HP Fortran Programmer's Guide (March 2010)

Calling C routines from HP Fortran
C strings
Chapter 8 203
C strings
C strings differ from Fortran character variables in two important respects:
• C expects strings to be null-terminated.
• For each character variable or character constant that Fortran passes to a C routine, it
also passes a hidden length argument.
The following sections discuss these differences and explain how to code for them. The last
section includes an example program.
C null-terminated string
Unlike HP Fortran programs written in C expect strings to be null-terminated; that is, the
last character of a string must be the null character ('\0'). To pass a string from Fortran to C,
you must do the following:
• Declare the character variable that is large enough to include the null character.
• Explicitly assign the null character to the final element of the character array or use the
concatenation operator, as in the following example:
CALL csub ('a string'//CHAR(0))
If the Fortran program is going to use a string that has been passed back to it from C, then
either the C function or the Fortran subprogram should strip off the null character before
Fortran tries to use it. The example program in “Passing a string” on page 204 shows how to
do this in C.
Fortran hidden length argument
For each CHARACTER*n argument passed to a Fortran subprogram, two items are actually
passed as arguments:
• The address of the character argument in memory (that is, a pointer to the argument).
• The argument's length in bytes. This is the “hidden” length argument that is available to
the subprogram from the stack.
To pass a string argument from Fortran to C, you must explicitly prepare the C function to
receive the string address argument and the hidden argument. The order of the address
arguments in the argument list will be the same in C as in Fortran. The hidden length