read

Function

read invokes the read callable service to read a specified number of bytes from a file into a buffer that you provide. The number of bytes read is returned in RETVAL.

For more information about restrictions when running read in a conversion environment, see Pread() and pwrite() in z/OS® UNIX System Services Programming: Assembler Callable Services Reference

Parameters

fd

The file descriptor (a number) for the file to be read.

variable

The name of the buffer into which the data is to be read.

length

The maximum number of characters to read. After the read completes, the length of variable is the number of bytes read. This value is also returned in RETVAL.

Usage notes

Length:

The value of length is not checked against any system limit.

Access time:

A successful read updates the access time of the file read.

Origin of bytes read:

If the file specified by fd is a regular file, or any other type of file where a seek operation is possible, bytes are read from the file offset associated with the file descriptor. A successful read increments the file offset by the number of bytes read.

For files where no seek operation is possible, there is no file offset associated with the file descriptor. Reading begins at the current position in the file.

Number of bytes read:

When a read request completes, the RETVAL field shows the number of bytes that are read—a number less than or equal to the number specified as length. The following are some reasons why the number of bytes read might be less than the number of bytes requested:

• Fewer than the requested number of bytes remained in the file; the end of file was reached before length bytes were read.
• The service was interrupted by a signal after some but not all of the requested bytes were read. (If no bytes were read, the return value is set to -1 and an error is reported.)
• The file is a pipe, FIFO, or special file and fewer bytes than length specified were available for reading.

There are several reasons why a read request might complete successfully with no bytes read (that is, with RETVAL set to 0). For example, zero bytes are read in these cases:

• The call specified a length of zero.
• The starting position for the read was at or beyond the end of the file.
• The file that is being read is a FIFO file or a pipe, and no process has the pipe open for writing.
• The file being read is a subsidiary pseudo-terminal and a zero-length canonical file was written to the manager pseudoterminal.

Nonblocking:

If a process has a pipe open for reading with nonblocking specified, a request to read from the file ends with a return value of -1 a return code of 0, and ERRNO of EAGAIN. But if nonblocking was not specified, the read request is blocked (does not return) until some data is written or the pipe is closed by all other processes that have the pipe open for writing.

Both manager and subsidiary pseudo-terminals operate this way, too, except that how they act depends on how they were opened. If the manager or the subsidiary is opened blocking, the reads are blocked if there is no data. If it is opened nonblocking, EAGAIN is returned if there is no data.

SIGTTOU processing:

The read service causes signal SIGTTIN to be sent under the following conditions:

• The process is attempting to read from its controlling terminal, and
• The process is running in a background process group, and
• The SIGTTIN signal is not blocked or ignored, and
• The process group of the process is not orphaned.

If these conditions are met, SIGTTIN is sent. If SIGTTIN has a handler, the handler gets control and the read ends with the return code set to EINTRO. If SIGTTIN is set to default, the process stops in the read and continues when the process is moved to the foreground.