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CERT C: Rec. ARR01-C

Do not apply the sizeof operator to a pointer when taking the size of an array

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Description

Rule Definition

Do not apply the sizeof operator to a pointer when taking the size of an array.1

Polyspace Implementation

The rule checker checks for the issue Possible use of sizeof on array.

Examples

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Issue

The issue Possible use of sizeof on array occurs when Polyspace® Bug Finder™ detects possibly unintended results from the use of sizeof operator on an array. For instance:

  • You use the sizeof operator on an array parameter name, expecting the array size. However, the array parameter name by itself is a pointer. The sizeof operator returns the size of that pointer.

  • You use the sizeof operator on an array element, expecting the array size. However, the operator returns the size of the array element.

Risk

Incorrect use of the sizeof operator can cause the following issues:

  • If you expect the sizeof operator to return array size and use the return value to constrain a loop, the number of loop runs are smaller than what you expect.

  • If you use the return value of sizeof operator to allocate a buffer, the buffer size is smaller than what you require. Insufficient buffer can lead to resultant weaknesses such as buffer overflows.

  • If you use the return value of sizeof operator incorrectly in a function call, the function does not behave as you expect.

Fix

Do not use the sizeof operator on an array parameter name or array element to determine array size.

The best practice is to pass the array size as a separate function parameter and use that parameter in the function body.

Example - sizeof Used Incorrectly to Determine Array Size
#define MAX_SIZE 1024

void func(int a[MAX_SIZE]) {
    int i;

    for (i = 0; i < sizeof(a)/sizeof(int); i++)    { //Noncompliant
        a[i] = i + 1;
    }
}

In this example, sizeof(a) returns the size of the pointer a and not the array size.

Correction — Determine Array Size in Another Way

One possible correction is to use another means to determine the array size. For functions that operate on arrays, the best practice is to pass the size as an additional parameter.

#include<stddef.h>

void func(int a[], size_t aNumel) {
    int i;

    for (i = 0; i < aNumel; i++)    {
        a[i] = i + 1;
    }
}

Check Information

Group: Rec. 06. Arrays (ARR)

Version History

Introduced in R2019a

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See Also

Topics

External Websites

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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