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Using and Porting GNU Fortran

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15.1.8 Strange Behavior at Run Time

g77 code might fail at runtime with "segmentation violation", "bus error", or even something as subtle as a procedure call overwriting a variable or array element that it is not supposed to touch.

These can be symptoms of a wide variety of actual bugs that occurred earlier during the program's run, but manifested themselves as visible problems some time later.

Overflowing the bounds of an array--usually by writing beyond the end of it--is one of two kinds of bug that often occurs in Fortran code. (Compile your code with the `-fbounds-check' option to catch many of these kinds of errors at program run time.)

The other kind of bug is a mismatch between the actual arguments passed to a procedure and the dummy arguments as declared by that procedure.

Both of these kinds of bugs, and some others as well, can be difficult to track down, because the bug can change its behavior, or even appear to not occur, when using a debugger.

That is, these bugs can be quite sensitive to data, including data representing the placement of other data in memory (that is, pointers, such as the placement of stack frames in memory).

g77 now offers the ability to catch and report some of these problems at compile, link, or run time, such as by generating code to detect references to beyond the bounds of most arrays (except assumed-size arrays), and checking for agreement between calling and called procedures. Future improvements are likely to be made in the procedure-mismatch area, at least.

In the meantime, finding and fixing the programming bugs that lead to these behaviors is, ultimately, the user's responsibility, as difficult as that task can sometimes be.

One runtime problem that has been observed might have a simple solution. If a formatted WRITE produces an endless stream of spaces, check that your program is linked against the correct version of the C library. The configuration process takes care to account for your system's normal `libc' not being ANSI-standard, which will otherwise cause this behaviour. If your system's default library is ANSI-standard and you subsequently link against a non-ANSI one, there might be problems such as this one.

Specifically, on Solaris2 systems, avoid picking up the BSD library from `/usr/ucblib'.

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