segmentation fault 11

Question

I m having a problem with some program  I have searched about segmentation faults  by I don t understand them quite well  the only thing I know is that presumably I am trying to access some memory I shouldn t  The problem is that I see my code and don t understand what I am doing wrong     include lt stdio h gt   include lt math h gt   include lt stdlib h gt    define   lambda   2 0  define   g        1 0  define   Lx       100  define   F0       1 0  define   Tf       10  define   h       0 1  define   e       0 00001  FILE    file   double F 1000  1000000    void Inicio double D 1000  1000000     int i  for  i 399  i lt 600  i          D i  0  F0       void Iteration  double A 1000  1000000     long int i k  for  i 1  i lt 1000000  i          A 0  i   A 0  i-1    e  h h h h  g g  A 2  i-1  - 4 0 A 1  i-1    6 0 A 0  i-1 -4 0 A 998  i-1    A 997  i-1     2 0 g e  h h   A 1  i-1  - 2 A 0  i-1    A 998  i-1     e A 0  i-1   lambda-A 0  i-1  A 0  i-1        A 1  i   A 1  i-1    e  h h h h  g g  A 3  i-1  - 4 0 A 2  i-1    6 0 A 1  i-1 -4 0 A 0  i-1    A 998  i-1     2 0 g e  h h   A 2  i-1  - 2 A 1  i-1    A 0  i-1     e A 1  i-1   lambda-A 1  i-1  A 1  i-1        for  k 2  k lt 997  k              A k  i   A k  i-1    e  h h h h  g g  A k 2  i-1  - 4 0 A k 1  i-1    6 0 A k  i-1 -4 0 A k-1  i-1    A k-2  i-1     2 0 g e  h h   A k 1  i-1  - 2 A k  i-1    A k-1  i-1     e A k  i-1   lambda-A k  i-1  A k  i-1              A 997  i    A 997  i-1    e  h h h h  g g  A 0  i-1  - 4 A 998  i-1    6 A 997  i-1  - 4 A 996  i-1    A 995  i-1     2 0 g e  h h   A 998  i-1  - 2 A 997  i-1    A 996  i-1     e A 997  i-1   lambda-A 997  i-1  A 997  i-1        A 998  i    A 998  i-1    e  h h h h  g g  A 1  i-1  - 4 A 0  i-1    6 A 998  i-1  - 4 A 997  i-1    A 996  i-1     2 0 g e  h h   A 0  i-1  - 2 A 998  i-1    A 997  i-1     e A 998  i-1   lambda-A 998  i-1  A 998  i-1        A 999  i  A 0  i        main     long int i j  Inicio F   Iteration F   file   fopen  P1 txt   wt    for  i 0  i lt 1000000  i          for  j 0  j lt 1000  j              fprintf file   lf  t   4f  t  lf n   1 0 j 10 0  1 0 i  F j  i            fclose file       Thanks for your time

User · Accepted Answer

This declaration   double F 1000  1000000     would occupy 8   1000   1000000 bytes on a typical x86 system  This is about 7 45 GB  Chances are your system is running out of memory when trying to execute your code  which results in a segmentation fault

User · Answer

What system are you running on  Do you have access to some sort of debugger  gdb  visual studio s debugger  etc     That would give us valuable information  like the line of code where the program crashes    Also  the amount of memory may be prohibitive   Additionally  may I recommend that you replace the numeric limits by named definitions   As such    define DIM1 SZ 1000  define DIM2 SZ 1000000   Use those whenever you wish to refer to the array dimension limits  It will help avoid typing errors

User · Answer

Your array is occupying roughly 8 GB of memory  1 000 x 1 000 000 x sizeof double  bytes    That might be a factor in your problem   It is a global variable rather than a stack variable  so you may be OK  but you re pushing limits here   Writing that much data to a file is going to take a while   You don t check that the file was opened successfully  which could be a source of trouble  too  if it did fail  a segmentation fault is very likely    You really should introduce some named constants for 1 000 and 1 000 000  what do they represent   You should also write a function to do the calculation  you could use an inline function in C99 or later  or C      The repetition in the code is excruciating to behold   You should also use C99 notation for main    with the explicit return type  and preferably void for the argument list when you are not using argc or argv    int main void      Out of idle curiosity  I took a copy of your code  changed all occurrences of 1000 to ROWS  all occurrences of 1000000 to COLS  and then created enum   ROWS   1000  COLS   10000     thereby reducing the problem size by a factor of 100    I made a few minor changes so it would compile cleanly under my preferred set of compilation options  nothing serious  static in front of the functions  and the main array  file becomes a local to main  error check the fopen    etc     I then created a second copy and created an inline function to do the repeated calculation   and a second one to do subscript calculations    This means that the monstrous expression is only written out once     which is highly desirable as it ensure consistency    include  lt stdio h gt    define   lambda   2 0  define   g        1 0  define   F0       1 0  define   h        0 1  define   e        0 00001  enum   ROWS   1000  COLS   10000     static double F ROWS  COLS    static void Inicio double D ROWS  COLS         for  int i   399  i  lt  600  i       Magic numbers           D i  0    F0     enum   R   ROWS - 1     static inline int ko int k  int n        int rv   k   n      if  rv  gt   R          rv -  R      else if  rv  lt  0          rv    R      return rv      static inline void calculate value int i  int k  double A ROWS  COLS         int ks2   ko k  -2       int ks1   ko k  -1       int kp1   ko k   1       int kp2   ko k   2        A k  i    A k  i-1                e  h h h h    g g    A kp2  i-1  - 4 0 A kp1  i-1    6 0 A k  i-1  - 4 0 A ks1  i-1    A ks2  i-1                 2 0 g e  h h     A kp1  i-1  - 2 A k  i-1    A ks1  i-1                 e   A k  i-1     lambda - A k  i-1    A k  i-1       static void Iteration double A ROWS  COLS         for  int i   1  i  lt  COLS  i                  for  int k   0  k  lt  R  k                calculate value i  k  A           A 999  i    A 0  i            int main void        FILE  file   fopen  P2 txt   wt        if  file    0          return 1       Inicio F       Iteration F       for  int i   0  i  lt  COLS  i                  for  int j   0  j  lt  ROWS  j                          fprintf file   lf  t   4f  t  lf n   1 0 j 10 0  1 0 i  F j  i                        fclose file       return 0       This program writes to P2 txt instead of P1 txt   I ran both programs and compared the output files  the output was identical   When I ran the programs on a mostly idle machine  MacBook Pro  2 3 GHz Intel Core i7  16 GiB 1333 MHz RAM  Mac OS X 10 7 5  GCC 4 7 1   I got reasonably but not wholly consistent timing   Original   Modified 6 334s      6 367s 6 241s      6 231s 6 315s     10 778s 6 378s      6 320s 6 388s      6 293s 6 285s      6 268s 6 387s     10 954s 6 377s      6 227s 8 888s      6 347s 6 304s      6 286s 6 258s     10 302s 6 975s      6 260s 6 663s      6 847s 6 359s      6 313s 6 344s      6 335s 7 762s      6 533s 6 310s      9 418s 8 972s      6 370s 6 383s      6 357s   However  almost all that time is spent on disk I O   I reduced the disk I O to just the very last row of data  so the outer I O for loop became   for  int i   COLS - 1  i  lt  COLS  i      the timings were vastly reduced and very much more consistent   Original    Modified 0 168s      0 165s 0 145s      0 165s 0 165s      0 166s 0 164s      0 163s 0 151s      0 151s 0 148s      0 153s 0 152s      0 171s 0 165s      0 165s 0 173s      0 176s 0 171s      0 165s 0 151s      0 169s   The simplification in the code from having the ghastly expression written out just once is very beneficial  it seems to me   I d certainly far rather have to maintain that program than the original

User · Answer

Run your program with valgrind of linked to efence  That will tell you where the pointer is being dereferenced and most likely fix your problem if you fix all the errors they tell you about

[c] segmentation fault : 11

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