程式語言 - LeetCode - C - 注意事項 - Global Variable


題目如下：


使用Global Variable的寫法

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int idx = 0;
char *p = NULL;
 
void push(char ch)
{
    p[idx++] = ch;
}
 
void pop(void)
{
    if (idx > 0) {
        idx -= 1;
        p[idx] = 0;
    }
}
 
char* removeStars(char *s)
{
    int c0 = 0;
    int len = strlen(s);
 
    p = malloc(len + 1);
    memset(p, 0, len + 1);
 
    for (c0 = 0; c0 < len; c0++) {
        if (s[c0] != '*') {
            push(s[c0]);
        }
        else {
            pop();
        }
    }
    return p;
}

Test cases 

"aa"
"bb"

Exception 

=================================================================
==31==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000000096 at pc 0x55ca895660b8 bp 0x7ffdca7c0b40 sp 0x7ffdca7c0b30
WRITE of size 1 at 0x602000000096 thread T0
    #2 0x7f1f898d60b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
0x602000000096 is located 3 bytes to the right of 3-byte region [0x602000000090,0x602000000093)
allocated by thread T0 here:
    #0 0x7f1f8a51bdc6 in calloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10ddc6)
    #3 0x7f1f898d60b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
Shadow bytes around the buggy address:
  0x0c047fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff8000: fa fa 07 fa fa fa 07 fa fa fa 00 07 fa fa 03 fa
=>0x0c047fff8010: fa fa[03]fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==31==ABORTING

修改後即可正常執行

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void push(char *p, int *idx, char ch)
{
    p[(*idx)++] = ch;
}
 
void pop(char *p, int *idx)
{
    if ((*idx) > 0) {
        (*idx) -= 1;
        p[*idx] = 0;
    }
}
 
char* removeStars(char *s)
{
    int c0 = 0;
    int idx = 0;
    int len = strlen(s);
    char *p = malloc(len + 1);
 
    memset(p, 0, len + 1);
    for (c0 = 0; c0 < len; c0++) {
        if (s[c0] != '*') {
            push(p, &idx, s[c0]);
        }
        else {
            pop(p, &idx);
        }
    }
    return p;
}

P.S. 結論：不要使用Global Variable