pSOSystem System Calls - Read

fopen pREPC+ System Calls
Return Value
ab Append: open or create a binary file for writing at
the end-of-file.
r+ Open text file for updating, reading and writing at
the current file position.
w+ Truncate to zero length or create text file for
updating, reading and writing at the current file
position.
a+ Append: open or create text file for updating,
reading at the current file position and writing at
the end-of-file.
r+b or rb+ Open binary file for updating, reading and writing
at the current file position.
w+b or wb+ Truncate to zero length or create binary file for
updating, reading and writing at the current file
position.
a+b or ab+ Append: open or create a binary file for updating,
reading at current file position and writing at the
end-of-file.
Opening a disk file with read mode (r as the first character in mode
argument) fails if the file does not exist or cannot be read.
Opening a disk file with append mode (a as the first character in
mode argument) causes all subsequent writes to the then current
EOF, regardless of intervening calls to the fseek function.
When a disk file is opened with update mode (+ as the second or
third character in mode argument) both read and write operations
may be performed on the associated stream. However, output may
not be directly followed by input, or vice-versa, without an
interviewing call to the fflush function or to a file positioning
function, via fseek, fsetpos and rewind. The only exception to
the above rule is a read operation that encounters end-of-file.
If the open operation is successful, this function returns a pointer to the FILE
object that must be used in subsequent calls to specify this opened stream. If the
operation fails, a null pointer is returned and errno is set.
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