sendfile(2) — Linux manual page


sendfile(2)                System Calls Manual               sendfile(2)

NAME         top

       sendfile - transfer data between file descriptors

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/sendfile.h>

       ssize_t sendfile(int out_fd, int in_fd, off_t *_Nullable offset,
                        size_t count);

DESCRIPTION         top

       sendfile() copies data between one file descriptor and another.
       Because this copying is done within the kernel, sendfile() is
       more efficient than the combination of read(2) and write(2),
       which would require transferring data to and from user space.

       in_fd should be a file descriptor opened for reading and out_fd
       should be a descriptor opened for writing.

       If offset is not NULL, then it points to a variable holding the
       file offset from which sendfile() will start reading data from
       in_fd.  When sendfile() returns, this variable will be set to the
       offset of the byte following the last byte that was read.  If
       offset is not NULL, then sendfile() does not modify the file
       offset of in_fd; otherwise the file offset is adjusted to reflect
       the number of bytes read from in_fd.

       If offset is NULL, then data will be read from in_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

       The in_fd argument must correspond to a file which supports
       mmap(2)-like operations (i.e., it cannot be a socket).  Except
       since Linux 5.12 and if out_fd is a pipe, in which case
       sendfile() desugars to a splice(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, then sendfile()
       changes the file offset appropriately.

RETURN VALUE         top

       If the transfer was successful, the number of bytes written to
       out_fd is returned.  Note that a successful call to sendfile()
       may write fewer bytes than requested; the caller should be
       prepared to retry the call if there were unsent bytes.  See also

       On error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       EAGAIN Nonblocking I/O has been selected using O_NONBLOCK and the
              write would block.

       EBADF  The input file was not opened for reading or the output
              file was not opened for writing.

       EFAULT Bad address.

       EINVAL Descriptor is not valid or locked, or an mmap(2)-like
              operation is not available for in_fd, or count is

       EINVAL out_fd has the O_APPEND flag set.  This is not currently
              supported by sendfile().

       EIO    Unspecified error while reading from in_fd.

       ENOMEM Insufficient memory to read from in_fd.

              count is too large, the operation would result in
              exceeding the maximum size of either the input file or the
              output file.

       ESPIPE offset is not NULL but the input file is not seekable.

VERSIONS         top

       Other UNIX systems implement sendfile() with different semantics
       and prototypes.  It should not be used in portable programs.

STANDARDS         top


HISTORY         top

       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 Linux sendfile() system call was not designed to
       handle large file offsets.  Consequently, Linux 2.4 added
       sendfile64(), with a wider type for the offset argument.  The
       glibc sendfile() wrapper function transparently deals with the
       kernel differences.

NOTES         top

       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 use sendfile() 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 the TCP_CORK option, described
       in tcp(7), to minimize the number of packets and to tune

       Applications may wish to fall back to read(2) and write(2) in the
       case where sendfile() fails with EINVAL or ENOSYS.

       If out_fd refers to a socket or pipe with zero-copy support,
       callers must ensure the transferred portions of the file referred
       to by in_fd remain unmodified until the reader on the other end
       of out_fd has consumed the transferred data.

       The Linux-specific splice(2) call supports transferring data
       between arbitrary file descriptors provided one (or both) of them
       is a pipe.

SEE ALSO         top

       copy_file_range(2), mmap(2), open(2), socket(2), splice(2)

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                    sendfile(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)