CERT C: Rule FIO37-C

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

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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

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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: Rule 09. Input Output (FIO)

Version History

Introduced in R2019a

¹ This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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