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28 .\" @(#)read.2 8.4 (Berkeley) 2/26/94
45 .Fn read "int fd" "void *buf" "size_t nbytes"
47 .Fn pread "int fd" "void *buf" "size_t nbytes" "off_t offset"
50 .Fn readv "int fd" "const struct iovec *iov" "int iovcnt"
52 .Fn preadv "int fd" "const struct iovec *iov" "int iovcnt" "off_t offset"
59 of data from the object referenced by the descriptor
61 into the buffer pointed to by
66 performs the same action, but scatters the input data
69 buffers specified by the members of the
71 array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
77 perform the same functions, but read from the specified position in
78 the file without modifying the file pointer.
86 structure is defined as:
88 .Bd -literal -offset indent -compact
90 void *iov_base; /* Base address. */
91 size_t iov_len; /* Length. */
97 entry specifies the base address and length of an area
98 in memory where data should be placed.
102 will always fill an area completely before proceeding
105 On objects capable of seeking, the
108 given by the pointer associated with
114 the pointer is incremented by the number of bytes actually read.
116 Objects that are not capable of seeking always read from the current
118 The value of the pointer associated with such an
121 Upon successful completion,
127 return the number of bytes actually read and placed in the buffer.
128 The system guarantees to read the number of bytes requested if
129 the descriptor references a normal file that has that many bytes left
130 before the end-of-file, but in no other case.
138 syscalls are atomic with respect to each other in the effects on file
139 content, when they operate on regular files.
140 If two threads each call one of the
144 syscalls, each call will see either all of the changes of the other call,
148 kernel implements this guarantee by locking the file ranges affected by
152 number of bytes actually read is returned.
153 Upon reading end-of-file,
155 Otherwise, a -1 is returned and the global variable
157 is set to indicate the error.
172 is not a valid file or socket descriptor open for reading.
176 argument refers to a socket, and the remote socket end is
182 points outside the allocated address space.
184 An I/O error occurred while reading from the file system.
186 Corrupted data was detected while reading from the file system.
188 Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped
190 A read from a slow device
191 (i.e.\& one that might block for an arbitrary amount of time)
192 was interrupted by the delivery of a signal
193 before any data arrived.
195 The pointer associated with
199 The file was marked for non-blocking I/O,
200 and no data were ready to be read.
202 The file descriptor is associated with a directory.
203 Directories may only be read directly by root if the filesystem supports it and
205 .Dv security.bsd.allow_read_dir
206 sysctl MIB is set to a non-zero value.
207 For most scenarios, the
209 function should be used instead.
211 The file descriptor is associated with a file system and file type that
212 do not allow regular read operations on it.
214 The file descriptor is associated with a regular file,
218 is before the end-of-file, and
220 is greater than or equal to the offset maximum established
221 for this file system.
233 may return one of the following errors:
239 was less than or equal to 0, or greater than
252 array overflowed a 32-bit integer.
256 array points outside the process's allocated address space.
263 system calls may also return the following errors:
270 The file descriptor is associated with a pipe, socket, or FIFO.
275 .Xr getdirentries 2 ,
286 system call is expected to conform to
292 system calls are expected to conform to
297 system call appeared in
305 system call appeared in