gdb memory addresses not matching memory address printed out by program-CodePudding

I have a basic C program like this:

#include <stdio.h>
int main()
{
    int cookie;
    char buf[80];

    printf("A: %x B: %x\n", &buf, &cookie);
    gets(buf);

    if (cookie == 0x11111111){
        printf("You win!\n");
    } else {
        printf("THIS Didn't WORK");
    }
}

When I run it, the print statement prints A: efbffab0 B: efbffb08

Yet, if I were to open up this program in gdb and explore the memory address of any of the variables, I get a longer value then what the C program prints out:

Thread 2 hit Breakpoint 1, main () at challenge.c:10
10      if (cookie == 0x41424344){
(gdb) x/50x &buf
0x7ffeefbffab0: 0x41424344  0x41424344  0x41424344  0x41424344
0x7ffeefbffac0: 0x41424344  0x41424344  0x41424344  0x41424344
0x7ffeefbffad0: 0x41424344  0x41424344  0x41424344  0x41424344
0x7ffeefbffae0: 0x41424344  0x41424344  0x41424344  0x41424344
0x7ffeefbffaf0: 0x41424344  0x41424344  0x41424344  0x41424344
0x7ffeefbffb00: 0x41424344  0x00007f00  0x00011025  0x00000000
0x7ffeefbffb10: 0xefbffb28  0x00007ffe  0x20545f3d  0x00007fff
0x7ffeefbffb20: 0x20545f3d  0x00007fff  0x00000000  0x00000000
0x7ffeefbffb30: 0x00000001  0x00000000  0xefbffcb0  0x00007ffe
0x7ffeefbffb40: 0x00000000  0x00000000  0xefbffd09  0x00007ffe
0x7ffeefbffb50: 0xefbffd4b  0x00007ffe  0xefbffd65  0x00007ffe
0x7ffeefbffb60: 0xefbffd74  0x00007ffe  0xefbffd85  0x00007ffe
0x7ffeefbffb70: 0xefbffdce  0x00007ffe

How come the address from my C program, 0xefbffab0 doesn't match the address gdb gives me, 0x7ffeefbffab0? Where does the 0x7ffe prefix come from?

CodePudding user response：

You're using the wrong format specifier for printing a pointer. The x specifier is for unsigned int, which on your system is 32 bits, and thus prints only part of the address (however this is just a coincidence, using the wrong format specifier is undefined behavior). The one you want is p, which is for void *:

printf("A: %p B: %p\n", &buf, &cookie);