CERT C: Rec. MSC09-C

R2026b

Character encoding: Use subset of ASCII for safety

Since R2026b

Description

Character encoding: Use subset of ASCII for safety¹

Polyspace Implementation

Polyspace^® checks for the issue Unsafe characters in filename.

Examples

Unsafe characters in filename

Issue

The issue occurs when a filename passed to a file-opening function, such as open, fopen, or freopen, contains characters outside the ASCII subset [0-9a-zA-Z%&+,-.:=_].

Risk

Filenames with non-ASCII or special characters can cause portability issues across operating systems and file systems. Characters such as spaces and backslashes, or extended encoding sequences, can be misinterpreted, leading to unintended file access or file-not-found errors.

Fix

• Restrict filenames to the safe ASCII subset: digits, uppercase and lowercase letters, and the characters %, &, +, ,, -, ., :, =, and _.

• Sanitize user input intended to be used as filenames so that it uses only the safe ASCII subset.

Example — Non-ASCII Characters in Filename

The file name "\xe5ngstr\xf6m" contains non-ASCII characters (byte values outside the printable ASCII range). Polyspace reports a violation of this rule on the call to open().

#include <fcntl.h>
#include <sys/stat.h>

int main(void) {
    char *file_name = "\xe5ngstr\xf6m";
    mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
    int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, mode); // Noncompliant
    if (fd == -1) { }
    return 0;
}

Correction — Use Safe ASCII Filename

To fix this violation, use a filename that contains only these characters: [0-9a-zA-Z%&+,-.:=_].

#include <fcntl.h>
#include <sys/stat.h>

int main(void) {
    char *file_name = "angstrom.dat";
    mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
    int fd = open(file_name, O_CREAT | O_EXCL | O_WRONLY, mode); // Compliant
    if (fd == -1) { }
    return 0;
}

Check Information

Group: Rec. 48. Miscellaneous (MSC)

PQL Name: std.cert.MSC09_C

Version History

Introduced in R2026b