Hi all!
I have a very simple C code where I am initializing a char variable using
malloc and storing a string inside it using strcpy. Then I am simply trying
to change the first character of the string. Here is a snippet of what my
code looks like:
FILE *filp
char *fname = (char*)malloc(strlen("Working.txt")*sizeof(char)+1);
strcpy(fname,"Working.txt");
printf("Address of working.txt: %p\n", fname);
fname[0] = 'x';
My purpose is to access the value of the string using the pointer (i.e.
fname) inside of gem5. I am trying to do this from
src/cpu/o3/dyn_inst_impl.hh in the execute() function. After it calls
staticInst->execute(), I have created a condition where if the PC is the
instruction doing 'movb $0x78,(%rax)' (i.e. placing "x" at the data
pointer to by the address inside rax), I read the value from rax using
readIntReg() function and then I am trying to print the string
("Working.txt") using the pointer value I get from rax via this code:
char *ptr = (char*)addr_in_reg;
printf("File name at %p is %c\n",addr_in_reg, *ptr);
Where addr_in_reg holds the address of "Working.txt" (I cross-checked it by
printing the same address from my binary and it matches). At this point
gem5 segfaults.
Can someone please help me understand how gem5 deals with application
pointers and do they even point to something? When I debugged this using
gdb, it said that variable ptr cannot access the memory location at 0x3260.
P.S. I have also observed that for any application I run in gem5, the heap
variable addresses always start from 3260 and then increment according to
the size allocated to it.
P.P.S. I run my binary using this command:
build/X86/gem5.opt configs/example/se.py --cpu-type=DerivO3CPU --caches
--mem-type=DDR4_2400_8x8 --mem-size=8GB --cmd=test
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