Cirrus Logic CS485 Manual de usuario descargar pdf (Pagina 46)

CS485G Spring 2015 46

(b) The POSIX standard requires 117 functions, such as write(),

to be safe. But printf() is not safe.

51 Nonlocal jumps

1. Nonlocal jumps provide a powerful but dangerous method to trans-

fer control to an arbitrary location.

2. int setjmp(jmp buf jb): saves a “snapshot” in jmp buf. It

may return multiple times; the ﬁrst time it returns 0.

3. The snapshot contains register information, including pc and sp.

4. void longjmp(jmp buf jb, int val): causes a return from

the associated setjmp, with val as the return value.

52 Files

1. Unix ﬁles are sequences of bytes. The kernel does not distinguish

ﬁles built for various purposes (text, data, object) when a process

reads or write them, no matter what the ﬁle name might be.

2. Even non-ﬁle abstractions are made to look like ﬁles to allow pro-

cesses to treat them in a consistent way.

(a) Even devices look like ﬁles. They have names like /dev/tty1

and /dev/sda2. Some are character special, like terminals.

Others are block special, like disks.

(b) The kernel memory can be seen as a ﬁle: /dev/kmem.

(d) Directories are special, in that they are not read/written us-

ing read() and write(). Directories contain names of ﬁles,

building a hierarchical ﬁle arrangement.

(e) Pipes, used for inter-process communication, also look like ﬁles.

(f) A FIFO is like a pipe, but it has a name in the ﬁle system.

(g) A socket is used for cross-machine inter-process communica-

tion. Processes usually use recv() and send(), not read()

and write().

3. Basic I/O system calls. Always check the return value!

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Cirrus Logic CS485 Manual de usuario Pagina 46