RESOLVING SEGMENTATION FAULT (“CORE DUMPED”) IN UBUNTU

     

Overview

A segmentation fault (aka segfault) is a common condition that causes programs to crash; they are often associated with a tệp tin named core. Segfaults are caused by a program trying khổng lồ read or write an illegal memory location.

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Program memory is divided into different segments: a text segment for program instructions, a data segment for variables và arrays defined at compile time, a stack segment for temporary (or automatic) variables defined in subroutines and functions, và a heap segment for variables allocated during runtime by functions, such as malloc (in C) và allocate (in Fortran). For more, see About program segments.

A segfault occurs when a reference to lớn a variable falls outside the segment where that variable resides, or when a write is attempted lớn a location that is in a read-only segment. In practice, segfaults are almost always due to lớn trying to read or write a non-existent array element, not properly defining a pointer before using it, or (in C programs) accidentally using a variable"s value as an address (see the scanf example below).

Examples of common segfaults

For example, calling memset() as shown below would cause a program lớn segfault:

memset((char *)0x0, 1, 100);The following three cases illustrate the most common types of array-related segfaults:

Case ACase BCase C
/* "Array out of bounds" error valid indices for array foo are 0, 1, ... 999 */ int foo<1000>; for (int i = 0; i
/* Illegal memory access if value of n is not in the range 0, 1, ... 999 */ int n; int foo<1000>; for (int i = 0; i
/* Illegal memory access because no memory is allocated for foo2 */ float *foo, *foo2; foo = (float*)malloc(1000); foo2<0> = 1.0;

In case A, array foo is defined for index = 0,1, 2, ... 999. However, in the last iteration of the for loop, the program tries khổng lồ access foo<1000>. This will result in a segfault if that memory location lies outside the memory segment where foo resides. Even if it doesn"t cause a segfault, it is still a bug. In case B, integer n could be any random value. As in case A, if it is not in the range 0, 1, ... 999, it might cause a segfault. Whether it does or not, it is certainly a bug. In case C, allocation of memory for variable foo2 has been overlooked, so foo2 will point khổng lồ a random location in memory. Accessing foo2<0> will likely result in a segfault. Another common programming error that leads lớn segfaults is oversight in the use of pointers. For example, the C function scanf() expects the address of a variable as its second parameter; therefore, the following will likely cause the program to lớn crash with a segfault:int foo = 0; scanf("%d", foo); /* lưu ý missing & sign ; correct usage would have been &foo */The variable foo might be defined at memory location 1000, but the above function hotline would try to lớn read integer data into memory location 0 according to the definition of foo.

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A segfault will occur when a program attempts to operate on a memory location in a way that is not allowed (for example, attempts lớn write a read-only location would result in a segfault).Segfaults can also occur when your program runs out of stack space. This may not be a bug in your program, but may be due instead to your shell setting the stack kích cỡ limit too small.

Find out-of-bounds array references

Most Fortran compilers have an option that will insert code to vì chưng bounds checking on all array references during runtime. If an access falls outside the index range defined for an array, the program will halt & tell you where this occurs. For most Fortran compilers, the option is -C, or -check followed by a keyword. See your compiler"s user guide khổng lồ get the exact option. Use bounds checking only when debugging, since it will slow down your program. Some C compilers also have a bounds checking option.

Check shell limits

As noted in the last example above, some segfault problems are not due lớn bugs in your program, but are caused instead by system memory limits being phối too low. Usually it is the limit on stack form size that causes this kind of problem. To check memory limits, use the ulimit command in bash or ksh, or the limit command in csh or tcsh. Try setting the stacksize higher, & then re-run your program khổng lồ see if the segfault goes away.

Use debuggers to lớn diagnose segfaults

If you can"t find the problem any other way, you might try a debugger. For example, you could use GNU"s well-known debugger GDB khổng lồ view the backtrace of a bộ vi xử lý core file dumped by your program; whenever programs segfault, they usually dump the nội dung of (their section of the) memory at the time of the crash into a core file. Start your debugger with the command gdb core, & then use the backtrace command khổng lồ see where the program was when it crashed. This simple trick will allow you khổng lồ focus on that part of the code.

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If using backtrace on the coreg tệp tin doesn"t find the problem, you might have to lớn run the program under debugger control, & then step through the code one function, or one source code line, at a time. To bởi vì this, you will need khổng lồ compile your code without optimization, & with the -g flag, so information about source code lines will be embedded in the executable file. For more, see Step-by-step example for using GDB within Emacs khổng lồ debug a C orC++ program.