File, directory, network, and other OS/POSIX interfaces

Accessing processes, files, directories, communication pipes and other objects external to a Scheme system


POSIX interfaces

  A set of native Scheme procedures providing access to the common POSIX library facilities and UNIX system calls:
Time and timing
OS:time, OS:sleep, OS:cftime (presenting dates and times in various string formats), OS:string->time
Files and directories
OS:remove, OS:rename, OS:umask, OS:tmpnam, OS:file-exists?, OS:file-length
OS:chdir, OS:ch-mk-dir, OS:ch-parent-dir, OS:getcwd
OS:stderr->file (redirect the standard error to a file, which is opened in the append mode, and created, if necessary)
OS:ftell, OS:fseek-abs
Process/environment queries
OS:getuid, OS:geteuid, OS:getgid, OS:getegid, OS:getpid, OS:getppid, OS:getpgrp
OS:getenv, OS:putenv
Formatting functions
OS:flonum->sprintf, OS:int->sprintf
Interprocess communication and synchronization
OS:strerror, OS:system

The source code documents the interfaces in full detail.

 The current version is 3.3, Aug 11, 2003.

OS-spec.scm [24K]
The commented source code.

vipc.scm [2K]
Verification of the OS:within-critical-section and OS:within-timeout.

vext-io.scm [15K]
Verification of the OS:remove-at-exit, and of reading and writing through TCP, uni- and bidirectional pipes.

My own standard prelude


Scanning of a UNIX (POSIX) directory

and getting file status information

-- procedure+: OS:for-each-file-in-directory DIR-NAME PROC

The file-info object being given to the procedure PROC is the result of  
-- procedure+: OS:make-file-info FILE-PATH

The object accepts the following "messages"

- returns the file's name
- returns the link-count of the file
- #t if the file is a directory
- #t if the file is a block-special file, like those in /dev/dsk
- #t if the file is a char-special file, as /dev/tty
- #t if the file is a plain regular file
- #t if the file is a communication (FIFO) pipe
- returns the modification time of the file, a BIGNUM, generally, number, the number of seconds since Jan 1, 1970 (see man time(2))
- returns time of the last (read) access to the file
- returns the "creation" time of the file
- returns the file size
- returns owner's uid
- returns owner's gid (group id)
- returns a predicate that takes two symbol arguments, who ('owner, 'group, or 'others) and operation ('read, 'write, or 'exec), and tells if the file can be accessed by who with the operation

All i/o errors raise a ##SIGNAL.IO-ERROR, which can be ##caught.

Verification code vreaddir.scm is a good example of using these functions; note a test case that uses OS:for-each-file-in-directory to fake a "ls -l ." UNIX command

These function use Gambit's Scheme/C foreign function interface.

Current version: Apr 7, 1997
Click to download readdir.scm, and the verification code vreaddir.scm


Ports and the enhanced i/o


The following higher-level i/o procedures capture several frequently-occurring patterns of input and output. The procedures proved convenient in practice. They are also efficient: Each procedure has a slow and a fast path implementations. The slow path code works with any i/o port and on any R4RS+ Scheme system. The fast path is specific to Gambit and to stream ports. The procedures themselves chose the right (the most optimal) path of execution.

write-substring STR I J PORT
write (substring STR I J) into the output PORT. If PORT is an i/o stream port, an efficient fast path code will be executed.
write-string STR PORT
Write a string STR into the output PORT, using the write-substring procedure above.
copies characters from an IN-PORT to the OUT-PORT until the EOF or until the NUMBER of characters are copied -- whichever occurs first. The procedure returns the eof-object if EOF was seen; otherwise, it returns 0 (the rest counter). An i/o error signal may be raised as well.
If both IN-PORT and OUT-PORT are stream ports, a highly efficient fast path is used, which does not have to move any data between Scheme and C environments.
 The current version is 3.0, May 2, 2001.

OS-spec.scm [19K]
The commented source code.

vext-io.scm [15K]
Verification code and benchmarks. The benchmarks show that the fast path of port-copy can be faster than the slow path by a factor of 100!

My own standard prelude


Signals and threads


Although there is no standard way of handling Unix/Posix signals in Scheme, many implementations, e.g., Gambit, let a user specify a thunk to execute whenever a signal, a timer interrupt or other external condition occurs.

Incidentally this makes it possible to write thread scheduling systems and the like directly in Scheme. Indeed, when a signal handler returns the interrupted computation resumes. The signal handler can therefore use its own continuation for thread switching purposes. Listing 1 in the article below shows the example of this idea. It uses non-preemptive "interrupts" and thus should run on every Scheme system. However, if you happen to have a Gambit-C system, you can insert Listing 2 into Listing 1 where indicated. This buys you a preemptive re-scheduling upon arrival of a user-defined signal. You can do "kill -USR1 pid" to cause switching from one thread to another.

 The current version is 1.1, Feb 26, 2000.

A USENET article with the complete code and transcripts [plain text file]

From other archives
comp.lang.scheme newsgroup
an article Re: exception handling in scheme posted on Sat Feb 26 02:50:00 2000 GMT


Additions to Gambit-C's foreign function interface

provide the complete mapping between 32-bit C unsigned integers and Scheme numbers. Now, if a C integer turns out to be so big that it does not fit into Scheme's FIXNUM, the corresponding BIGNUM is created. Conversely, both BIGNUM and FIXNUM can be converted to C's unsigned int (if they fit, of course).
Current version: May 3, 1996
Click to download c-intf-add.c

Last updated July 4, 2004

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