CodeQL documentation

Use of potentially dangerous function

ID: cpp/potentially-dangerous-function
Kind: problem
Severity: warning
Precision: high
   - reliability
   - security
   - external/cwe/cwe-676
Query suites:
   - cpp-code-scanning.qls
   - cpp-security-extended.qls
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

This rule finds calls to functions that are dangerous to use. Currently, it checks for calls to gmtime, localtime, ctime and asctime.

The time related functions such as gmtime fill data into a tm struct or char array in shared memory and then returns a pointer to that memory. If the function is called from multiple places in the same program, and especially if it is called from multiple threads in the same program, then the calls will overwrite each other’s data.


Replace calls to gmtime with gmtime_r. With gmtime_r, the application code manages allocation of the tm struct. That way, separate calls to the function can use their own storage.

Similarly replace calls to localtime with localtime_r, calls to ctime with ctime_r and calls to asctime with asctime_r.


The following example checks the local time in two ways:

// BAD: using gmtime
int is_morning_bad() {
    const time_t now_seconds = time(NULL);
    struct tm *now = gmtime(&now_seconds);
    return (now->tm_hour < 12);

// GOOD: using gmtime_r
int is_morning_good() {
    const time_t now_seconds = time(NULL);
    struct tm now;
    gmtime_r(&now_seconds, &now);
    return (now.tm_hour < 12);

The first version uses gmtime, so it is vulnerable to its data being overwritten by another thread. Even if this code is not used in a multi-threaded context right now, future changes may make the program multi-threaded. The second version of the code uses gmtime_r. Since it allocates a new tm struct on every call, it is immune to other calls to gmtime or gmtime_r.