pca_lookup_file

pca_lookup_file(3)         Library Functions Manual         pca_lookup_file(3)



NAME
       pca_lookup_file, del_PathCache, del_PcaPathConf, new_PathCache,
       new_PcaPathConf, pca_last_error, pca_path_completions, pca_scan_path,
       pca_set_check_fn, ppc_file_start, ppc_literal_escapes - lookup a file
       in a list of directories

SYNOPSIS
       #include <libtecla.h>

       PathCache *new_PathCache(void);

       PathCache *del_PathCache(PathCache *pc);

       int pca_scan_path(PathCache *pc, const char *path);

       void pca_set_check_fn(PathCache *pc, CplCheckFn *check_fn,
                             void *data);

       char *pca_lookup_file(PathCache *pc, const char *name,
                             int name_len, int literal);

       const char *pca_last_error(PathCache *pc);

       CPL_MATCH_FN(pca_path_completions);



DESCRIPTION
       The PathCache object is part of the tecla library (see the libtecla(3)
       man page).

       PathCache objects allow an application to search for files in any colon
       separated list of directories, such as the unix execution PATH
       environment variable. Files in absolute directories are cached in a
       PathCache object, whereas relative directories are scanned as needed.
       Using a PathCache object, you can look up the full pathname of a simple
       filename, or you can obtain a list of the possible completions of a
       given filename prefix. By default all files in the list of directories
       are targets for lookup and completion, but a versatile mechanism is
       provided for only selecting specific types of files. The obvious
       application of this facility is to provide Tab-completion and lookup of
       executable commands in the unix PATH, so an optional callback which
       rejects all but executable files, is provided.


AN EXAMPLE
       Under UNIX, the following example program looks up and displays the
       full pathnames of each of the command names on the command line.

         #include <stdio.h>
         #include <stdlib.h>
         #include <libtecla.h>

         int main(int argc, char *argv[])
         {
           int i;
         /*
          * Create a cache for executable files.
          */
           PathCache *pc = new_PathCache();
           if(!pc)
             exit(1);
         /*
          * Scan the user's PATH for executables.
          */
           if(pca_scan_path(pc, getenv("PATH"))) {
             fprintf(stderr, "%s\n", pca_last_error(pc));
             exit(1);
           }
         /*
          * Arrange to only report executable files.
          */
          pca_set_check_fn(pc, cpl_check_exe, NULL);
         /*
          * Lookup and display the full pathname of each of the
          * commands listed on the command line.
          */
           for(i=1; i<argc; i++) {
             char *cmd = pca_lookup_file(pc, argv[i], -1, 0);
             printf("The full pathname of '%s' is %s\n", argv[i],
                    cmd ? cmd : "unknown");
           }
           pc = del_PathCache(pc);  /* Clean up */
           return 0;
         }

       The following is an example of what this does on my laptop under linux:

         $ ./example less more blob
         The full pathname of 'less' is /usr/bin/less
         The full pathname of 'more' is /bin/more
         The full pathname of 'blob' is unknown
         $


FUNCTION DESCRIPTIONS
       In order to use the facilities of this module, you must first allocate
       a PathCache object by calling the new_PathCache() constructor function.

         PathCache *new_PathCache(void)

       This function creates the resources needed to cache and lookup files in
       a list of directories. It returns NULL on error.


POPULATING THE CACHE
       Once you have created a cache, it needs to be populated with files.  To
       do this, call the pca_scan_path() function.

         int pca_scan_path(PathCache *pc, const char *path);

       Whenever this function is called, it discards the current contents of
       the cache, then scans the list of directories specified in its path
       argument for files. The path argument must be a string containing a
       colon-separated list of directories, such as
       "/usr/bin:/home/mcs/bin:.". This can include directories specified by
       absolute pathnames such as "/usr/bin", as well as sub-directories
       specified by relative pathnames such as "." or "bin". Files in the
       absolute directories are immediately cached in the specified PathCache
       object, whereas sub-directories, whose identities obviously change
       whenever the current working directory is changed, are marked to be
       scanned on the fly whenever a file is looked up.

       On success this function return 0. On error it returns 1, and a
       description of the error can be obtained by calling pca_last_error(pc).


LOOKING UP FILES
       Once the cache has been populated with files, you can look up the full
       pathname of a file, simply by specifying its filename to
       pca_lookup_file().

         char *pca_lookup_file(PathCache *pc, const char *name,
                               int name_len, int literal);

       To make it possible to pass this function a filename which is actually
       part of a longer string, the name_len argument can be used to specify
       the length of the filename at the start of the name[] argument. If you
       pass -1 for this length, the length of the string will be determined
       with strlen(). If the name[] string might contain backslashes that
       escape the special meanings of spaces and tabs within the filename,
       give the literal argument, the value 0. Otherwise, if backslashes
       should be treated as normal characters, pass 1 for the value of the
       literal argument.


FILENAME COMPLETION
       Looking up the potential completions of a filename-prefix in the
       filename cache, is achieved by passing the provided
       pca_path_completions() callback function to the cpl_complete_word()
       function (see the cpl_complete_word(3) man page).

         CPL_MATCH_FN(pca_path_completions);

       This callback requires that its data argument be a pointer to a
       PcaPathConf object. Configuration objects of this type are allocated by
       calling new_PcaPathConf().

         PcaPathConf *new_PcaPathConf(PathCache *pc);

       This function returns an object initialized with default configuration
       parameters, which determine how the cpl_path_completions() callback
       function behaves. The functions which allow you to individually change
       these parameters are discussed below.

       By default, the pca_path_completions() callback function searches
       backwards for the start of the filename being completed, looking for
       the first un-escaped space or the start of the input line. If you wish
       to specify a different location, call ppc_file_start() with the index
       at which the filename starts in the input line. Passing start_index=-1
       re-enables the default behavior.

         void ppc_file_start(PcaPathConf *ppc, int start_index);

       By default, when pca_path_completions() looks at a filename in the
       input line, each lone backslash in the input line is interpreted as
       being a special character which removes any special significance of the
       character which follows it, such as a space which should be taken as
       part of the filename rather than delimiting the start of the filename.
       These backslashes are thus ignored while looking for completions, and
       subsequently added before spaces, tabs and literal backslashes in the
       list of completions. To have unescaped backslashes treated as normal
       characters, call ppc_literal_escapes() with a non-zero value in its
       literal argument.

         void ppc_literal_escapes(PcaPathConf *ppc, int literal);

       When you have finished with a PcaPathConf variable, you can pass it to
       the del_PcaPathConf() destructor function to reclaim its memory.

         PcaPathConf *del_PcaPathConf(PcaPathConf *ppc);



BEING SELECTIVE
       If you are only interested in certain types or files, such as, for
       example, executable files, or files whose names end in a particular
       suffix, you can arrange for the file completion and lookup functions to
       be selective in the filenames that they return.  This is done by
       registering a callback function with your PathCache object. Thereafter,
       whenever a filename is found which either matches a filename being
       looked up, or matches a prefix which is being completed, your callback
       function will be called with the full pathname of the file, plus any
       application-specific data that you provide, and if the callback returns
       1 the filename will be reported as a match, and if it returns 0, it
       will be ignored.  Suitable callback functions and their prototypes
       should be declared with the following macro. The CplCheckFn typedef is
       also provided in case you wish to declare pointers to such functions.

         #define CPL_CHECK_FN(fn) int (fn)(void *data, \
                                           const char *pathname)
         typedef CPL_CHECK_FN(CplCheckFn);

       Registering one of these functions involves calling the
       pca_set_check_fn() function. In addition to the callback function,
       passed via the check_fn argument, you can pass a pointer to anything
       via the data argument. This pointer will be passed on to your callback
       function, via its own data argument, whenever it is called, so this
       provides a way to pass appplication specific data to your callback.

         void pca_set_check_fn(PathCache *pc, CplCheckFn *check_fn,
                               void *data);

       Note that these callbacks are passed the full pathname of each matching
       file, so the decision about whether a file is of interest can be based
       on any property of the file, not just its filename. As an example, the
       provided cpl_check_exe() callback function looks at the executable
       permissions of the file and the permissions of its parent directories,
       and only returns 1 if the user has execute permission to the file. This
       callback function can thus be used to lookup or complete command names
       found in the directories listed in the user's PATH environment
       variable. The example program given earlier in this man page provides a
       demonstration of this.

       Beware that if somebody tries to complete an empty string, your
       callback will get called once for every file in the cache, which could
       number in the thousands. If your callback does anything time consuming,
       this could result in an unacceptable delay for the user, so callbacks
       should be kept short.

       To improve performance, whenever one of these callbacks is called, the
       choice that it makes is cached, and the next time the corresponding
       file is looked up, instead of calling the callback again, the cached
       record of whether it was accepted or rejected is used. Thus if somebody
       tries to complete an empty string, and hits tab a second time when
       nothing appears to happen, there will only be one long delay, since the
       second pass will operate entirely from the cached dispositions of the
       files. These cached dipositions are discarded whenever pca_scan_path()
       is called, and whenever pca_set_check_fn() is called with changed
       callback function or data arguments.


ERROR HANDLING
       If pca_scan_path() reports that an error occurred by returning 1, you
       can obtain a terse description of the error by calling
       pca_last_error(pc). This returns an internal string containing an error
       message.

         const char *pca_last_error(PathCache *pc);



CLEANING UP
       Once you have finished using a PathCache object, you can reclaim its
       resources by passing it to the del_PathCache() destructor function.
       This takes a pointer to one of these objects, and always returns NULL.

         PathCache *del_PathCache(PathCache *pc);


THREAD SAFETY
       In multi-threaded programs, you should use the libtecla_r.a version of
       the library. This uses POSIX reentrant functions where available (hence
       the _r suffix), and disables features that rely on non-reentrant system
       functions. In the case of this module, the only disabled feature is
       username completion in ~username/ expressions, in
       cpl_path_completions().

       Using the libtecla_r.a version of the library, it is safe to use the
       facilities of this module in multiple threads, provided that each
       thread uses a separately allocated PathCache object. In other words, if
       two threads want to do path searching, they should each call
       new_PathCache() to allocate their own caches.


FILES
       libtecla.a    -    The tecla library
       libtecla.h    -    The tecla header file.


SEE ALSO
       libtecla(3), gl_get_line(3), ef_expand_file(3), cpl_complete_word(3)


AUTHOR
       Martin Shepherd  (mcs@astro.caltech.edu)



                                                            pca_lookup_file(3)