C Language, Questions on Unix File/Directory Manipulation using C

Hon-Chi N
#1 / 25

Questions on Unix File/Directory Manipulation using C

1. How can I rename (move) files across filesystems?

I need a C function that can move files across machines/filesystems,
just as user-command "mv". The program I'm writing should create
files in /tmp/ and choose based on input to copy some of the files
to current directory.

"rename" can't work across filesystems, as explained in rename(2).
I tried to write my own version using "link", and neither does link
work, as explained in link(2).

int myRename(char *old_file, char *new_file) {
if (rename(old_file, new_file) == 0) {
return 0;
}
else if (errno == EXDEV) {
/* link() can't work across filesystems too. */
if (link(old_file, new_file) != 0)
return -1;
return unlink(old_file);
}
else {
perror("myRename");
return -1;
}
}

The only solution I can think of is to copy files, and then delete
the old files.

int myRename(char *old_file, char *new_file) {
if (rename(old_file, new_file) == 0) {
return 0;
}
else if (errno == EXDEV) {
FILES *old_fd, *new_fd;
char buf[MAX_BUF];
int file_size;
old_fd = fopen(old_file, "r");
new_fd = fopen(new_file, "w");
file_size = fread(buf, sizeof(char), MAX_BUF, old_fd);
fwrite(buf, sizeof(char), file_size, new_fd);

return unlink(old_file);
}
else {
perror("myRename");
return -1;
}
}

But, I think the solution above is inefficient and has serious
limitation, namely what if the "old_file" has size larger than
MAX_BUF. It means I have to implement a while loop to complete the
copy. Does someone know a better (more efficient) way to implement
myRename? Or is there a C function that does the same job as
user-command "cp"?

2. Where (which lib) does basename() function reside in Solaris 2.5.1,
and how to link its object using gcc?

Here is a portion of my code that uses basename().

#include <libgen.h>

char myProjFile[] = "/proj/foo/bar.c";
printf("The basename of '%s' is '%s'\n", myProjFile,
basename(myProjFile));

I compiled it with both gcc 2.8.1 and gcc 2.95.2 on Solaris 2.5.1,
both returned the following errors.

Undefined first referenced
symbol in file
basename /var/tmp/cca002G51.o
ld: fatal: Symbol referencing errors. No output written to foo

However, if I compile it on Solaris 2.7 using either version of gcc,
it works fine. Hence, I assume it's not gcc problem.

Is this a common/known problem for Solaris 2.5.1, or is the problem
specific to my Solaris 2.5.1 machine? In either case, how can I fix
it?

Thanks. I appreciate your help.

Hon-Chi

Sent via Deja.com http://www.*-*-*.com/
Before you buy.

Sat, 29 Jun 2002 03:00:00 GMT

Andy Knigh
#2 / 25

Questions on Unix File/Directory Manipulation using C

Quote:

> 1. How can I rename (move) files across filesystems?

> I need a C function that can move files across machines/filesystems,
> just as user-command "mv". The program I'm writing should create
> files in /tmp/ and choose based on input to copy some of the files
> to current directory.

> "rename" can't work across filesystems, as explained in rename(2).
> I tried to write my own version using "link", and neither does link
> work, as explained in link(2).

> int myRename(char *old_file, char *new_file) {
> if (rename(old_file, new_file) == 0) {
> return 0;
> }
> else if (errno == EXDEV) {
> /* link() can't work across filesystems too. */
> if (link(old_file, new_file) != 0)
> return -1;
> return unlink(old_file);
> }
> else {
> perror("myRename");
> return -1;
> }
> }

> The only solution I can think of is to copy files, and then delete
> the old files.

> int myRename(char *old_file, char *new_file) {
> if (rename(old_file, new_file) == 0) {
> return 0;
> }
> else if (errno == EXDEV) {
> FILES *old_fd, *new_fd;
> char buf[MAX_BUF];
> int file_size;
> old_fd = fopen(old_file, "r");
> new_fd = fopen(new_file, "w");
> file_size = fread(buf, sizeof(char), MAX_BUF, old_fd);
> fwrite(buf, sizeof(char), file_size, new_fd);

> return unlink(old_file);
> }
> else {
> perror("myRename");
> return -1;
> }
> }

> But, I think the solution above is inefficient and has serious
> limitation, namely what if the "old_file" has size larger than
> MAX_BUF. It means I have to implement a while loop to complete the
> copy. Does someone know a better (more efficient) way to implement
> myRename? Or is there a C function that does the same job as
> user-command "cp"?

> 2. Where (which lib) does basename() function reside in Solaris 2.5.1,
> and how to link its object using gcc?

> Here is a portion of my code that uses basename().

> #include <libgen.h>

> char myProjFile[] = "/proj/foo/bar.c";
> printf("The basename of '%s' is '%s'\n", myProjFile,
> basename(myProjFile));

> I compiled it with both gcc 2.8.1 and gcc 2.95.2 on Solaris 2.5.1,
> both returned the following errors.

> Undefined first referenced
> symbol in file
> basename /var/tmp/cca002G51.o
> ld: fatal: Symbol referencing errors. No output written to foo

> However, if I compile it on Solaris 2.7 using either version of gcc,
> it works fine. Hence, I assume it's not gcc problem.

> Is this a common/known problem for Solaris 2.5.1, or is the problem
> specific to my Solaris 2.5.1 machine? In either case, how can I fix
> it?

> Thanks. I appreciate your help.

> Hon-Chi

> Sent via Deja.com http://www.deja.com/
> Before you buy.

1. IMO the best solution is to write a hybrid function that firstly
tries to use the link()/unlink() method. If you attempt a cross-device
link, link() will fail and errno will be set to EXDEV. If this happens
you must then revert to the copy technique. AFAIK this is how mv is
usually implemented.

2. Include stdgen.h and link with -lgen.

Sat, 29 Jun 2002 03:00:00 GMT

Nick Maclar
#3 / 25

Questions on Unix File/Directory Manipulation using C

Yes. But (to the original poster and people following up), please
do not cross-post this to comp.std.c or even comp.lang.c. It is not
a C language question, still less one about C standardisation. It
is a Unix question.

If I could remember the details or be bothered to look them up,
I could give you a response in terms of the MVS primitives. This
would not be helpful, but is equally valid in C terms :-)

Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QG, England.

Tel.: +44 1223 334761 Fax: +44 1223 334679

Sat, 29 Jun 2002 03:00:00 GMT

Kenneth C Stah
#4 / 25