Debugging Segmentation Faults

If a program gets a segmentation fault (SIGSEGV), it references a memory address outside of the memory available to it.

The most frequent causes for a segmentation fault are:

• An array index is outside the declared range.

• The name of an array index is misspelled.

• The calling routine has a REAL argument, which the called routine has as INTEGER.

• An array index is miscalculated.

• The calling routine has fewer arguments than required.

• A pointer is used before it has been defined.

Using dbx to Locate Problems

Use dbx to find the source code line where a segmentation fault has occurred.

Use a program to generate a segmentation fault:

demo% cat WhereSEGV.f
    INTEGER a(5)
    j = 2000000
    DO 9 i = 1,5
        a(j) = (i * 10)
9    CONTINUE
    PRINT *, a
    END
demo%

Use dbx to find the line number of a dbx segmentation fault:

demo% f95 -g -silent WhereSEGV.f
demo% a.out
Segmentation fault
demo% dbx a.out
Reading symbolic information for a.out
program terminated by signal SEGV (segmentation violation)
(dbx) run
Running: a.out
signal SEGV (no mapping at the fault address)
    in MAIN at line 4 in file "WhereSEGV.f"
    4                   a(j) = (i * 10)
(dbx)