MISRA C:2025 Rule 21.6

R2026b

The Standard Library input/output functions shall not be used

Since R2026b

Description

The Standard Library input/output functions shall not be used ¹ .

Rationale

This rule applies to the functions that are provided by <stdio.h> and in C99, their character-wide equivalents provided by <wchar.h>. Using these functions can cause unspecified, undefined and implementation-defined behavior.

Polyspace Implementation

Polyspace^® reports a violation if you call the file and stream input/output functions from the <stdio.h> and <wchar.h> headers. If the Standard Library function is a macro and the macro is expanded in the code, this rule is violated. Polyspace assumes that rule 21.2 is not violated.

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

Use of Input and Output Functions from <stdio.h> and <wchar.h>

In this example, the function func() calls various file and stream input/output functions from the headers <stdio.h> and <wchar.h>. Polyspace reports violations of this rule on their uses.

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

void mixed_io_example() {
    // Define a buffer for wide characters
    wchar_t wbuffer[50];

    // Read a wide string from standard input
    wprintf(L"Enter a wide string: "); // Noncompliant
    fgetws(wbuffer, 50, stdin); // Noncompliant

    // Print the wide string using wide character output
    wprintf(L"You entered: %ls", wbuffer); // Noncompliant

    // Define a buffer for regular characters
    char buffer[50];

    // Read a regular string from standard input
    printf("Enter a regular string: "); // Noncompliant
    fgets(buffer, 50, stdin); // Noncompliant

    // Print the regular string using regular character output
    printf("You entered: %s", buffer); // Noncompliant
}

Check Information

Group: Standard Libraries

Category: Required

AGC Category: Required

PQL Name: std.misra_c_2025.R21_6

Version History

Introduced in R2026b