In Chapter 4 of the book "Advanced Programming in the Unix Environment," which covers files and directories, there is a code sample which aims to be like the ftw
command and traverse a file hierarchy. It uses a pointer to an absolute file path, as well as a recursive function with a callback to traverse the directory, using calls to opendir()
and readdir()
in the process.
There is an exercise in which readers are asked to use chdir()
and file names instead of using the absolute paths to accomplish the same task and to compare the times of the two programs. I wrote a program using chdir()
and did not notice a difference in the time. Is this expected? I would have thought that the additional call to chdir()
would add some overhead. Is it maybe a relatively trivial call? Any insight would be appreciated.
Here's the recursive function using absolute paths:
static int /* we return whatever func() returns */
dopath(Myfunc* func)
{
struct stat statbuf;
struct dirent *dirp;
DIR *dp;
int ret;
char *ptr;
if (lstat(fullpath, &statbuf) < 0) /* stat error */
return(func(fullpath, &statbuf, FTW_NS));
if (S_ISDIR(statbuf.st_mode) == 0) /* not a directory */
return(func(fullpath, &statbuf, FTW_F));
/*
* It's a directory. First call func() for the directory,
* then process each filename in the directory.
*/
if ((ret = func(fullpath, &statbuf, FTW_D)) != 0)
return(ret);
ptr = fullpath + strlen(fullpath); /* point to end of fullpath */
*ptr++ = '/';
*ptr = 0;
if ((dp = opendir(fullpath)) == NULL) /* can't read directory */
return(func(fullpath, &statbuf, FTW_DNR));
while ((dirp = readdir(dp)) != NULL) {
if (strcmp(dirp->d_name, ".") == 0 ||
strcmp(dirp->d_name, "..") == 0)
continue; /* ignore dot and dot-dot */
strcpy(ptr, dirp->d_name); /* append name after slash */
if ((ret = dopath(func)) != 0) /* recursive */
break; /* time to leave */
}
ptr[-1] = 0; /* erase everything from slash onwards */
if (closedir(dp) < 0)
err_ret("can't close directory %s", fullpath);
return(ret);
}
And here's the function with my changes:
static int /* we return whatever func() returns */
dopath(Myfunc* func, char* path)
{
struct stat statbuf;
struct dirent *dirp;
DIR *dp;
int ret;
if (lstat(path, &statbuf) < 0) /* stat error */
return(func(path, &statbuf, FTW_NS));
if (S_ISDIR(statbuf.st_mode) == 0) /* not a directory */
return(func(path, &statbuf, FTW_F));
/*
* It's a directory. First call func() for the directory,
* then process each filename in the directory.
*/
if ((ret = func(path, &statbuf, FTW_D)) != 0)
return(ret);
if ( chdir(path) < 0 )
return(func(path, &statbuf, FTW_DNR));
if ((dp = opendir(".")) == NULL) /* can't read directory */
return(func(path, &statbuf, FTW_DNR));
while ((dirp = readdir(dp)) != NULL) {
if (strcmp(dirp->d_name, ".") == 0 ||
strcmp(dirp->d_name, "..") == 0)
continue; /* ignore dot and dot-dot */
if ((ret = dopath(func, dirp->d_name)) != 0) /* recursive */
break; /* time to leave */
}
if ( chdir("..") < 0 )
err_ret("can't go up directory");
if (closedir(dp) < 0)
err_ret("can't close directory %s", fullpath);
return(ret);
}