[LINUX] Follow the file hierarchy with fts

A note on how to navigate the file hierarchy using fts.

sample

Create a sample program that scans the directory given by the argument and outputs the size and pathname.

sample.cpp

#include <cstdio>
#include <fts.h>
#include <sys/stat.h>

int main(int argc, char** argv)
{
    if (argc != 2) {
        std::fprintf(stderr, "Usage: %s <dir>\n", argv[0]);
        return -1;
    }

    char* const paths[] = {argv[1], nullptr};

    FTS* fts = fts_open(paths, 0, nullptr);
    if (fts == nullptr) {
        std::fprintf(stderr, "open failed.\n");
        return -1;
    }

    FTSENT* ent = nullptr;
    while ((ent = fts_read(fts)) != nullptr) {
        std::printf("%-5lu %s\n", ent->fts_statp->st_size, ent->fts_path);
    }

    if (fts_close(fts) != 0) {
        std::fprintf(stderr, "close failed.\n");
        return -1;
    }

    return 0;
}

The directory and file size paths can be output recursively.

$ g++ -std=c++14 sample.cpp -o sample
$ ./sample testdir
4096  testdir
4096  testdir/subdir
114   testdir/subdir/fff.txt
110   testdir/subdir/eee.txt
49    testdir/subdir/ddd.txt
4096  testdir/subdir
64    testdir/bbb.txt
48    testdir/aaa.txt
8     testdir/ccc.txt
4096  testdir

Change file hierarchy scan order

By giving a function pointer to fts_open, it seems possible to change the order in which the file hierarchy is scanned.

$ man fts
... 
       FTS *fts_open(char * const *path_argv, int options,
                     int (*compar)(const FTSENT **, const FTSENT **));
... 
       The argument compar() specifies a user-defined function which may be used to order the traversal of the hierarchy.  
       It takes two pointers to pointers to FTSENT structures as arguments and should return a negative value, zero, or a positive value to indicate if the file referenced by its first argument comes before, in any order with respect to, or after, the file referenced by its second argument.
...

So, modify it to give a function pointer so that the size is output in ascending order, and execute it again.

sample2.cpp

#include <cstdio>
#include <fts.h>
#include <sys/stat.h>

static int compar(const FTSENT** rhs, const FTSENT** lhs)
{
    const FTSENT* r = *rhs;
    const FTSENT* l = *lhs;

    return (r->fts_statp->st_size - l->fts_statp->st_size);
}

int main(int argc, char** argv)
{
    if (argc != 2) {
        std::fprintf(stderr, "Usage: %s <dir>\n", argv[0]);
        return -1;
    }

    char* const paths[] = {argv[1], nullptr};

    FTS* fts = fts_open(paths, 0, compar);
    if (fts == nullptr) {
        std::fprintf(stderr, "open failed.\n");
        return -1;
    }

    FTSENT* ent = nullptr;
    while ((ent = fts_read(fts)) != nullptr) {
        std::printf("%-5lu %s\n", ent->fts_statp->st_size, ent->fts_path);
    }

    if (fts_close(fts) != 0) {
        std::fprintf(stderr, "close failed.\n");
        return -1;
    }

    return 0;
}

Files in the directory are now output in ascending order of size.

$ g++ -std=c++14 sample2.cpp -o sample2
$ ./sample2 testdir/
4096  testdir/
8     testdir/ccc.txt
48    testdir/aaa.txt
64    testdir/bbb.txt
4096  testdir/subdir
49    testdir/subdir/ddd.txt
110   testdir/subdir/eee.txt
114   testdir/subdir/fff.txt
4096  testdir/subdir
4096  testdir/