C言語 ファイルやパス関係の最大値をプログラムから取得する方法 Mac編

取得方法

unistd.hで定義されているpathconfという関数を使います。
直接に環境変数の中には定義されておらず、pathconf関数を実行して取得する必要があります。

SYNOPSIS

     #include <unistd.h>

     long
     fpathconf(int fildes, int name);

     long
     pathconf(const char *path, int name);

一覧

変数	説明
_PC_LINK_MAX	The maximum file link count.
_PC_MAX_CANON	The maximum number of bytes in terminal canonical input line.
_PC_MAX_INPUT	The minimum maximum number of bytes for which space is available in a terminal input queue.
_PC_NAME_MAX	The maximum number of bytes in a file name.
_PC_PATH_MAX	The maximum number of bytes in a pathname.
_PC_PIPE_BUF	The maximum number of bytes which will be written atomically to a pipe.
_PC_CHOWN_RESTRICTED	Return 1 if appropriate privileges are required for the chown(2) system call, otherwise 0.
_PC_NO_TRUNC	Return 1 if file names longer than KERN_NAME_MAX are truncated.
_PC_VDISABLE	Returns the terminal character disabling value.
_PC_XATTR_SIZE_BITS	Returns the number of bits used to store maximum extended attribute size in bytes. For example, if the maximum attribute size supported by a file system is 128K, the value returned will be 18. However a value 18 can mean that the maximum attribute size can be anywhere from (256KB - 1) to 128KB. As a special case, the resource fork can have much larger size, and some file system specific extended attributes can have smaller and preset size; for example, Finder Info is always 32 bytes.
_PC_MIN_HOLE_SIZE	If a file system supports the reporting of holes (see lseek(2)), pathconf() and fpathconf() return a positive number that represents the minimum hole size returned in bytes. The offsets of holes returned will be aligned to this same value. A special value of 1 is returned if the file system does not specify the minimum hole size but still reports holes.

使用例

#include <unistd.h>
#include <stdio.h>

int main()
{
    printf("_PC_LINK_MAX         : %lu\n", pathconf(".", _PC_LINK_MAX));
    printf("_PC_MAX_CANON        : %lu\n", pathconf(".", _PC_MAX_CANON));
    printf("_PC_MAX_INPUT        : %lu\n", pathconf(".", _PC_MAX_INPUT));
    printf("_PC_NAME_MAX         : %lu\n", pathconf(".", _PC_NAME_MAX));
    printf("_PC_PATH_MAX         : %lu\n", pathconf(".", _PC_PATH_MAX));
    printf("_PC_PIPE_BUF         : %lu\n", pathconf(".", _PC_PIPE_BUF));
    printf("_PC_CHOWN_RESTRICTED : %lu\n", pathconf(".", _PC_CHOWN_RESTRICTED));
    printf("_PC_NO_TRUNC         : %lu\n", pathconf(".", _PC_NO_TRUNC));
    printf("_PC_VDISABLE         : %lu\n", pathconf(".", _PC_VDISABLE));
    printf("_PC_XATTR_SIZE_BITS  : %lu\n", pathconf(".", _PC_XATTR_SIZE_BITS));
    printf("_PC_MIN_HOLE_SIZE    : %lu\n", pathconf(".", _PC_MIN_HOLE_SIZE));
    return (0);
}

実行結果

_PC_LINK_MAX         : 32767
_PC_MAX_CANON        : 18446744073709551615
_PC_MAX_INPUT        : 18446744073709551615
_PC_NAME_MAX         : 255
_PC_PATH_MAX         : 1024
_PC_PIPE_BUF         : 512
_PC_CHOWN_RESTRICTED : 200112
_PC_NO_TRUNC         : 200112
_PC_VDISABLE         : 18446744073709551615
_PC_XATTR_SIZE_BITS  : 64
_PC_MIN_HOLE_SIZE    : 4096

エラー処理について

ちゃんとエラー処理をする書き方だと、以下のようになります。

pathconf() - 構成可能パス名変数の判別から借りてきました。

/*

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>

int 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);
}