Valgrind中的未初始化字節錯誤

Question

我是C語言的新手，我正在與Valgrind一起做一些練習。

我的代碼：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    int i;
    int ints[10];
    char myString[50]; 

    for (i=0; i<=10; i++) {
        ints[i] = rand() % 10;
    }

    printf("''string'' is %lu characters long\n", strlen(myString));
    strcpy(myString, "This is a medium sized string");
    printf("''string'' is %lu characters long\n", strlen(myString));

    return 0;
}

我的Valgrind輸出：

==5425== Syscall param write(buf) points to uninitialised byte(s)
==5425==    at 0x260E9A: write$NOCANCEL (in /usr/lib/system/libsystem_kernel.dylib)
==5425==    by 0x17E5E8: __sflush (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x180CAA: __sfvwrite (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x18B191: __vfprintf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x1AF2DA: __v2printf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x1AF6AF: __xvprintf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x186B29: vfprintf_l (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x18496F: printf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x100000E73: main (test.c:15)
==5425==  Address 0x10001236e is 14 bytes inside a block of size 4,096 alloc'd
==5425==    at 0x6DEB: malloc (in /usr/local/Cellar/valgrind/3.9.0/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==5425==    by 0x181855: __smakebuf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x196217: __swsetup (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x1AF158: __v2printf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x1AF6AF: __xvprintf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x186B29: vfprintf_l (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x18496F: printf (in /usr/lib/system/libsystem_c.dylib)
==5425==    by 0x100000E73: main (test.c:15)
==5425==  Uninitialised value was created by a stack allocation
==5425==    at 0x100000DF0: main (test.c:6)

我以為該錯誤是由於在我進行第一個printf調用時myString是“空”的事實引起的，但是，我不確定如何解決它。

謝謝！

Answer 1

當您在此處進行strlen調用時，字符串myString未初始化：

printf("''string'' is %lu characters long\n", strlen(myString));

當strlen函數通過myString搜索'\\0' ，它將讀取未初始化的數據。 這就是觸發valgrind中的錯誤的原因。

要解決此問題，請添加初始化：

char myString[50] = {0};

Answer 2

我認為該錯誤是由於myString為“空”

並不是的。 myString不為空，未初始化，其內容未定義，因此使用strlen(myString)是非法的。

您可以將其初始化為一個空字符串，如下所示：

char myString[50] = ""; 

Answer 3

不能說myString為空，它有一些內容。 只是內容是任意的。

這意味着在該處可能沒有零字節來終止該字符串，因此在其上調用printf或strlen 並不是一個好主意，因為它可能跑出緩沖區的末端並引起問題。

您可以通過確保它是有效的字符串來簡單地使其工作，例如：

*myString = '\0';