C.3.2.2 C Code for chdir and stat

Here is the C code for these extensions. They were written for GNU/Linux. The code needs some more work for complete portability to other POSIX-compliant systems:(65)

#include "awk.h" #include <sys/sysmacros.h> /* do_chdir --- provide dynamically loaded chdir() builtin for gawk */ static NODE * do_chdir(tree) NODE *tree; { NODE *newdir; int ret = -1; newdir = get_argument(tree, 0);

The file includes the "awk.h" header file for definitions for the gawk internals. It includes <sys/sysmacros.h> for access to the major and minor macros.

By convention, for an awk function foo, the function that implements it is called `do_foo'. The function should take a `NODE *' argument, usually called tree, that represents the argument list to the function. The newdir variable represents the new directory to change to, retrieved with get_argument. Note that the first argument is numbered zero.

This code actually accomplishes the chdir. It first forces the argument to be a string and passes the string value to the chdir system call. If the chdir fails, ERRNO is updated. The result of force_string has to be freed with free_temp:

if (newdir != NULL) { (void) force_string(newdir); ret = chdir(newdir->stptr); if (ret < 0) update_ERRNO(); free_temp(newdir); }

Finally, the function returns the return value to the awk level, using set_value. Then it must return a value from the call to the new built-in (this value ignored by the interpreter):

/* Set the return value */ set_value(tmp_number((AWKNUM) ret)); /* Just to make the interpreter happy */ return tmp_number((AWKNUM) 0); }

The stat built-in is more involved. First comes a function that turns a numeric mode into a printable representation (e.g., 644 becomes `-rw-r--r--'). This is omitted here for brevity:

/* format_mode --- turn a stat mode field into something readable */ static char * format_mode(fmode) unsigned long fmode; { ... }

Next comes the actual do_stat function itself. First come the variable declarations and argument checking:

/* do_stat --- provide a stat() function for gawk */ static NODE * do_stat(tree) NODE *tree; { NODE *file, *array; struct stat sbuf; int ret; char *msg; NODE **aptr; char *pmode; /* printable mode */ char *type = "unknown"; /* check arg count */ if (tree->param_cnt != 2) fatal( "stat: called with %d arguments, should be 2", tree->param_cnt);

Then comes the actual work. First, we get the arguments. Then, we always clear the array. To get the file information, we use lstat, in case the file is a symbolic link. If there's an error, we set ERRNO and return:

/* * directory is first arg, * array to hold results is second */ file = get_argument(tree, 0); array = get_argument(tree, 1); /* empty out the array */ assoc_clear(array); /* lstat the file, if error, set ERRNO and return */ (void) force_string(file); ret = lstat(file->stptr, & sbuf); if (ret < 0) { update_ERRNO(); set_value(tmp_number((AWKNUM) ret)); free_temp(file); return tmp_number((AWKNUM) 0); }

Now comes the tedious part: filling in the array. Only a few of the calls are shown here, since they all follow the same pattern:

/* fill in the array */ aptr = assoc_lookup(array, tmp_string("name", 4), FALSE); *aptr = dupnode(file); aptr = assoc_lookup(array, tmp_string("mode", 4), FALSE); *aptr = make_number((AWKNUM) sbuf.st_mode); aptr = assoc_lookup(array, tmp_string("pmode", 5), FALSE); pmode = format_mode(sbuf.st_mode); *aptr = make_string(pmode, strlen(pmode));

When done, we free the temporary value containing the file name, set the return value, and return:

free_temp(file); /* Set the return value */ set_value(tmp_number((AWKNUM) ret)); /* Just to make the interpreter happy */ return tmp_number((AWKNUM) 0); }

Finally, it's necessary to provide the "glue" that loads the new function(s) into gawk. By convention, each library has a routine named dlload that does the job:

/* dlload --- load new builtins in this library */ NODE * dlload(tree, dl) NODE *tree; void *dl; { make_builtin("chdir", do_chdir, 1); make_builtin("stat", do_stat, 2); return tmp_number((AWKNUM) 0); }

And that's it! As an exercise, consider adding functions to implement system calls such as chown, chmod, and umask.