--- name: "copy_file_range" section: "2" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/copy_file_range" --- # copy\_file\_range(2) Copy a range of data from one file to another ## 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 ``` **ssize\_t copy\_file\_range(int** *fd\_in***, off\_t \*\_Nullable** *off\_in***,** **int** *fd\_out***, off\_t \*\_Nullable** *off\_out***,** **size\_t** *size***, 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 *size* 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 size 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)](/open/2.md)) 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 behavior and requirements of Linux 5.19, that was also backported to earlier stable kernels. ## STANDARDS Linux, GNU. ## HISTORY Linux 4.5. Before glibc 2.30, starting with glibc 2.27, the glibc wrapper function provided a user-space fallback implementation when the kernel did not implement this system call. Since glibc 2.30, that fallback has been removed; the function now fails with ENOSYS if the kernel lacks support for **copy\_file\_range**(). ## 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)](/lseek/2.md) **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 #include #include #include #include #include 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 \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); } ``` ## SEE ALSO [**lseek**(2)](/lseek/2.md), [**sendfile**(2)](/sendfile/2.md), [**splice**(2)](/splice/2.md)