NAME

       sendfile - transfer data between file descriptors

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/sendfile.h>

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

DESCRIPTION

       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 descriptors.

       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

       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 NOTES.

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

ERRORS

       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 negative.

       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.

       EOVERFLOW
              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

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

STANDARDS

       None.

HISTORY

       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

       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 performance.

       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

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