pathconf() — Determine configurable path name variables

The varcode argument may be any one of the following symbols, defined in the unistd.h header file, each standing for a configuration variable:

_PC_LINK_MAX

Represents LINK_MAX, the maximum number of links the file can have. If pathname is a directory, pathconf() returns the maximum number of links that can be established to the directory itself.

_PC_MAX_CANON

Represents MAX_CANON, the maximum number of bytes in a terminal canonical input line. pathname must refer to a character special file for a terminal.

_PC_MAX_INPUT

Represents MAX_INPUT, the maximum number of bytes for which space is available in a terminal input queue. That is, it refers to the maximum number of bytes that a portable application can have the user enter before the application actually reads the input. pathname must refer to a character special file for a terminal.

_PC_NAME_MAX

Represents NAME_MAX, the maximum number of characters in a file name (not including any terminating NULL at the end if the file name is stored as a string). This symbol refers only to the file name itself, that is, the last component of the file's path name. pathconf() returns the maximum length of file names.

_PC_PATH_MAX

Represents PATH_MAX, the maximum number of characters in a complete path name (not including any terminating NULL at the end if the path name is stored as a string). pathconf() returns the maximum length of a relative path name.

_PC_PIPE_BUF

Represents PIPE_BUF, the maximum number of bytes that can be written “atomically” to a pipe. If more than this number of bytes is written to a pipe, the operation may take more than one physical write operation and physical read operation to read the data on the other end of the pipe. If pathname is a FIFO special file, pathconf() returns the value for the file itself. If pathname is a directory, pathconf() returns the value for any FIFOs that exist or that can be created under the directory. If pathname is any other kind of file, an errno of EINVAL will be returned, indicating an invalid path name was specified.

_PC_CHOWN_RESTRICTED

Represents _POSIX_CHOWN_RESTRICTED defined in the unistd.h header file, and restricts use of chown() to a process with appropriate privileges. It also changes the group ID of a file to the effective group ID of the process or to one of its supplementary group IDs. If pathname is a directory, pathconf() returns the value for any kind of file under the directory, but not for subdirectories of the directory.

_PC_NO_TRUNC

Represents _POSIX_NO_TRUNC defined in the unistd.h header file, and generates an error if a file name is longer than NAME_MAX. If pathname refers to a directory, the value returned by pathconf() applies to all files under that directory.

_PC_VDISABLE

Represents _POSIX_VDISABLE defined in the unistd.h header file. This symbol indicates that terminal special characters can be disabled using this character value, if it is defined; see the callable service tcsetattr() for details. pathname must refer to a character special file for a terminal.

_PC_ACL

Returns 1 if an access control mechanism is supported by the security product.

_PC_ACL_ENTRIES_MAX

Returns the maximum number of ACL entries in an ACL for a file or directory that supports ACLs.

If pathconf() cannot determine an appropriate value, it sets errno to one of the following values:

Error Code

Description

EACCES

The process does not have search permission on some component of the pathname.

EINVAL

varcode is not a valid variable code, or the given variable cannot be associated with the specified file.

• If varcode refers to MAX_CANON, MAX_INPUT, or _POSIX_VDISABLE, and pathname does not refer to a character special file, pathconf() returns -1 and sets errno to EINVAL.
• If varcode refers to NAME_MAX, PATH_MAX, or POSIX_NO_TRUNC, and pathname does not refer to a directory, pathconf() returns the requested information.
• If varcode refers to PC_PIPE_BUF and pathname refers to a pipe or a FIFO, the value returned applies to the referenced object itself. If pathname refers to a directory, the value returned applies to any FIFOs that exist or can be created within the directory. If pathname refers to any other type of file, the function sets errno to EINVAL.

ELOOP

A loop exists in symbolic links. This error is issued if more than POSIX_SYMLOOPS symbolic links are detected in the resolution of pathname.

ENAMETOOLONG

pathname is longer than PATH_MAX characters, or some component of pathname is longer than NAME_MAX while _POSIX_NO_TRUNC is in effect.

For symbolic links, the length of the path name string substituted for a symbolic link exceeds PATH_MAX.

ENOENT

There is no file named pathname, or the pathname argument is an empty string.

ENOTDIR

Some component of the pathname is not a directory.

CELEBP01

⁄* CELEBP01

   This example determines the maximum number of characters in a file name.

 *⁄
#define _POSIX_SOURCE
#include <errno.h>
#include <unistd.h>
#undef _POSIX_SOURCE
#include <stdio.h>

main() {
  long result;

  errno = 0;
  puts("examining NAME_MAX limit for root filesystem");
  if ((result = pathconf("⁄", _PC_NAME_MAX)) == -1)
    if (errno == 0)
      puts("There is no limit to NAME_MAX.");
    else perror("pathconf() error");
  else
    printf("NAME_MAX is %ld\n", result);
}

Output:

examining NAME_MAX limit for root file system
NAME_MAX is 255