segfault in pointer to string program Post: 302556107

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Top Forums Programming segfault in pointer to string program Post 302556107 by Corona688 on Friday 16th of September 2011 02:59:51 PM

09-16-2011

Registered User

Quote:

Originally Posted by zius_oram

got it

thank you so much. do C through segfault for any particular type of error?? i wanna know when exactly C throws segfault?

All 'segfault' means is that your program attempted to access a memory page that either
1) doesn't exist, or
2) your program doesn't have permissions to access.

There's almost endless reasons why a program could end up doing that but it's often a logic error of some sort -- not checking the return value of something and dereferencing a NULL, going beyond array bounds and accidentally overwriting nearby variables, reusing a pointer you already free()'d and mangling whatever data(if anything) ended up in it later... You can even corrupt the stack frame itself so return jumps the program to invalid memory and bombs out long after the actual error was made. And lots more.

You went beyond array bounds, which would have started modifying the values of local stack variables. This can go wrong in many interesting ways... it could have kept going until you'd mangled your entire stack and hit the bottom of memory. Or (more likely) when you started mangling local stack variables, the value of s was set to some bizzare value which pointed to invalid memory.

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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)