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Full Discussion: Unclear pointer and array
Top Forums Programming Unclear pointer and array Post 302884179 by Don Cragun on Saturday 18th of January 2014 07:08:41 AM
Old 01-18-2014
The declarations:
Code:
    char *text;
    char *a;

allocate space on the stack for two pointers to characters. And the size of a is the size of a pointer (4 bytes in a 32-bit application; 8 bytes in a 64-bit application).

With these declarations, the initial value of these pointers is whatever random bytes happen to have been on the stack. When you copy data or read data into an area pointed to by an uninitialized pointer, you will get a memory fault, a bus error, or overwrite data at some random location depending on what random bytes on the stack happen to underly your pointers. If you malloc() space for arrays of characters and assign the pointers that malloc() returned to your pointers, then you won't be overwriting a random location in memory, but you would still have a problem because the sizeof(a) in:
Code:
       substring(start, end, text, a, sizeof(a)));

is the size of the pointer; not the number of bytes allocated by malloc() to the array pointed to by a.

The declarations:
Code:
    char text[100];
    char a[100];

allocate two arrays of 100 bytes each on your stack. And the size of a is 100 bytes.
 

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LEARN ABOUT MINIX

chmem

CHMEM(1)						      General Commands Manual							  CHMEM(1)

NAME

       chmem - change memory allocation

SYNOPSIS

       chmem [+] [-] [=] amount file

EXAMPLES

       chmem =50000 a.out  # Give a.out 50K of stack space

       chmem -4000 a.out   # Reduce the stack space by 4000 bytes

       chmem +1000 file1   # Increase each stack by 1000 bytes

DESCRIPTION

       When  a	program is loaded into memory, it is allocated enough memory for the text and data+bss segments, plus an area for the stack.  Data
       segment growth using malloc , brk , or sbrk eats up stack space from the low end.  The amount of stack space to allocate is derived from  a
       field  in  the executable program's file header.  If the combined stack and data segment growth exceeds the stack space allocated, the pro-
       gram will be terminated.

       It is therefore important to set the amount of stack space carefully.  If too little is provided, the program may crash.  If  too  much	is
       provided,  memory  will	be  wasted, and fewer programs will be able to fit in memory and run simultaneously.  MINIX does not swap, so that
       when memory is full, subsequent attempts to fork will fail.  The compiler sets the stack space to the largest possible value (for the Intel
       CPUs, 64K - text - data).  For many programs, this value is far too large.  Nonrecursive programs that do not call brk , sbrk , or malloc ,
       and do not have any local arrays usually do not need more than 8K of stack space.

       The chmem command changes the value of the header field that determines the stack allocation, and thus indirectly the total memory required
       to run the program.  The = option sets the stack size to a specific value; the + and - options increment and decrement the current value by
       the indicated amount.  The old and new stack sizes are printed.

SEE ALSO

       install(1), brk(2).

																	  CHMEM(1)
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