XL Fortran Enterprise Edition for AIX : User's Guide - IBM

-qinitauto Option
Syntax
-qinitauto[=hex_value] | -qnoinitauto
Initializes each byte or word (4 bytes) of storage for automatic variables to a
specific value, depending on the length of the hex_value. This helps you to locate
variables that are referenced before being defined. For example, by using both the
-qinitauto option to initialize REAL variables with a NaNS value and the -qflttrap
option, it is possible to identify references to uninitialized REAL variables at run
time. Prior to XL Fortran Version 5.1.1, you could only use this option to initialize
each byte of storage.
Setting hex_value to zero ensures that all automatic variables are cleared before
being used. Some programs assume that variables are initialized to zero and do not
work when they are not. Other programs may work if they are not optimized but
fail when they are optimized. Typically, setting all the variables to all zero bytes
prevents such run-time errors. It is better to locate the variables that require
zeroing and insert code in your program to do so than to rely on this option to do
it for you. Using this option will generally zero more things than necessary and
may result in slower programs.
To locate and fix these errors, set the bytes to a value other than zero, which will
consistently reproduce incorrect results. This method is especially valuable in cases
where adding debugging statements or loading the program into a symbolic
debugger makes the error go away.
Setting hex_value to FF (255) gives REAL and COMPLEX variables an initial value
of “negative not a number”, or -NaNQ. Any operations on these variables will also
result in NaNQ values, making it clear that an uninitialized variable has been used
in a calculation.
This option can help you to debug programs with uninitialized variables in
subprograms; for example, you can use it to initialize REAL variables with a NaNS
value. You can initialize 8-byte REAL variables to double-precision NaNS values
by specifying an 8-digit hexadecimal number, that, when repeated, has a
double-precision NaNS value. For example, you could specify a number such as
7FBFFFFF, that, when stored in a REAL(4) variable, has a single-precision NaNS
value. The value 7FF7FFFF, when stored in a REAL(4) variable, has a
single-precision NaNQ value. If the same number is stored twice in a REAL(8)
variable (7FF7FFFF7FF7FFFF), it has a double-precision NaNS value.
Arguments
v The hex_value is a 1-digit to 8-digit hexadecimal (0-F) number.
v
v
To initialize each byte of storage to a specific value, specify 1 or 2 digits for the
hex_value. If you specify only 1 digit, the compiler pads the hex_value on the left
with a zero.
To initialize each word of storage to a specific value, specify 3 to 8 digits for the
hex_value. If you specify more than 2 but fewer than 8 digits, the compiler pads
the hex_value on the left with zeros.
v In the case of word initialization, if automatic variables are not a multiple of 4
bytes in length, the hex_value may be truncated on the left to fit. For example, if
you specify 5 digits for the hex_value and an automatic variable is only 1 byte
long, the compiler truncates the 3 digits on the left-hand side of the hex_value
and assigns the two right-hand digits to the variable.
XL Fortran Compiler-Option Reference 177