Mainframe, MVS and zOS Discussion

C doesn't have anything but errno for fopen failures, but it also doesn't have all the complex mode string extensions that exist in mainframe C.

On the mainframe, if an fopen("dd:...", modestring) fails due to a fopen mode string mismatch relative to the JCL that cataloged and/or allocated the DDNAME/DSNAME in question, is there a way to get extended error information about the precise nature of that fopen mode mismatch?

I'm not a mainframe C expert, however are you using #include <errno.h> to trap the errors. It might help.