read(2) — Linux manual page


READ(2)                   Linux Programmer's Manual                  READ(2)

NAME         top

       read - read from a file descriptor

SYNOPSIS         top

       #include <unistd.h>

       ssize_t read(int fd, void *buf, size_t count);

DESCRIPTION         top

       read() attempts to read up to count bytes from file descriptor fd
       into the buffer starting at buf.

       On files that support seeking, the read operation commences at the
       file offset, and the file offset is incremented by the number of
       bytes read.  If the file offset is at or past the end of file, no
       bytes are read, and read() returns zero.

       If count is zero, read() may detect the errors described below.  In
       the absence of any errors, or if read() does not check for errors, a
       read() with a count of 0 returns zero and has no other effects.

       According to POSIX.1, if count is greater than SSIZE_MAX, the result
       is implementation-defined; see NOTES for the upper limit on Linux.

RETURN VALUE         top

       On success, the number of bytes read is returned (zero indicates end
       of file), and the file position is advanced by this number.  It is
       not an error if this number is smaller than the number of bytes
       requested; this may happen for example because fewer bytes are
       actually available right now (maybe because we were close to end-of-
       file, or because we are reading from a pipe, or from a terminal), or
       because read() was interrupted by a signal.  See also NOTES.

       On error, -1 is returned, and errno is set appropriately.  In this
       case, it is left unspecified whether the file position (if any)

ERRORS         top

       EAGAIN The file descriptor fd refers to a file other than a socket
              and has been marked nonblocking (O_NONBLOCK), and the read
              would block.  See open(2) for further details on the
              O_NONBLOCK flag.

              The file descriptor fd refers to a socket and has been marked
              nonblocking (O_NONBLOCK), and the read would block.
              POSIX.1-2001 allows either error to be returned for this case,
              and does not require these constants to have the same value,
              so a portable application should check for both possibilities.

       EBADF  fd is not a valid file descriptor or is not open for reading.

       EFAULT buf is outside your accessible address space.

       EINTR  The call was interrupted by a signal before any data was read;
              see signal(7).

       EINVAL fd is attached to an object which is unsuitable for reading;
              or the file was opened with the O_DIRECT flag, and either the
              address specified in buf, the value specified in count, or the
              file offset is not suitably aligned.

       EINVAL fd was created via a call to timerfd_create(2) and the wrong
              size buffer was given to read(); see timerfd_create(2) for
              further information.

       EIO    I/O error.  This will happen for example when the process is
              in a background process group, tries to read from its
              controlling terminal, and either it is ignoring or blocking
              SIGTTIN or its process group is orphaned.  It may also occur
              when there is a low-level I/O error while reading from a disk
              or tape.  A further possible cause of EIO on networked
              filesystems is when an advisory lock had been taken out on the
              file descriptor and this lock has been lost.  See the Lost
              locks section of fcntl(2) for further details.

       EISDIR fd refers to a directory.

       Other errors may occur, depending on the object connected to fd.

CONFORMING TO         top

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES         top

       The types size_t and ssize_t are, respectively, unsigned and signed
       integer data types specified by POSIX.1.

       On Linux, read() (and similar system calls) 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

       On NFS filesystems, reading small amounts of data will update the
       timestamp only the first time, subsequent calls may not do so.  This
       is caused by client side attribute caching, because most if not all
       NFS clients leave st_atime (last file access time) updates to the
       server, and client side reads satisfied from the client's cache will
       not cause st_atime updates on the server as there are no server-side
       reads.  UNIX semantics can be obtained by disabling client-side
       attribute caching, but in most situations this will substantially
       increase server load and decrease performance.

BUGS         top

       According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread
       Interactions with Regular File Operations"):

           All of the following functions shall be atomic with respect to
           each other in the effects specified in POSIX.1-2008 when they
           operate on regular files or symbolic links: ...

       Among the APIs subsequently listed are read() and readv(2).  And
       among the effects that should be atomic across threads (and
       processes) are updates of the file offset.  However, on Linux before
       version 3.14, this was not the case: if two processes that share an
       open file description (see open(2)) perform a read() (or readv(2)) at
       the same time, then the I/O operations were not atomic with respect
       updating the file offset, with the result that the reads in the two
       processes might (incorrectly) overlap in the blocks of data that they
       obtained.  This problem was fixed in Linux 3.14.

SEE ALSO         top

       close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2),
       readdir(2), readlink(2), readv(2), select(2), write(2), fread(3)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2018-02-02                          READ(2)

Pages that refer to this page: egrep(1)fgrep(1)grep(1)pv(1)strace(1)telnet-probe(1)close(2)creat(2)epoll_ctl(2)eventfd2(2)eventfd(2)fanotify_init(2)fcntl(2)fcntl64(2)getpgid(2)getpgrp(2)getrandom(2)inotify_add_watch(2)ioctl_tty(2)_newselect(2)open(2)openat(2)perf_event_open(2)perfmonctl(2)pidfd_open(2)pipe2(2)pipe(2)prctl(2)pread(2)pread64(2)preadv2(2)preadv(2)pselect(2)pselect6(2)ptrace(2)pwrite(2)pwrite64(2)pwritev2(2)pwritev(2)readahead(2)readv(2)recv(2)recvfrom(2)recvmsg(2)seccomp(2)select(2)select_tut(2)sendfile(2)sendfile64(2)setpgid(2)setpgrp(2)signalfd(2)signalfd4(2)socket(2)socketpair(2)syscalls(2)timerfd_create(2)timerfd_gettime(2)timerfd_settime(2)userfaultfd(2)write(2)writev(2)aiocb(3)aio_error(3)aio_read(3)aio_return(3)cfgetispeed(3)cfgetospeed(3)cfmakeraw(3)cfsetispeed(3)cfsetospeed(3)cfsetspeed(3)clockid_t(3)clock_t(3)curs_getch(3x)db(3)dbopen(3)dev_t(3)div_t(3)double_t(3)eventfd_read(3)eventfd_write(3)fd_clr(3)FD_CLR(3)fd_isset(3)FD_ISSET(3)fdopen(3)fd_set(3)FD_SET(3)fd_zero(3)FD_ZERO(3)fenv_t(3)fexcept_t(3)fgetc(3)fgets(3)file(3)FILE(3)float_t(3)fopen(3)fread(3)freopen(3)fwrite(3)getc(3)getch(3x)getchar(3)getdelim(3)getline(3)gets(3)gid_t(3)has_key(3x)id_t(3)imaxdiv_t(3)int16_t(3)int32_t(3)int64_t(3)int8_t(3)intmax_t(3)intn_t(3)intN_t(3)intptr_t(3)lconv(3)ldiv_t(3)libexpect(3)lldiv_t(3)mkfifo(3)mkfifoat(3)mpool(3)mvgetch(3x)mvwgetch(3x)off_t(3)pid_t(3)ptrdiff_t(3)readdir(3)regex_t(3)regmatch_t(3)regoff_t(3)rtime(3)siginfo_t(3)sigset_t(3)sigval(3)size_t(3)ssize_t(3)stderr(3)stdin(3)stdio(3)stdout(3)suseconds_t(3)tcdrain(3)tcflow(3)tcflush(3)tcgetattr(3)tcsendbreak(3)tcsetattr(3)termios(3)timer_t(3)timespec(3)time_t(3)timeval(3)uid_t(3)uint16_t(3)uint32_t(3)uint64_t(3)uint8_t(3)uintmax_t(3)uintn_t(3)uintN_t(3)uintptr_t(3)ungetc(3)ungetch(3x)va_list(3)void(3)wgetch(3x)xdr(3)xdr_array(3)xdr_bool(3)xdr_bytes(3)xdr_char(3)xdr_destroy(3)xdr_double(3)xdr_enum(3)xdr_float(3)xdr_free(3)xdr_getpos(3)xdr_inline(3)xdr_int(3)xdr_long(3)xdrmem_create(3)xdr_opaque(3)xdr_pointer(3)xdrrec_create(3)xdrrec_endofrecord(3)xdrrec_eof(3)xdrrec_skiprecord(3)xdr_reference(3)xdr_setpos(3)xdr_short(3)xdrstdio_create(3)xdr_string(3)xdr_u_char(3)xdr_u_int(3)xdr_u_long(3)xdr_union(3)xdr_u_short(3)xdr_vector(3)xdr_void(3)xdr_wrapstring(3)xfsctl(3)dsp56k(4)fuse(4)lirc(4)null(4)random(4)rtc(4)st(4)tty_ioctl(4)urandom(4)zero(4)proc(5)procfs(5)systemd.exec(5)aio(7)cpuset(7)epoll(7)fanotify(7)inode(7)inotify(7)pipe(7)signal(7)signal-safety(7)socket(7)spufs(7)system_data_types(7)vsock(7)x25(7)fuse(8)mount.fuse3(8)netsniff-ng(8)xfs_io(8)