我已经编写了一个函数来比较版本c的字符串，如"1.2.3"、"1.123.9"等。
int compare_versions(char* first, char* second)
{
size_t first_last_dot1 = 0;
size_t first_last_dot2 = strcspn(first, ".");
size_t second_last_dot1 = 0;
size_t second_last_dot2 = strcspn(second, ".");
while(first_last_dot2 || second_last_dot2)
{
if(first_last_dot2 && !second_last_dot2) return 1;
if(!first_last_dot2 && second_last_dot2) return -1;
char* first_c = (char*)calloc(first_last_dot2 + 1, sizeof(char));
strncat(first_c, first + first_last_dot1, first_last_dot2);
int first_n = atoi(first_c);
first_last_dot1 += first_last_dot2 + 1;
first_last_dot2 = strcspn(first + first_last_dot1, ".");
free(first_c);
char* second_c = (char*)calloc(second_last_dot2 + 1, sizeof(char));
strncat(second_c, second + second_last_dot1, second_last_dot2);
int second_n = atoi(second_c);
second_last_dot1 += second_last_dot2 + 1;
second_last_dot2 = strcspn(second + second_last_dot1, ".");
free(second_c);
if (first_n != second_n)
{
if(first_n < second_n) return -1;
return 1;
}
}
return 0;
}
valgrind根本不会抱怨我的"测试"：
assert(0 == compare_versions("1.12345678.2", "1.12345678.2"));
valgrind输出：
==24002== Memcheck, a memory error detector
==24002== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==24002== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==24002== Command: ./a.out
==24002==
==24002==
==24002== HEAP SUMMARY:
==24002== in use at exit: 0 bytes in 0 blocks
==24002== total heap usage: 10 allocs, 10 frees, 48 bytes allocated
==24002==
==24002== All heap blocks were freed -- no leaks are possible
==24002==
==24002== For counts of detected and suppressed errors, rerun with: -v
==24002== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 6)
现在我想重用分配的内存，即：当我分配一个大小为4的内存块时，我可以将其重用于大小不超过4的所有c字符串（包括终止的null字符）。
所以我写了这个版本的相同功能：
int compare_versions(char* first, char* second)
{
size_t first_last_dot1 = 0;
size_t first_last_dot2 = strcspn(first, ".");
size_t second_last_dot1 = 0;
size_t second_last_dot2 = strcspn(second, ".");
char* first_c = (char*)calloc(first_last_dot2 + 1, sizeof(char));
char* second_c = (char*)calloc(second_last_dot2 + 1, sizeof(char));
int first_max = first_last_dot2;
int second_max = second_last_dot2;
while(first_last_dot2 || second_last_dot2)
{
// first longer than second ( different only by last segment )
if(first_last_dot2 && !second_last_dot2) {
free(first_c);
free(second_c);
return 1;
}
// second longer than first ( different only by last segment )
if(!first_last_dot2 && second_last_dot2) {
free(first_c);
free(second_c);
return -1;
}
if(first_last_dot2 > first_max) {
first_max = first_last_dot2;
first_c = (char*)realloc(first_c, first_last_dot2 + 1);
memset(first_c, 0, first_last_dot2);
}
strncat(first_c, first + first_last_dot1, first_last_dot2);
int first_n = atoi(first_c);
first_last_dot1 += first_last_dot2 + 1;
first_last_dot2 = strcspn(first + first_last_dot1, ".");
if(second_last_dot2 > second_max) {
second_max = second_last_dot2;
second_c = (char*)realloc(second_c, second_last_dot2 + 1);
memset(second_c, 0, second_last_dot2);
}
strncat(second_c, second + second_last_dot1, second_last_dot2);
int second_n = atoi(second_c);
second_last_dot1 += second_last_dot2 + 1;
second_last_dot2 = strcspn(second + second_last_dot1, ".");
if (first_n != second_n)
{
free(first_c);
free(second_c);
if(first_n < second_n) return -1;
return 1;
}
}
free(first_c);
free(second_c);
return 0;
}
但我会从valgrind得到以下输出：
==24039== Memcheck, a memory error detector
==24039== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==24039== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==24039== Command: ./a.out
==24039==
==24039== Invalid write of size 1
==24039== at 0x4A07E83: strncat (mc_replace_strmem.c:304)
==24039== by 0x400828: compare_versions (main.c:46)
==24039== by 0x400986: main (main.c:94)
==24039== Address 0x4c330e9 is 0 bytes after a block of size 9 alloc'd
==24039== at 0x4A06C20: realloc (vg_replace_malloc.c:662)
==24039== by 0x4007F1: compare_versions (main.c:43)
==24039== by 0x400986: main (main.c:94)
==24039==
==24039== Invalid read of size 1
==24039== at 0x39AA436FF3: ____strtol_l_internal (strtol_l.c:438)
==24039== by 0x39AA433C5F: atoi (atoi.c:28)
==24039== by 0x400834: compare_versions (main.c:47)
==24039== by 0x400986: main (main.c:94)
==24039== Address 0x4c330e9 is 0 bytes after a block of size 9 alloc'd
==24039== at 0x4A06C20: realloc (vg_replace_malloc.c:662)
==24039== by 0x4007F1: compare_versions (main.c:43)
==24039== by 0x400986: main (main.c:94)
==24039==
...
==24039==
==24039== HEAP SUMMARY:
==24039== in use at exit: 0 bytes in 0 blocks
==24039== total heap usage: 4 allocs, 4 frees, 22 bytes allocated
==24039==
==24039== All heap blocks were freed -- no leaks are possible
==24039==
==24039== For counts of detected and suppressed errors, rerun with: -v
==24039== ERROR SUMMARY: 34 errors from 8 contexts (suppressed: 6 from 6)
这里有什么问题？
Valgrind说了所有的-我只看最初的错误：
===24039===大小为1的无效写入===24039===在0x4A07E83:strncat（mc_replace_strmem.c:304）===24039===通过0x400828:compare_versions（main.c:46）===24039===通过0x400986:main（main.c:94）===24039===地址0x4c330e9在分配大小为9的块之后为0字节===24039===在0x4A06C20:realloc（vg_replace_malloc.c:662）===24039===通过0x4007F1:compare_versions（main.c:43）===24039===通过0x400986:main（main.c:94）
线路compare_versions (main.c:46)为：
strncat(first_c, first + first_last_dot1, first_last_dot2);
线路compare_versions (main.c:43)为：
first_c = (char*)realloc(first_c, first_last_dot2 + 1);
错误发生在第三个循环中。first_last_dot2已在线的第二个循环周期中设置
first_last_dot2 = strcspn(first + first_last_dot1, ".");
值1（版本字符串最后一部分的长度，即2）；先前first_last_dot2在第二个循环周期中被设置为值8（版本字符串的中间部分的长度，即12345678），因此realloc分配的大小8+1=9。由于现在if条件(first_last_dot2 > first_max)不成立，因此realloc和
memset(first_c, 0, first_last_dot2);
被跳过；由于不执行后者，first_c仍然包含12345678，strncat将2附加到它，将终止的空字节写入first_c[9]，是在大小为9的块分配之后的0字节
如果将两个memset（也是second_c的一个）移动到if块之后，则没有错误。