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Top Forums Programming predict the output Post 86904 by mbb on Tuesday 18th of October 2005 08:45:20 AM
Old 10-18-2005
Might be an interview question ...

The program might do anything from print "Hello world" to core dump (or GPF or whatever).

You won't really know until you compile it or run it and even then the result may be different depending on all sorts of factors e.g. compiler used and Operating System. And worse, it may not run in the same way twice either.

Why? Because a[2] is not declared. The syntax is valid so the C compiler will produce an executable that will reference the memory location &a+2, but that access is not legitimate. You could be changing another, unrelated variable or accessing the code section of the process, or the stack or the heap. You may even be accessing an memory area outside of your program's allotted segment.

On a small program you may not experience any problems at all, but as the program becomes larger and more complex the likelihood in encountering problems increases. Your program may not out and out crash, but may become dysfunctional for no reason that can be explained. Or unpredicatable, sometimes referred to as 'unstable'.

C is a powerful language precisely because it allows these freedoms with memory access, but bad memory management in your program is the primary risk with C too!
 

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BRK(2)								System Calls Manual							    BRK(2)

NAME
brk, sbrk - change data segment size SYNOPSIS
#include <unistd.h> char *brk(char *addr) char *sbrk(int incr) DESCRIPTION
Brk sets the system's idea of the lowest data segment location not used by the program (called the break) to addr. Locations greater than addr and below the stack pointer are not in the address space and will thus cause a memory violation if accessed. In the alternate function sbrk, incr more bytes are added to the program's data space and a pointer to the start of the new area is returned. When a program begins execution via execve the break is set at the highest location defined by the program and data storage areas. Ordi- narily, therefore, only programs with growing data areas need to use sbrk. RETURN VALUE
The address of the new break is returned if brk succeeds; -1 if the program requests more memory than the system limit. Sbrk returns -1 if the break could not be set. ERRORS
Sbrk will fail and no additional memory will be allocated if one of the following are true: [ENOMEM] The maximum possible size of a data segment (as set by chmem(1)) was exceeded. [ENOMEM] Insufficient virtual memory space existed to support the expansion. (Minix-vmd) SEE ALSO
chmem(1), execve(2), malloc(3), end(3). NOTES
Minix-vmd rounds a small data segment limit up to 3 megabytes. BUGS
Setting the break may fail due to a temporary lack of virtual memory under Minix-vmd. It is not possible to distinguish this from a fail- ure caused by exceeding the maximum size of the data segment. 4th Berkeley Distribution May 22, 1986 BRK(2)
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