What Causes a Segmentation Fault in Fortran Programs?

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  • Thread starter Thread starter pandroza
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Discussion Overview

The discussion revolves around the causes of segmentation faults in Fortran programs, particularly focusing on a code snippet provided by a user. Participants explore potential issues related to memory management, syntax errors, and debugging techniques within the context of Fortran 90 programming.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that a syntax error in the read statement (specifically a misplaced comma) might be contributing to the segmentation fault.
  • Another participant proposes that the segmentation fault could be a classical memory problem that may require increasing memory limits before running the executable.
  • A different participant inquires about the exact point of failure in the program, sharing their own experience with a typo in function declarations leading to a similar error.
  • One participant recommends using STOP statements to isolate the location of the error, suggesting a step-by-step approach to identify the problematic code segment.
  • Another participant notes that segmentation faults often occur when accessing memory outside of array bounds, particularly referencing attempts to access the 0 element.
  • It is mentioned that compiling the code with a debug flag could help identify where the fault occurs.

Areas of Agreement / Disagreement

Participants express various hypotheses about the causes of the segmentation fault, with no consensus reached on a single definitive cause. Multiple competing views and suggestions remain present throughout the discussion.

Contextual Notes

Participants highlight potential limitations in the provided code, such as the need for proper memory management and the importance of debugging techniques. Specific assumptions about the environment and compiler settings are also noted but remain unresolved.

pandroza
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Hi, I am new in fortran.
I have this code in fortran 90, I get segmentation fault when I run my program, can you help me please, I am desperate!

SUBROUTINE COUNTX(NX,ncov)
IMPLICIT NONE
INTEGER, INTENT(IN)::ncov
INTEGER,DIMENSION(ncov),INTENT(INOUT)::NX(:)
INTEGER:: AUX,i

DO i=1,ncov
print *,NX(i)
END DO






END SUBROUTINE



PROGRAM minSC
IMPLICIT NONE
INTEGER::ncov,i
INTEGER,ALLOCATABLE::NX(:)

PRINT "(A)","Give me the value ncov "
read(*,*),ncov
allocate(NX(ncov))

DO i=1,Ncov
NX(i)=0

END DO
CALL COUNTX(NX,ncov)


END PROGRAM
 
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pandroza said:
Hi, I am new in fortran.
I have this code in fortran 90, I get segmentation fault when I run my program, can you help me please, I am desperate!

SUBROUTINE COUNTX(NX,ncov)
IMPLICIT NONE
INTEGER, INTENT(IN)::ncov
INTEGER,DIMENSION(ncov),INTENT(INOUT)::NX(:)
INTEGER:: AUX,i

DO i=1,ncov
print *,NX(i)
END DO






END SUBROUTINE



PROGRAM minSC
IMPLICIT NONE
INTEGER::ncov,i
INTEGER,ALLOCATABLE::NX(:)

PRINT "(A)","Give me the value ncov "
read(*,*),ncov
allocate(NX(ncov))

DO i=1,Ncov
NX(i)=0

END DO
CALL COUNTX(NX,ncov)


END PROGRAM

I don't believe you should have that second comma in your read statement. IOW, I believe you should have this:
Code: 
     read(*,*) ncov

There might be other things causing your segmentation fault, but my knowledge of Fortran is from F77.
 
I think you have compiled the code. If the problem occurs when you run executable file then it is classical memory problem. So may need to increase memory limits before running the exe-file. there are several type of commands ex:

limit stacksize unlimited
limit memoryuse unlimited
limit vmemoryuse unlimited
ulimit -s 8000000

may be more.

Just try!
 
Where does the segmentation fault occur exactly? Does your program print anything first, or does it happen right at the beginning?

I actually got this same error yesterday and stumbled upon this thread while trying to solve it. In my case, it turns out that I had a simple typo when I was declaring external functions. So when I ran my program, it attempted to access a function that had not been declared, which means that it was trying to call a function with a memory address of 0, which belongs to the kernel. The program isn't allowed to access this memory location, hence the error.
 
I can't see what it wrong here, but the way to solve these problems is isolate where the error is happening.

If you don't have a development environment with a debugger, insert STOP statements to find out where the problem is.

First try

Code: 
...
PRINT "(A)","Give me the value ncov "
STOP
...

Obviously that won't do anything useful, but it should run without crashing!

Then do
Code: 
...
read(*,*),ncov
print *, ncov
STOP
...

Note, also print out ncov so you know the program has read what you expected it to read.

Then do
Code: 
...
allocate(NX(ncov))
STOP
...

etc.

This might seem tedious, but it doesn't take long to do. When you know exactly which statement is causing the crash, it is a lot easier to figure out what is wrong.
 
just to add to previous replies seg faults are usually caused when adressing a location outside array bounds (many times you try to reach the 0 element)

also if you compile your code using the debug flag -g it will tell you where the fault occures.
 

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