1COPY_FILE_RANGE(2) Linux Programmer's Manual COPY_FILE_RANGE(2)
2
6 copy_file_range - Copy a range of data from one file to another
7
9 #define _GNU_SOURCE
10 #include <unistd.h>
11 ssize_t copy_file_range(int fd_in, off64_t *off_in,
13 int fd_out, off64_t *off_out,
14 size_t len, unsigned int flags);
15
17 The copy_file_range() system call performs an in-kernel copy between
18 two file descriptors without the additional cost of transferring data
19 from the kernel to user space and then back into the kernel. It copies
20 up to len bytes of data from the source file descriptor fd_in to the
21 target file descriptor fd_out, overwriting any data that exists within
22 the requested range of the target file.
23 The following semantics apply for off_in, and similar statements apply
25 to off_out:
26 * If off_in is NULL, then bytes are read from fd_in starting from the
28 file offset, and the file offset is adjusted by the number of bytes
29 copied.
30 * If off_in is not NULL, then off_in must point to a buffer that spec‐
32 ifies the starting offset where bytes from fd_in will be read. The
33 file offset of fd_in is not changed, but off_in is adjusted appro‐
34 priately.
35 fd_in and fd_out can refer to the same file. If they refer to the same
37 file, then the source and target ranges are not allowed to overlap.
38 The flags argument is provided to allow for future extensions and cur‐
40 rently must be set to 0.
41
43 Upon successful completion, copy_file_range() will return the number of
44 bytes copied between files. This could be less than the length origi‐
45 nally requested. If the file offset of fd_in is at or past the end of
46 file, no bytes are copied, and copy_file_range() returns zero.
47 On error, copy_file_range() returns -1 and errno is set to indicate the
49 error.
50
52 EBADF One or more file descriptors are not valid.
53 EBADF fd_in is not open for reading; or fd_out is not open for writ‐
55 ing.
56 EBADF The O_APPEND flag is set for the open file description (see
58 open(2)) referred to by the file descriptor fd_out.
59 EFBIG An attempt was made to write at a position past the maximum file
61 offset the kernel supports.
62 EFBIG An attempt was made to write a range that exceeds the allowed
64 maximum file size. The maximum file size differs between
65 filesystem implementations and can be different from the maximum
66 allowed file offset.
67 EFBIG An attempt was made to write beyond the process's file size re‐
69 source limit. This may also result in the process receiving a
70 SIGXFSZ signal.
71 EINVAL The flags argument is not 0.
73 EINVAL fd_in and fd_out refer to the same file and the source and tar‐
75 get ranges overlap.
76 EINVAL Either fd_in or fd_out is not a regular file.
78 EIO A low-level I/O error occurred while copying.
80 EISDIR Either fd_in or fd_out refers to a directory.
82 ENOMEM Out of memory.
84 ENOSPC There is not enough space on the target filesystem to complete
86 the copy.
87 EOVERFLOW
89 The requested source or destination range is too large to repre‐
90 sent in the specified data types.
91 EPERM fd_out refers to an immutable file.
93 ETXTBSY
95 Either fd_in or fd_out refers to an active swap file.
96 EXDEV The files referred to by fd_in and fd_out are not on the same
98 mounted filesystem (pre Linux 5.3).
99
101 The copy_file_range() system call first appeared in Linux 4.5, but
102 glibc 2.27 provides a user-space emulation when it is not available.
103 A major rework of the kernel implementation occurred in 5.3. Areas of
105 the API that weren't clearly defined were clarified and the API bounds
106 are much more strictly checked than on earlier kernels. Applications
107 should target the behaviour and requirements of 5.3 kernels.
108 First support for cross-filesystem copies was introduced in Linux 5.3.
110 Older kernels will return -EXDEV when cross-filesystem copies are at‐
111 tempted.
112
114 The copy_file_range() system call is a nonstandard Linux and GNU exten‐
115 sion.
116
118 If fd_in is a sparse file, then copy_file_range() may expand any holes
119 existing in the requested range. Users may benefit from calling
120 copy_file_range() in a loop, and using the lseek(2) SEEK_DATA and
121 SEEK_HOLE operations to find the locations of data segments.
122 copy_file_range() gives filesystems an opportunity to implement "copy
124 acceleration" techniques, such as the use of reflinks (i.e., two or
125 more inodes that share pointers to the same copy-on-write disk blocks)
126 or server-side-copy (in the case of NFS).
127
129 #define _GNU_SOURCE
130 #include <fcntl.h>
131 #include <stdio.h>
132 #include <stdlib.h>
133 #include <sys/stat.h>
134 #include <unistd.h>
135 int
137 main(int argc, char *argv[])
138 {
139 int fd_in, fd_out;
140 struct stat stat;
141 off64_t len, ret;
142 if (argc != 3) {
144 fprintf(stderr, "Usage: %s <source> <destination>\n", argv[0]);
145 exit(EXIT_FAILURE);
146 }
147 fd_in = open(argv[1], O_RDONLY);
149 if (fd_in == -1) {
150 perror("open (argv[1])");
151 exit(EXIT_FAILURE);
152 }
153 if (fstat(fd_in, &stat) == -1) {
155 perror("fstat");
156 exit(EXIT_FAILURE);
157 }
158 len = stat.st_size;
160 fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
162 if (fd_out == -1) {
163 perror("open (argv[2])");
164 exit(EXIT_FAILURE);
165 }
166 do {
168 ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
169 if (ret == -1) {
170 perror("copy_file_range");
171 exit(EXIT_FAILURE);
172 }
173 len -= ret;
175 } while (len > 0 && ret > 0);
176 close(fd_in);
178 close(fd_out);
179 exit(EXIT_SUCCESS);
180 }
181
183 lseek(2), sendfile(2), splice(2)
184
186 This page is part of release 5.13 of the Linux man-pages project. A
187 description of the project, information about reporting bugs, and the
188 latest version of this page, can be found at
189 https://www.kernel.org/doc/man-pages/.
190Linux 2021-08-27 COPY_FILE_RANGE(2)