read bytes from a file
#include <unistd.h> #include <sys/uio.h> ssize_t read( int fildes, void *buf, size_t nbyte ); int readv( int fildes, const struct iovec *iov, int iovcnt );
The read() function attempts to read nbyte bytes from the file associated with the open file descriptor, fildes, into the buffer pointed to by buf.
If nbyte is zero, read() returns zero and has no other effect.
On a regular file or other file capable of seeking, read() starts at a position in the file given by the file offset associated with fildes. Before successfully returning from read(), the file offset is incremented by the number of bytes actually read.
The readv() function performs the same action as read(), but places the data into the iovcnt buffers specified by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1].
For readv(), the iovec structure contains the following members:
readv() always fills one buffer completely before proceeding to the next. The maximum number of entries in the iov array is UIO_MAXIOV.
The read() call ignores advisory locks that may have been set by the fcntl(), lock(), or locking() functions. |
On a file not capable of seeking, read() starts at the current position.
When read() returns successfully, its return value is the number of bytes actually read and placed in the buffer. This number will never be greater than nbyte, although it may be less than nbyte for one of the following reasons:
If read() is interrupted by a signal before it reads any data, it returns -1, and errno is set to EINTR. However, if read() is interrupted by a signal after it has successfully read some data, it will return the number of bytes read.
No data is transferred past the current end-of-file. If the starting position is at or after the end-of-file, read() returns zero. If the file is a device special file, the result of subsequent calls to read() will work, based on the then current state of the device (that is, the end of file is transitory).
If the value of nbyte is greater than INT_MAX, read() returns -1, and sets errno to EINVAL. See <limits.h>.
When attempting to read from an empty pipe or FIFO:
When attempting to read from a file (other than a pipe or FIFO) that support nonblocking reads and has no data currently available:
If read() is called on a portion of a file prior to the end-of-file that hasn't been written, it returns bytes with the value zero.
If read() succeeds, the st_atime field of the file is marked for update.
When read() is called and the O_CACHE flag (a QNX extension) is set, internal file system caching behavior may be modified. See the open() documentation for a discussion of the effect of O_CACHE on I/O calls.
The number of bytes actually read. If not successful, -1 is returned, errno is set to indicate the error, and the content of the buffer pointed to by buf is indeterminate.
If any of the following conditions occurs, the read() function returns -1 and sets errno to the corresponding value:
#include <sys/types.h> #include <fcntl.h> #include <unistd.h> void main() { int fd; int size_read; char buffer[80]; /* Open a file for input */ fd = open( "myfile.dat", O_RDONLY ); /* Read the text */ size_read = read( fd, buffer, sizeof( buffer ) ); /* Test for error */ if( size_read == -1 ) { perror( "Error reading myfile.dat" ); } /* Close the file */ close( fd ); }
POSIX 1003.1
Safety: | |
---|---|
Interrupt handler | No |
Signal handler | Yes, but modifies errno |
Thread | Yes |
block_read(), close(), creat(), dup(), dup2(), errno, fcntl(), lseek(), open(), pipe(), select(), write(), writev()