Manpage of SENDFILE
SENDFILESection: Linux Programmer's Manual (2)
NAMEsendfile - transfer data between file descriptors
DESCRIPTIONsendfile() 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_fdshould be a file descriptor opened for reading and out_fdshould be a descriptor opened for writing.
If offsetis 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 offsetis 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 offsetis NULL, then data will be read from in_fdstarting at the file offset, and the file offset will be updated by the call.
countis the number of bytes to copy between the file descriptors.
The in_fdargument must correspond to a file which supports mmap(2)-like operations (i.e., it cannot be a socket).
RETURN VALUEIf the transfer was successful, the number of bytes written to out_fdis 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.
- Nonblocking I/O has been selected using O_NONBLOCKand the write would block.
- The input file was not opened for reading or the output file was not opened for writing.
- Bad address.
- Descriptor is not valid or locked, or an mmap(2)-like operation is not available for in_fd, or countis negative.
- out_fdhas the O_APPENDflag set. This is not currently supported by sendfile().
- Unspecified error while reading from in_fd.
- Insufficient memory to read from in_fd.
- countis too large, the operation would result in exceeding the maximum size of either the input file or the output file.
- offsetis not NULL but the input file is not seek(2)-able.
VERSIONSsendfile() first appeared in Linux 2.2. The include file <sys/sendfile.h>is present since glibc 2.1.
CONFORMING TONot specified in POSIX.1-2001, nor in other standards.
NOTESsendfile() 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 tcp(7), to minimize the number of packets and to tune performance., you will find it useful to employ the TCP_CORKoption, described in
In Linux 2.4 and earlier, out_fdcould 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 offsetargument. The glibc sendfile() wrapper function transparently deals with the kernel differences.
If out_fdrefers to a socket or pipe with zero-copy support, callers must ensure the transferred portions of the file referred to by in_fdremain unmodified until the reader on the other end of out_fdhas 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 ALSOcopy_file_range(2), mmap(2), open(2), socket(2), splice(2)
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