Traversing a Filesystem with fts(3) - c

Question

I have a question on fts(3). I am getting a segmentation fault whenever I try to access any members of the fts_children() function. When I read the man page at http://www.kernel.org/doc/man-pages/online/pages/man3/fts.3.html it claims to fill itself after the read function runs and returns a linked list linked through the link field in the structure. My suspicion is that the child_function is returning nothing but I feel like that doesn't line up with the man page. Am I supposed to be adding these files to the child buffer because I thought that was being done automatically? My code is below,
Thanks!
#include<stdlib.h>
#include<stdio.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fts.h>
#include<string.h>
int compare (const FTSENT**, const FTSENT**);
int main(int argc, char* const argv[])
{
FTS* file_system = NULL;
FTSENT* child = NULL;
FTSENT* parent = NULL;
FTSENT* temp = NULL;
file_system = fts_open(argv + 1,FTS_COMFOLLOW | FTS_NOCHDIR,&compare);
while( (parent = fts_read(file_system)) != NULL)
{
child = fts_children(file_system,0);
printf("%s\n", child->fts_path);
}
// while (child ->fts_link != NULL)
// child = child->fts_link;
fts_close(file_system);
return 0;
}
int compare(const FTSENT** one, const FTSENT** two){
return (strcmp((*one)->fts_name, (*two)->fts_name));
}
"test_fs.c" 43L, 1108C

Answer 1

If you are only interested in traversing all directories and files for the specified path(s), just repeatedly call fts_read.
If you want to only iterate through the stream, #sehe's example could be rewritten as:
#include<stdlib.h>
#include<stdio.h>
#include<sys/types.h>
#include<fts.h>
#include<string.h>
#include<errno.h>
int compare (const FTSENT**, const FTSENT**);
void indent (int i);
int main(int argc, char* const argv[])
{
FTS* file_system = NULL;
FTSENT *node, = NULL;
if (argc<2)
{
printf("Usage: %s <path-spec>\n", argv[0]);
exit(255);
}
file_system = fts_open(argv + 1,FTS_COMFOLLOW|FTS_NOCHDIR,&compare);
if (NULL != file_system)
{
while( (node = fts_read(file_system)) != NULL)
{
switch (node->fts_info)
{
case FTS_D :
case FTS_F :
case FTS_SL:
indent(node->fts_level);
printf("%s\n", node->fts_name);
break;
default:
break;
}
}
fts_close(file_system);
}
return 0;
}
int compare(const FTSENT** one, const FTSENT** two)
{
return (strcmp((*one)->fts_name, (*two)->fts_name));
}
void indent(int i)
{
for (; i > 0; i--)
printf(" ");
}
When you run it, it iterates through the stream and lists all files and directories in order:
★ mkdir -p test/A/1 test/A/2 test/B/1 test/B/2
★ tree test
test
├── A
│   ├── 1
│   └── 2
└── B
├── 1
└── 2
★ ./fts test
test
A
1
2
B
1
2
Call fts_children only if you want a list of child nodes of a specific directory. In that case you must call fts_read at least once before calling fts_children; otherwise fts_children will only return nodes specified in argv parameter to fts_open.

Answer 2

You simply need to add a NULL check.
You might want to
add one for file_system
check for command line arguments
Add more errorhandling:
The fts_children() function returns a pointer to an FTSENT structure describing the first entry in a NULL terminated linked list of files in the directory, if successful. The fts_children() function may fail and set errno for any of the errors that the chdir(), malloc(), opendir(), readdir(), and stat() functions specify.
Update To the new question(s) in the comment:
The while loop for linked list traversal was misplaced (outside the outer loop?)
The printf displayed only the path... not the filename.
while you're at it:
#include<stdlib.h>
#include<stdio.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fts.h>
#include<string.h>
#include<errno.h>
int compare (const FTSENT**, const FTSENT**);
int main(int argc, char* const argv[])
{
FTS* file_system = NULL;
FTSENT* child = NULL;
FTSENT* parent = NULL;
if (argc<2)
{
printf("Usage: %s <path-spec>\n", argv[0]);
exit(255);
}
file_system = fts_open(argv + 1,FTS_COMFOLLOW | FTS_NOCHDIR,&compare);
if (NULL != file_system)
{
while( (parent = fts_read(file_system)) != NULL)
{
child = fts_children(file_system,0);
if (errno != 0)
{
perror("fts_children");
}
while ((NULL != child)
&& (NULL != child->fts_link))
{
child = child->fts_link;
printf("%s%s\n", child->fts_path, child->fts_name);
}
}
fts_close(file_system);
}
return 0;
}
int compare(const FTSENT** one, const FTSENT** two)
{
return (strcmp((*one)->fts_name, (*two)->fts_name));
}
Sample output fragment:
./.profiles/sehe/.opera/icons/cache/g_0000
./.profiles/sehe/.opera/icons/cache/g_0000/opr00002.tmp
./.profiles/sehe/.opera/icons/cache/g_0000/opr00003.tmp
./.profiles/sehe/home/sehe/.mozilla
fts_children: Permission denied
./.vbox-sehe-ipc/lock