Safe and Unsafe Interfaces

This chapter defines MT-safety levels for functions and libraries.

• "Thread Safety"

• "MT Interface Safety Levels"

• "Async-Signal-Safe Functions"

• "MT Safety Levels for Libraries"

Thread Safety

Thread safety is the avoidance of data races--situations in which 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 it is logically correct when executed simultaneously by several threads. At a practical level, it is convenient to recognize three levels of safety.

• Unsafe

• Thread safe - Serializable

• Thread safe - MT-Safe

An unsafe procedure can be made thread safe and serializable by surrounding it with statements to lock and unlock a mutex. Example 6-1 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, this is stronger synchronization than is usually necessary. When two threads are sending output to different files using fputs(), one need not wait for the other--the threads need synchronization only when they are sharing an output file.

The last version is MT-safe. It 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 it is thread safe and its execution does not negatively affect performance.

Example 6-1 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) {
    static mutex_t mut;
    char *p;
    mutex_lock(&m[fileno(stream)]);
    for (p=s; *p; p++)
        putc((int)*p, stream);
    mutex_unlock(&m[fileno(stream)]0;
}

MT Interface Safety Levels

The threads man pages, man(3THR), use the safety level categories listed in Table 6-1 to describe how well an interface supports threads (these categories are explained more fully in the Intro(3) reference manual page).

Table 6-1 Interface Safety Levels

See the section 3 manual pages for the safety levels of library routines.

Some functions have purposely not been made safe for the following reasons.

• Making the interface MT-Safe would have negatively affected the performance of single-threaded applications.

• The library has an Unsafe interface. For example, a function might return a pointer to a buffer in the stack. You can use reentrant counterparts for some of these functions. The reentrant function name is the original function name with "_r" appended.

Caution - There is no way to be certain that a function with a name not ending in "_r" is MT-Safe other than by checking its reference manual page. Use of a function identified as not MT-Safe must be protected by a synchronizing device or by restriction to the initial thread.

Reentrant Functions for Unsafe Interfaces

For most functions with Unsafe interfaces, an MT-Safe version of the routine exists. The name of the new MT-Safe routine is always the name of the old Unsafe routine with "_r" appended. The Table 6-2 "_r" routines are supplied in the Solaris environment.

Table 6-2 Reentrant Functions