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14.1 Working Directory

Each process has associated with it a directory, called its current working directory or simply working directory, that is used in the resolution of relative file names (see File Name Resolution).

When you log in and begin a new session, your working directory is initially set to the home directory associated with your login account in the system user database. You can find any user's home directory using the getpwuid or getpwnam functions; see User Database.

Users can change the working directory using shell commands like cd. The functions described in this section are the primitives used by those commands and by other programs for examining and changing the working directory. Prototypes for these functions are declared in the header file unistd.h.

— Function: char * getcwd (char *buffer, size_t size)

The getcwd function returns an absolute file name representing the current working directory, storing it in the character array buffer that you provide. The size argument is how you tell the system the allocation size of buffer.

The GNU library version of this function also permits you to specify a null pointer for the buffer argument. Then getcwd allocates a buffer automatically, as with malloc (see Unconstrained Allocation). If the size is greater than zero, then the buffer is that large; otherwise, the buffer is as large as necessary to hold the result.

The return value is buffer on success and a null pointer on failure. The following errno error conditions are defined for this function:

EINVAL
The size argument is zero and buffer is not a null pointer.
ERANGE
The size argument is less than the length of the working directory name. You need to allocate a bigger array and try again.
EACCES
Permission to read or search a component of the file name was denied.

You could implement the behavior of GNU's getcwd (NULL, 0) using only the standard behavior of getcwd: 

     char *
     gnu_getcwd ()
     {
       size_t size = 100;
     
       while (1)
         {
           char *buffer = (char *) xmalloc (size);
           if (getcwd (buffer, size) == buffer)
             return buffer;
           free (buffer);
           if (errno != ERANGE)
             return 0;
           size *= 2;
         }
     }

See Malloc Examples, for information about xmalloc, which is not a library function but is a customary name used in most GNU software.

— Deprecated Function: char * getwd (char *buffer)

This is similar to getcwd, but has no way to specify the size of the buffer. The GNU library provides getwd only for backwards compatibility with BSD.

The buffer argument should be a pointer to an array at least PATH_MAX bytes long (see Limits for Files). In the GNU system there is no limit to the size of a file name, so this is not necessarily enough space to contain the directory name. That is why this function is deprecated.

— Function: char * get_current_dir_name (void)

This get_current_dir_name function is basically equivalent to getcwd (NULL, 0). The only difference is that the value of the PWD variable is returned if this value is correct. This is a subtle difference which is visible if the path described by the PWD value is using one or more symbol links in which case the value returned by getcwd can resolve the symbol links and therefore yield a different result.

This function is a GNU extension.

— Function: int chdir (const char *filename)

This function is used to set the process's working directory to filename.

The normal, successful return value from chdir is 0. A value of -1 is returned to indicate an error. The errno error conditions defined for this function are the usual file name syntax errors (see File Name Errors), plus ENOTDIR if the file filename is not a directory.

— Function: int fchdir (int filedes)

This function is used to set the process's working directory to directory associated with the file descriptor filedes.

The normal, successful return value from fchdir is 0. A value of -1 is returned to indicate an error. The following errno error conditions are defined for this function:

EACCES
Read permission is denied for the directory named by dirname.
EBADF
The filedes argument is not a valid file descriptor.
ENOTDIR
The file descriptor filedes is not associated with a directory.
EINTR
The function call was interrupt by a signal.
EIO
An I/O error occurred.