C: Providing regular expression as argument to open() ? - c

Question

Is there a way to open a file with the open() without knowing its full name?
The linux shell provides an easy way to do that (in some sense), by accepting regular expressions as input.
For example, if you have a folder containing the files:
a.out file1 file2 file3 file4 file.txt test
and you want to list only the files with the prefix file you can do so by:
$ ls file*
file1 file2 file3 file4 file.txt
Or:
$ ls file[1-9]
file1 file2 file3 file4
To list only numbered files and so on...
I need to open the same file whenever my program launches.
The problem is, the file it needs to open is of the form: X*Y, meaning it starts with an X and ends with Y, but it could be anything in between.
For example, it could be X-Toshiba_12.45y9-Y, or it might be X-Dell-5.44s-Y.
I want to be able to open this file without having to consider the model.
The file may reside with some other files in that folder, but the X prefix and Y postfix are unique.
I could iterate the files in that folder and try to find my file by matching strings, but I'd rather avoid it.
Is there a way to provide open() with a regular expression somehow?

Answer 1

These are not regular expressions! You are talking about glob patterns.
You can use the POSIX.1-2001 glob() function to expand a glob pattern (like *.* or foo-*.?a* or *.[a-z]* and so on) to an array of filenames/pathnames that match the given pattern (starting at the current working directory, unless the pattern specifies an absolute path). This is basically what most shells use when they expand file name patterns.
If you were hell-bent on using regular expressions to specify file names (say, you need find-type behaviour, but with regular expressions), use SUSv4 nftw() function to traverse a directory tree -- it even works with the corner cases, like fewer descriptors than tree depth, or files modified, renamed, or moved while tree traversal --, and POSIX regex functions to filter on the file names. Note: regcomp() and regexec() etc. are built-in to POSIX.1-2001 -supporting C libraries, and that includes just about all current C library implementations for Linux. No external libraries are needed at all.
It makes me very sad to see example code using opendir()/readdir() to traverse a directory tree, when nftw() is available and much smarter and more robust. Just define _XOPEN_SOURCE 700 and _POSIX_C_SOURCE 200809L to get all these nice features in Linux and many *BSD variants, too.

Answer 2

Check this example
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <dirent.h>
int
startswith(const char *const haystack, const char *const needle)
{
size_t haystackLength;
size_t needleLength;
if ((haystack == NULL) || (needle == NULL))
return 0;
haystackLength = strlen(haystack);
needleLength = strlen(needle);
if (haystackLength < needleLength)
return 0;
return (memcmp(haystack, needle, needleLength) == 0);
}
int
endswith(const char *const haystack, const char *const needle)
{
size_t haystackLength;
size_t needleLength;
if ((haystack == NULL) || (needle == NULL))
return 0;
haystackLength = strlen(haystack);
needleLength = strlen(needle);
if (haystackLength < needleLength)
return 0;
return (memcmp(haystack + haystackLength - needleLength, needle, needleLength) == 0);
}
void
searchdir(const char *const directory, const char *const starts, const char *const ends)
{
DIR *dir;
struct dirent *entry;
dir = opendir(directory);
if (dir == NULL)
return;
while ((entry = readdir(dir)) != NULL)
{
struct stat statbuf;
char filepath[PATH_MAX];
size_t length;
const char *name;
name = entry->d_name;
if ((strcmp(name, ".") == 0) || (strcmp(name, "..") == 0))
continue;
length = snprintf(filepath, sizeof(filepath), "%s/%s", directory, name);
if (length >= sizeof(filepath))
{
fprintf(stderr, "unexpected error\n");
closedir(dir);
return;
}
if (stat(filepath, &statbuf) == -1)
{
fprintf(stderr, "cannot stat `%s'\n", filepath);
continue;
}
/* if the entry is a directory, probably recures */
if (S_ISDIR(statbuf.st_mode) != 0)
saerchdir(filepath, starts, ends);
/* or just, continue? */
/* The file name does not match */
if ((startswith(name, starts) == 0) || (endswith(name, ends) == 0))
continue;
/* Do whatever you want with the file */
fprintf(stdout, "%s\n", filepath);
}
closedir(dir);
}
int
main(int argc, char **argv)
{
if (argc < 4)
{
fprintf(stderr, "usage: %s directory startpattern endpattern\n", argv[0]);
fprintf(stderr, "\tex. %s /home/${USER} X Y\n", argv[0]);
return -1;
}
searchdir(argv[1], argv[2], argv[3]);
return 0;
}
Do whatever you want with the file, can go from pushing it into a char * array, to passing a function pointer to the function and executing that function on the file path.