C, Fortran, and single-character strings

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By Jonathan Corbet June 20, 2019 The calling interfaces between programming languages are, by their nature, ripe for misunderstandings; different languages can have subtly different ideas of how data should be passed around. Such misunderstandings often have the effect of making things break right away; these are quickly fixed. Others can persist for years or even decades before jumping out of the shadows and making things fail. A problem of the latter variety recently turned up in how some C programs are passing strings to Fortran subroutines, with unpleasant effects on widely used packages like LAPACK.

The C language famously does not worry much about the length of strings, which simply extend until the null byte at the end. Fortran, though, likes to know the sizes of the strings it is dealing with. When strings are passed as arguments to functions or subroutines, the GCC Fortran argument-passing conventions state that the length of each string is to be appended to the list of arguments. Consider a Fortran subroutine defined something like this:

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