string when successful

Description

Rule Definition

Do not assume that fgets() or fgetws() returns a nonempty string when successful.¹

Polyspace Implementation

The rule checker checks for Use of indeterminate string.

Examples

Use of indeterminate string

Issue

Use of indeterminate string occurs when you do not check the validity of the buffer returned from fgets-family functions. The checker raises a defect when such a buffer is used as:

• An argument in standard functions that print or manipulate strings or wide strings.

• A return value.

• An argument in external functions with parameter type const char * or const wchar_t *.

Risk

If an fgets-family function fails, the content of its output buffer is indeterminate. Use of such a buffer has undefined behavior and can result in a program that stops working or other security vulnerabilities.

Fix

Reset the output buffer of an fgets-family function to a known string value when the function fails.

Example - Output of fgets() Passed to External Function

#include <stdio.h>
#include <wchar.h>
#include <string.h>
#include <stdlib.h>

#define SIZE20 20

extern void display_text(const char *txt);

void func(void) {
    char buf[SIZE20];
	
	/* Check fgets() error */
    if (fgets (buf, sizeof (buf), stdin) == NULL)
    {
        /* 'buf' may contain an indeterminate string.  */
        ;
    }
	/* 'buf passed to external function */
    display_text(buf); //Noncompliant
}
        
      

In this example, the output buf is passed to the external function display_text(), but its value is not reset if fgets() fails.

Correction — Reset fgets() Output on Failure

If fgets() fails, reset buf to a known value before you pass it to an external function.

#include <stdio.h>
#include <wchar.h>
#include <string.h>
#include <stdlib.h>

#define SIZE20 20

extern void display_text(const char *txt);

void func1(void) {
    char buf[SIZE20];
	/* Check fgets() error */
    if (fgets (buf, sizeof (buf), stdin) == NULL)
    {
		/* value of 'buf' reset after fgets() failure. */
        buf[0] = '\0';
    }
	/* 'buf' passed to external function */
    display_text(buf); 
} 

Check Information

Group: 07. Input Output (FIO)

Version History

Introduced in R2019a