plucky (2) copy_file_range.2.gz

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NAME

       copy_file_range - Copy a range of data from one file to another

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #define _GNU_SOURCE
       #define _FILE_OFFSET_BITS 64
       #include <unistd.h>

       ssize_t copy_file_range(int fd_in, off_t *_Nullable off_in,
                               int fd_out, off_t *_Nullable off_out,
                               size_t len, unsigned int flags);

DESCRIPTION

       The  copy_file_range()  system  call  performs an in-kernel copy between two file descriptors without the
       additional cost of transferring data from the kernel to user space and then back  into  the  kernel.   It
       copies  up  to  len  bytes  of  data  from the source file descriptor fd_in to the target file descriptor
       fd_out, overwriting any data that exists within the requested range of the target file.

       The following semantics apply for off_in, and similar statements apply to off_out:

       •  If off_in is NULL, then bytes are read from fd_in starting from the file offset, and the  file  offset
          is adjusted by the number of bytes copied.

       •  If  off_in  is  not  NULL, then off_in must point to a buffer that specifies the starting offset where
          bytes from fd_in will be read.  The file offset of fd_in  is  not  changed,  but  off_in  is  adjusted
          appropriately.

       fd_in  and fd_out can refer to the same file.  If they refer to the same file, then the source and target
       ranges are not allowed to overlap.

       The flags argument is provided to allow for future extensions and currently must be set to 0.

RETURN VALUE

       Upon successful completion, copy_file_range() will return the number of bytes copied between files.  This
       could be less than the length originally requested.  If the file offset of fd_in is at or past the end of
       file, no bytes are copied, and copy_file_range() returns zero.

       On error, copy_file_range() returns -1 and errno is set to indicate the error.

ERRORS

       EBADF  One or more file descriptors are not valid.

       EBADF  fd_in is not open for reading; or fd_out is not open for writing.

       EBADF  The O_APPEND flag is set for the open file description (see  open(2))  referred  to  by  the  file
              descriptor fd_out.

       EFBIG  An attempt was made to write at a position past the maximum file offset the kernel supports.

       EFBIG  An attempt was made to write a range that exceeds the allowed maximum file size.  The maximum file
              size differs between filesystem implementations and can be different from the maximum allowed file
              offset.

       EFBIG  An  attempt was made to write beyond the process's file size resource limit.  This may also result
              in the process receiving a SIGXFSZ signal.

       EINVAL The flags argument is not 0.

       EINVAL fd_in and fd_out refer to the same file and the source and target ranges overlap.

       EINVAL Either fd_in or fd_out is not a regular file.

       EIO    A low-level I/O error occurred while copying.

       EISDIR Either fd_in or fd_out refers to a directory.

       ENOMEM Out of memory.

       ENOSPC There is not enough space on the target filesystem to complete the copy.

       EOPNOTSUPP (since Linux 5.19)
              The filesystem does not support this operation.

       EOVERFLOW
              The requested source or destination range is too large to represent in the specified data types.

       EPERM  fd_out refers to an immutable file.

       ETXTBSY
              Either fd_in or fd_out refers to an active swap file.

       EXDEV (before Linux 5.3)
              The files referred to by fd_in and fd_out are not on the same filesystem.

       EXDEV (since Linux 5.19)
              The files referred to by fd_in and fd_out are not on the  same  filesystem,  and  the  source  and
              target filesystems are not of the same type, or do not support cross-filesystem copy.

VERSIONS

       A major rework of the kernel implementation occurred in Linux 5.3.  Areas of the API that weren't clearly
       defined were clarified and the API bounds are much more strictly checked than on earlier kernels.

       Since Linux 5.19, cross-filesystem copies can be achieved when both filesystems are of the same type, and
       that filesystem implements support for it.  See BUGS for behavior prior to Linux 5.19.

       Applications  should  target  the  behaviour  and requirements of Linux 5.19, that was also backported to
       earlier stable kernels.

STANDARDS

       Linux, GNU.

HISTORY

       Linux 4.5, but glibc 2.27 provides a user-space emulation when it is not available.

NOTES

       If fd_in is a sparse file, then copy_file_range() may expand any holes existing in the  requested  range.
       Users  may  benefit  from  calling  copy_file_range()  in  a  loop,  and using the lseek(2) SEEK_DATA and
       SEEK_HOLE operations to find the locations of data segments.

       copy_file_range() gives filesystems an opportunity to implement "copy acceleration" techniques,  such  as
       the  use of reflinks (i.e., two or more inodes that share pointers to the same copy-on-write disk blocks)
       or server-side-copy (in the case of NFS).

       _FILE_OFFSET_BITS should be defined to be 64 in code that uses non-null off_in or off_out or  that  takes
       the  address of copy_file_range, if the code is intended to be portable to traditional 32-bit x86 and ARM
       platforms where off_t's width defaults to 32 bits.

BUGS

       In Linux 5.3 to Linux 5.18, cross-filesystem copies were implemented by the kernel, if the operation  was
       not  supported by individual filesystems.  However, on some virtual filesystems, the call failed to copy,
       while still reporting success.

EXAMPLES

       #define _GNU_SOURCE
       #define _FILE_OFFSET_BITS 64
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/stat.h>
       #include <sys/types.h>
       #include <unistd.h>

       int
       main(int argc, char *argv[])
       {
           int          fd_in, fd_out;
           off_t        len, ret;
           struct stat  stat;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s <source> <destination>\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           fd_in = open(argv[1], O_RDONLY);
           if (fd_in == -1) {
               perror("open (argv[1])");
               exit(EXIT_FAILURE);
           }

           if (fstat(fd_in, &stat) == -1) {
               perror("fstat");
               exit(EXIT_FAILURE);
           }

           len = stat.st_size;

           fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
           if (fd_out == -1) {
               perror("open (argv[2])");
               exit(EXIT_FAILURE);
           }

           do {
               ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
               if (ret == -1) {
                   perror("copy_file_range");
                   exit(EXIT_FAILURE);
               }

               len -= ret;
           } while (len > 0 && ret > 0);

           close(fd_in);
           close(fd_out);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       lseek(2), sendfile(2), splice(2)