getcwd - get the pathname of the current working directory
#include <unistd.h>
char *getcwd(char *buf, size_t size);
The getcwd() function shall place an absolute pathname of the current working directory in the array pointed to by buf, and return buf. The pathname shall contain no components that are dot or dot-dot, or are symbolic links.
If there are multiple pathnames that getcwd() could place in the array pointed to by buf, one beginning with a single <slash> character and one or more beginning with two <slash> characters, then getcwd() shall place the pathname beginning with a single <slash> character in the array. The pathname shall not contain any unnecessary <slash> characters after the leading one or two <slash> characters.
The size argument is the size in bytes of the character array pointed to by the buf argument. If buf is a null pointer, the behavior of getcwd() is unspecified.
Upon successful completion, getcwd() shall return the buf argument. Otherwise, getcwd() shall return a null pointer and set errno to indicate the error. The contents of the array pointed to by buf are then undefined.
The getcwd() function shall fail if:
- [EINVAL]
- The size argument is 0.
- [ERANGE]
- The size argument is greater than 0, but is smaller than the length of the string +1.
The getcwd() function may fail if:
- [EACCES]
- Search permission was denied for the current directory, or read or search permission was denied for a directory above the current directory in the file hierarchy.
- [ENOMEM]
- Insufficient storage space is available.
The following example uses {PATH_MAX} as the initial buffer size (unless it is indeterminate or very large), and calls getcwd() with progressively larger buffers until it does not give an [ERANGE] error.
#include <stdlib.h> #include <errno.h> #include <unistd.h>
...
long path_max; size_t size; char *buf; char *ptr;
path_max = pathconf(".", _PC_PATH_MAX); if (path_max == -1) size = 1024; else if (path_max > 10240) size = 10240; else size = path_max;
for (buf = ptr = NULL; ptr == NULL; size *= 2) { if ((buf = realloc(buf, size)) == NULL) { ... handle error ... }
ptr = getcwd(buf, size); if (ptr == NULL && errno != ERANGE) { ... handle error ... } } ... free (buf);
If the pathname obtained from getcwd() is longer than {PATH_MAX} bytes, it could produce an [ENAMETOOLONG] error if passed to chdir(). Therefore, in order to return to that directory it may be necessary to break the pathname into sections shorter than {PATH_MAX} bytes and call chdir() on each section in turn (the first section being an absolute pathname and subsequent sections being relative pathnames). A simpler way to handle saving and restoring the working directory when it may be deeper than {PATH_MAX} bytes in the file hierarchy is to use a file descriptor and fchdir(), rather than getcwd() and chdir(). However, the two methods do have some differences. The fchdir() approach causes the program to restore a working directory even if it has been renamed in the meantime, whereas the chdir() approach restores to a directory with the same name as the original, even if the directories were renamed in the meantime. Since the fchdir() approach does not access parent directories, it can succeed when getcwd() would fail due to permissions problems. In applications conforming to earlier versions of this standard, it was not possible to use the fchdir() approach when the working directory is searchable but not readable, as the only way to open a directory was with O_RDONLY, whereas the getcwd() approach can succeed in this case.
Having getcwd() take no arguments and instead use the malloc() function to produce space for the returned argument was considered. The advantage is that getcwd() knows how big the working directory pathname is and can allocate an appropriate amount of space. But the programmer would have to use the free() function to free the resulting object, or each use of getcwd() would further reduce the available memory. Finally, getcwd() is taken from the SVID where it has the two arguments used in this volume of POSIX.1-2008.
The older function getwd() was rejected for use in this context because it had only a buffer argument and no size argument, and thus had no way to prevent overwriting the buffer, except to depend on the programmer to provide a large enough buffer.
On some implementations, if buf is a null pointer, getcwd() may obtain size bytes of memory using malloc(). In this case, the pointer returned by getcwd() may be used as the argument in a subsequent call to free(). Invoking getcwd() with buf as a null pointer is not recommended in conforming applications.
Earlier implementations of getcwd() sometimes generated pathnames like "../../../subdirname" internally, using them to explore the path of ancestor directories back to the root. If one of these internal pathnames exceeded {PATH_MAX} in length, the implementation could fail with errno set to [ENAMETOOLONG]. This is no longer allowed.
If a program is operating in a directory where some (grand)parent directory does not permit reading, getcwd() may fail, as in most implementations it must read the directory to determine the name of the file. This can occur if search, but not read, permission is granted in an intermediate directory, or if the program is placed in that directory by some more privileged process (for example, login). Including the [EACCES] error condition makes the reporting of the error consistent and warns the application developer that getcwd() can fail for reasons beyond the control of the application developer or user. Some implementations can avoid this occurrence (for example, by implementing getcwd() using pwd, where pwd is a set-user-root process), thus the error was made optional. Since this volume of POSIX.1-2008 permits the addition of other errors, this would be a common addition and yet one that applications could not be expected to deal with without this addition.
None.
XBD <unistd.h>
First released in Issue 1. Derived from Issue 1 of the SVID.
The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:
The [ENOMEM] optional error condition is added.
Austin Group Interpretation 1003.1-2001 #140 is applied, changing the text for consistency with the pwd utility, adding text to address the case where the current directory is deeper in the file hierarchy than {PATH_MAX} bytes, and adding the requirements relating to pathnames beginning with two <slash> characters.
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