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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |VERSIONS |STANDARDS |HISTORY |NOTES |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
copy_file_range(2) System Calls Manualcopy_file_range(2)copy_file_range - Copy a range of data from one file to another
Standard C library (libc,-lc)
#define _GNU_SOURCE#define _FILE_OFFSET_BITS 64#include <unistd.h>ssize_t copy_file_range(intfd_in, off_t *_Nullableoff_in,intfd_out, off_t *_Nullableoff_out,size_tsize, unsigned intflags);
Thecopy_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 tosize bytes of data from the source file descriptorfd_in to the target file descriptorfd_out, overwriting any data that exists within the requested range of the target file. The following semantics apply foroff_in, and similar statements apply tooff_out: • Ifoff_in is NULL, then bytes are read fromfd_in starting from the file offset, and the file offset is adjusted by the number of bytes copied. • Ifoff_in is not NULL, thenoff_in must point to a buffer that specifies the starting offset where bytes fromfd_in will be read. The file offset offd_in is not changed, butoff_in is adjusted appropriately.fd_in andfd_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. Theflags argument is provided to allow for future extensions and currently must be set to 0.
Upon successful completion,copy_file_range() will return the number of bytes copied between files. This could be less than the size originally requested. If the file offset offd_in is at or past the end of file, no bytes are copied, andcopy_file_range() returns zero. On error,copy_file_range() returns -1 anderrno is set to indicate the error.
EBADFOne or more file descriptors are not valid.EBADFfd_in is not open for reading; orfd_out is not open for writing.EBADFTheO_APPENDflag is set for the open file description (seeopen(2)) referred to by the file descriptorfd_out.EFBIGAn attempt was made to write at a position past the maximum file offset the kernel supports.EFBIGAn 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.EFBIGAn attempt was made to write beyond the process's file size resource limit. This may also result in the process receiving aSIGXFSZsignal.EINVALTheflags argument is not 0.EINVALfd_in andfd_out refer to the same file and the source and target ranges overlap.EINVALEitherfd_in orfd_out is not a regular file.EIOA low-level I/O error occurred while copying.EISDIREitherfd_in orfd_out refers to a directory.ENOMEMOut of memory.ENOSPCThere 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.EPERMfd_out refers to an immutable file.ETXTBSY Eitherfd_in orfd_out refers to an active swap file.EXDEV(before Linux 5.3) The files referred to byfd_in andfd_out are not on the same filesystem.EXDEV(since Linux 5.19) The files referred to byfd_in andfd_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.
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.
Linux, GNU.
Linux 4.5, but glibc 2.27 provides a user-space emulation when it is not available.
Iffd_in is a sparse file, thencopy_file_range() may expand any holes existing in the requested range. Users may benefit from callingcopy_file_range() in a loop, and using thelseek(2)SEEK_DATAandSEEK_HOLEoperations 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_BITSshould be defined to be 64 in code that uses non-nulloff_in oroff_out or that takes the address ofcopy_file_range, if the code is intended to be portable to traditional 32-bit x86 and ARM platforms whereoff_t's width defaults to 32 bits.
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.
#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 size, 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); } size = 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, size, 0); if (ret == -1) { perror("copy_file_range"); exit(EXIT_FAILURE); } size -= ret; } while (size > 0 && ret > 0); close(fd_in); close(fd_out); exit(EXIT_SUCCESS); }lseek(2),sendfile(2),splice(2)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17copy_file_range(2)Pages that refer to this page:sendfile(2), splice(2), syscalls(2), off_t(3type), proc_pid_io(5), feature_test_macros(7), xfs_io(8)
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