Книга: Programming with POSIX® Threads

9.3.10 Stdio

9.3.10 Stdio

Pthreads provides some new functions, and new versions of old functions, to access ANSI C stdio features safely from a threaded process. For safety reasons, the old forms of single-character access to stdio buffers have been altered to lock the file stream, which can decrease performance. You can change old code to instead lock the file stream manually and, within that locked region, use new character access operations that do not lock the file stream.

flockfile

void flockfile (

FILE *file);

Increase the lock count for a stdio file stream to gain exclusive access to the file stream. If the file stream is currently locked by another thread, the calling thread is blocked until the lock count for the file stream becomes zero. If the calling thread already owns the file stream lock, the lock count is incremented — an identical number of calls to funlockfile is required to release the file stream lock.

Although most stdio functions, such as printf and fgets, are thread-safe, you may sometimes find that it is important that a sequence of printf calls, for example, from one thread cannot be separated by calls made from another thread. Also, a few stdio functions are not thread-safe and can only be used while the file stream is locked by the caller.

References: 6.4.1 Headers: <stdio.h>

Hint: Use to protect a sequence of stdio operations.

ftrylockfile

int ftrylockfile (

FILE *file);

If the file stream is currently locked by another thread, return a nonzero value. Otherwise, increase the lock count for the file stream, and return the value zero.

References: 6.4.1 Headers: <stdio.h>

Hint: Use to protect a sequence of stdio operations.

funlockfile

void funlockfile (

FILE *file);

Decrease the lock count for a stdio file stream that was previously locked by a corresponding call to funlockfile. If the lock count becomes 0, release the lock so that another thread can lock it.

References: 6.4.1 Headers: <stdio.h>

Hint: Use to protect a sequence of stdio operations.

getc_unlocked

int getc_unlocked (

FILE *file);

Return a single character from the stdio stream file, without locking the file stream. This operation must only be used while the file stream has been locked by calling flockfile, or when you know that no other thread may access the file stream concurrently. Returns EOF for read errors or end-of-file condition.

References: 6.4.2 Headers: <stdio.h>

Hint: Replace old calls to getc to retain fastest access.

getchar_unlocked

int getc_unlocked (void);

Return a single character from the stdio stream stdin without locking the file stream. This operation must only be used while the file stream has been locked by calling flockfile, or when you know that no other thread may access the file stream concurrently. Returns EOF for read errors or end-of-file condition.

References: 6.4.2 Headers: <stdio.h>

Hint: Replace old calls to getchar to retain fastest access.

putc_unlocked

int putc_unlocked (

int c,

FILE *file);

Write a single character c (interpreted as an unsigned char) to the stdio stream file without locking the file stream. This operation must only be used while the file stream has been locked by calling flockfile, or when you know that no other thread may access the file stream concurrently. Returns the character or the value EOF if an error occurred.

References: 6.4.2 Headers: <stdio.h>

Hint: Replace old calls to putc to retain fastest access.

putchar_unlocked

int putchar_unlocked (

int c);

Write a single character c (interpreted as an unsigned char) to the stdio stream stdout without locking the file stream. This operation must only be used while the file stream has been locked by calling flockfile, or when you know that no other thread may access the file stream concurrently. Returns the character or the value EOF if an error occurred.

References: 6.4.2 Headers: <stdio.h>

Hint: Replace old calls to putchar to retain fastest access.

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