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MISRA C:2023 Rule 21.18

The size_t argument passed to any function in <string.h> shall have an appropriate value

Since R2024a

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Description

Rule Definition

The size_t argument passed to any function in <string.h> shall have an appropriate value.

Rationale

The value of a size_t argument passed to a function defined in <string.h> must be positive and not greater than the size of the smallest object passed by pointer to the function. Consider this code, which compares the strings str1 and str2 by using the strncmp() function :

strncmp(lhs_string, rhs_string, num)
The size_t argument num must be positive and must not be greater than the size of str1 or str2, whichever is smaller.

Otherwise, using the function can result in read or write access beyond the bounds of the function arguments.

Polyspace Implementation

Polyspace® reports a violation if a call to a memory or string function from <string.h> results in read or write access beyond the bounds of its arguments. For example:

  • When arguments of a memory or string function are buffers lacking null-termination, violations are reported when the value passed to the size parameter is greater than the size of at least one of the buffers. Consider this code:

    char buf1[ 3 ] = "abc";
char buf2[ 6 ] = "123456";
(void) memcpy(buf2, buf1, 5); // Violation
(void) memcpy(buf1, buf2, 4); // Violation
    The calls to memcpy() results in write access of memory locations beyond the bounds of buf1.

  • When arguments of a memory or string functions are null-terminated buffers or C-strings and the destination string is smaller than the source string, violation is reported if the value passed to the size argument is larger than the size of the destination string. Consider this code:

    char str1[] = "abc";
char str2[] = "123456";
(void) strncpy(str1, str2, 5); // Violation
    Copying 5 characters from str2 to str1 results in write access of memory locations beyond the bounds of str1. For null-terminated buffers or C-strings, violations are not reported when the destination string is larger than the source string, regardless of the size argument.

Polyspace checks for violations of this rule when memory and string functions from <string.h> are called, including: memchr(), memcmp(), memcpy(), memmove(), memset(), strncat(), strncmp(), strncpy(), strxfrm(). If the sizes of the argument buffers or strings are not known when these functions are called, Polyspace does not report a violation.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

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#include <string.h>
char buf1[ 5 ] = "12345";
char buf2[ 10 ] = "1234567890";

void f(void)
{
    if (memcmp(buf1, buf2, 5) == 0) {    /* Compliant */

    }
    if (memcmp(buf1, buf2, 6) == 0) {    /* Non-compliant */

    }
}

In this example, the first if statement is compliant because copying the first five elements from buf2 to buf1 does not result in read or write access beyond the bounds of these buffers.

The second if statement is noncompliant because the value of the size argument is six, which greater than the size of the destination buffer buf1. This call to memcmp() results in write access beyond the bounds of buf1.

Check Information

Group: Standard libraries
Category: Mandatory
AGC Category: Mandatory

Version History

Introduced in R2024a

See Also

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