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exchangedata(2) [osx man page]

EXCHANGEDATA(2) 					      BSD System Calls Manual						   EXCHANGEDATA(2)

NAME
exchangedata -- atomically exchange data between two files SYNOPSIS
#include <unistd.h> #include <sys/attr.h> int exchangedata(const char * path1, const char * path2, unsigned int options); DESCRIPTION
The exchangedata() function swaps the contents of the files referenced by path1 and path2 in an atomic fashion. That is, all concurrent pro- cesses will either see the pre-exchanged state or the post-exchanged state; they can never see the files in an inconsistent state. The data in all forks is swapped in this way. The options parameter lets you control specific aspects of the function's behaviour. Open file descriptors follow the swapped data. Thus, a descriptor that previously referenced path1 will now reference the data that's acces- sible via path2, and vice versa. In general, the file attributes (metadata) are not exchanged. Specifically, the object identifier attributes (that is, the ATTR_CMN_OBJID and ATTR_CMN_OBJPERMANENTID attributes as defined by the getattrlist(2) function) are not swapped. An exception to this general rule is that the modification time attribute ( ATTR_CMN_MODTIME ) is swapped. When combined, these features allow you to implement a 'safe save' function that does not break references to the file (for example, aliases). You first save the new contents to a temporary file and then exchange the data of the original file and the temporary. Programs that reference the file via an object identifier will continue to reference the original file, but now it has the new data. The path1 and path2 parameters must both reference valid files. All directories listed in the path names leading to these files must be searchable. You must have write access to the files. The options parameter is a bit set that controls the behaviour of exchangedata(). The following option bits are defined. FSOPT_NOFOLLOW If this bit is set, exchangedata() will not follow a symlink if it occurs as the last component of path1 or path2. RETURN VALUES
Upon successful completion a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error. COMPATIBILITY
Not all volumes support exchangedata(). You can test whether a volume supports exchangedata() by using getattrlist(2) to get the volume capabilities attribute ATTR_VOL_CAPABILITIES, and then testing the VOL_CAP_INT_EXCHANGEDATA flag. ERRORS
exchangedata() will fail if: [ENOTSUP] The volume does not support exchangedata(). [ENOTDIR] A component of the path prefix is not a directory. [ENAMETOOLONG] A component of a path name exceeded NAME_MAX characters, or an entire path name exceeded PATH_MAX characters. [ENOENT] Either file does not exist. [EACCES] Search permission is denied for a component of the path prefix. [ELOOP] Too many symbolic links were encountered in translating the pathname. [EFAULT] path1 or path2 points to an invalid address. [EXDEV] path1 and path2 are on different volumes (mounted file systems). [EINVAL] path1 or path2 reference the same file. [EINVAL] You try to exchange something other than a regular file (for example, a directory). [EIO] An I/O error occurred while reading from or writing to the file system. SEE ALSO
getattrlist(2) HISTORY
A exchangedata() function call appeared in Darwin 1.3.1 (Mac OS X version 10.0). Darwin December 15, 2003 Darwin

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SYMLINK(2)						      BSD System Calls Manual							SYMLINK(2)

NAME
symlink, symlinkat -- make symbolic link to a file SYNOPSIS
#include <unistd.h> int symlink(const char *path1, const char *path2); int symlinkat(const char *name1, int fd, const char *name2); DESCRIPTION
A symbolic link path2 is created to path1 (path2 is the name of the file created, path1 is the string used in creating the symbolic link). Either name may be an arbitrary path name; the files need not be on the same file system. The symlinkat() system call is equivalent to symlink() except in the case where name2 specifies a relative path. In this case the symbolic link is created relative to the directory associated with the file descriptor fd instead of the current working directory. If symlinkat() is passed the special value AT_FDCWD in the fd parameter, the current working directory is used and the behavior is identical to a call to symlink(). RETURN VALUES
Upon successful completion, a zero value is returned. If an error occurs, the error code is stored in errno and a -1 value is returned. ERRORS
The symbolic link succeeds unless: [EACCES] Write permission is denied in the directory where the symbolic link is being created. [EACCES] A component of the path2 path prefix denies search permission. [EDQUOT] The directory in which the entry for the new symbolic link is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted. [EDQUOT] The new symbolic link cannot be created because the user's quota of disk blocks on the file system that will contain the symbolic link has been exhausted. [EDQUOT] The user's quota of inodes on the file system on which the symbolic link is being created has been exhausted. [EEXIST] Path2 already exists. [EFAULT] Path1 or path2 points outside the process's allocated address space. [EIO] An I/O error occurs while making the directory entry or allocating the inode. [EIO] An I/O error occurs while making the directory entry for path2, or allocating the inode for path2, or writing out the link contents of path2. [ELOOP] Too many symbolic links are encountered in translating the pathname. This is taken to be indicative of a looping symbolic link. [ENAMETOOLONG] A component of a pathname exceeds {NAME_MAX} characters, or an entire path name exceeds {PATH_MAX} characters. [ENOENT] A component of path2 does not name an existing file or path2 is an empty string. [ENOSPC] The directory in which the entry for the new symbolic link is being placed cannot be extended because there is no space left on the file system containing the directory. [ENOSPC] The new symbolic link cannot be created because there there is no space left on the file system that will contain the sym- bolic link. [ENOSPC] There are no free inodes on the file system on which the symbolic link is being created. [ENOTDIR] A component of the path2 prefix is not a directory. [EROFS] The file path2 would reside on a read-only file system. In addition to the errors returned by the symlink(), the symlinkat() may fail if: [EBADF] The name2 argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor open for searching. [ENOTDIR] The name2 argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory. SEE ALSO
ln(1), link(2), unlink(2), symlink(7) STANDARDS
The symlinkat() system call is expected to conform to POSIX.1-2008 . HISTORY
The symlink() function call appeared in 4.2BSD. The symlinkat() system call appeared in OS X 10.10 4.2 Berkeley Distribution June 4, 1993 4.2 Berkeley Distribution

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