|READ(2)||Linux Programmer's Manual||READ(2)|
NAME¶read - read from a file descriptor
#include <unistd.h>ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION¶read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
RETURN VALUE¶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.
- 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.
- EAGAIN or EWOULDBLOCK
- 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.
- fd is not a valid file descriptor or is not open for reading.
- buf is outside your accessible address space.
- The call was interrupted by a signal before any data was read; see signal(7).
- 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.
- 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.
- 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.
- fd refers to a directory.
CONFORMING TO¶SVr4, 4.3BSD, POSIX.1-2001.
NOTES¶The types size_t and ssize_t are, respectively, unsigned and signed integer data types specified by POSIX.1.
BUGS¶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: ...
SEE ALSO¶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¶This page is part of release 4.10 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 https://www.kernel.org/doc/man-pages/.