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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |VERSIONS |STANDARDS |HISTORY |NOTES |SEE ALSO |COLOPHON | |
sendfile(2) System Calls Manualsendfile(2)sendfile - transfer data between file descriptors
Standard C library (libc,-lc)
#include <sys/sendfile.h>ssize_t sendfile(intout_fd, intin_fd, off_t *_Nullableoffset,size_tcount);
sendfile() copies data between one file descriptor and another. Because this copying is done within the kernel,sendfile() is more efficient than the combination ofread(2) andwrite(2), which would require transferring data to and from user space.in_fd should be a file descriptor opened for reading andout_fd should be a descriptor opened for writing. Ifoffset is not NULL, then it points to a variable holding the file offset from whichsendfile() will start reading data fromin_fd. Whensendfile() returns, this variable will be set to the offset of the byte following the last byte that was read. Ifoffset is not NULL, thensendfile() does not modify the file offset ofin_fd; otherwise the file offset is adjusted to reflect the number of bytes read fromin_fd. Ifoffset is NULL, then data will be read fromin_fd starting at the file offset, and the file offset will be updated by the call.count is the number of bytes to copy between the file descriptors. Thein_fd argument must correspond to a file which supportsmmap(2)-like operations (i.e., it cannot be a socket). Except since Linux 5.12 and ifout_fd is a pipe, in which casesendfile() desugars to asplice(2) and its restrictions apply. Before Linux 2.6.33,out_fd must refer to a socket. Since Linux 2.6.33 it can be any file. If it's seekable, thensendfile() changes the file offset appropriately.
If the transfer was successful, the number of bytes written toout_fd is returned. Note that a successful call tosendfile() may write fewer bytes than requested; the caller should be prepared to retry the call if there were unsent bytes. See also NOTES. On error, -1 is returned, anderrno is set to indicate the error.
EAGAINNonblocking I/O has been selected usingO_NONBLOCKand the write would block.EBADFThe input file was not opened for reading or the output file was not opened for writing.EFAULTBad address.EINVALDescriptor is not valid or locked, or anmmap(2)-like operation is not available forin_fd, orcount is negative.EINVALout_fd has theO_APPENDflag set. This is not currently supported bysendfile().EIOUnspecified error while reading fromin_fd.ENOMEMInsufficient memory to read fromin_fd.EOVERFLOWcount is too large, the operation would result in exceeding the maximum size of either the input file or the output file.ESPIPEoffset is not NULL but the input file is not seekable.
Other UNIX systems implementsendfile() with different semantics and prototypes. It should not be used in portable programs.
None.
Linux 2.2, glibc 2.1. In Linux 2.4 and earlier,out_fd could also refer to a regular file; this possibility went away in the Linux 2.6.x kernel series, but was restored in Linux 2.6.33. The original Linuxsendfile() system call was not designed to handle large file offsets. Consequently, Linux 2.4 addedsendfile64(), with a wider type for theoffset argument. The glibcsendfile() wrapper function transparently deals with the kernel differences.
sendfile() will transfer at most 0x7ffff000 (2,147,479,552) bytes, returning the number of bytes actually transferred. (This is true on both 32-bit and 64-bit systems.) If you plan to usesendfile() for sending files to a TCP socket, but need to send some header data in front of the file contents, you will find it useful to employ theTCP_CORKoption, described intcp(7), to minimize the number of packets and to tune performance. Applications may wish to fall back toread(2) andwrite(2) in the case wheresendfile() fails withEINVALorENOSYS. Ifout_fd refers to a socket or pipe with zero-copy support, callers must ensure the transferred portions of the file referred to byin_fd remain unmodified until the reader on the other end ofout_fd has consumed the transferred data. The Linux-specificsplice(2) call supports transferring data between arbitrary file descriptors provided one (or both) of them is a pipe.
copy_file_range(2),mmap(2),open(2),socket(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 2025-05-17sendfile(2)Pages that refer to this page:hardlink(1), copy_file_range(2), send(2), splice(2), syscalls(2), proc_pid_io(5), socket(7), tcp(7)
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