Thread Safety

Thread safety is the avoidance of data races. Data races occur when data are set to either correct or incorrect values, depending upon the order in which multiple threads access and modify the data.

When no sharing is intended, give each thread a private copy of the data. When sharing is important, provide explicit synchronization to make certain that the program behaves in a deterministic manner.

A procedure is thread safe when the procedure is logically correct when executed simultaneously by several threads. At a practical level, safety falls into the following recognized levels.

• Unsafe

• Thread safe, Serializable

• Thread safe, MT-Safe

An unsafe procedure can be made thread safe and able to be serialized by surrounding the procedure with statements to lock and unlock a mutex. Example 7–1 shows three simplified implementations of fputs() , initially thread unsafe.

Next is a serializable version of this routine with a single mutex protecting the procedure from concurrent execution problems. Actually, the single mutex is stronger synchronization than is usually necessary. When two threads are sending output to different files by using fputs(), one thread need not wait for the other thread. The threads need synchronization only when sharing an output file.

The last version is MT-safe. This version uses one lock for each file, allowing two threads to print to different files at the same time. So, a routine is MT-safe when the routine is thread safe, and the routine's execution does not negatively affect performance.

Example 7–1 Degrees of Thread Safety

/* not thread-safe */
fputs(const char *s, FILE *stream) {
    char *p;
    for (p=s; *p; p++)
        putc((int)*p, stream);
}

/* serializable */
fputs(const char *s, FILE *stream) {
    static mutex_t mut;
    char *p;
    mutex_lock(&m);
    for (p=s; *p; p++)
        putc((int)*p, stream);

    mutex_unlock(&m);
}

/* MT-Safe */
mutex_t m[NFILE];
fputs(const char *s, FILE *stream) {
    char *p;
    mutex_lock(&m[fileno(stream)]);
    for (p=s; *p; p++)
        putc((int)*p, stream);
    mutex_unlock(&m[fileno(stream)]0;
}