Question

这是我写的一个小C程序：

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

/* Prototypes */
int sum(int *summands, unsigned int n);
void increment(char *string, int n);
void copy(char *src, char *dst);

int main(void)
{
    int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    char str1[] = "HAL";
    char str2[] = "SRIKANT";
    char str3[] = "A string to be copied.";
    char *str4 = malloc(sizeof(str3));

    printf("The sum of all the elements in ARR is %d.\n", sum(arr, 9));

    printf("STR1 is: %s. STR2 is: %s.\n", str1, str2);
    printf("Incrementing all letters...\n");
    increment(str1, 3);
    increment(str2, 7);
    printf("Incremented!\n");
    printf("STR1 is now: %s. STR2 is now: %s.\n", str1, str2);

    copy(str3, str4);
    printf("STR4 is: %s\n", str4);
    free(str4);

    return 0;
}

/* Returns the sum of all the elements in SUMMANDS. */
int sum(int *summands, unsigned int n)
{
    int sum = 0;
    for (int i = 0; i < (int) n; i++)
    {
        sum += *(summands + i);
    }
    return sum;
}

/* Increments all the letters in the string STRING, held in an array of length N.
 * Does not modify any other memory which has been previously allocated. */
void increment(char *string, int n)
{
    for (int i = 0; i < n; i++)
    {
        (*(string + i))++;
    }
}

/* Copies the string SRC to DST. */
// void copy(char *src, char *dst)
// {
//     while (*src)
//     {
//         *dst++ = *src++;
//     }
//     *dst = '\0';
// }

void copy(char *src, char *dst)
{
    while ((*dst++ = *src++));
}

当我在运行Linux Mint 18的计算机上运行Valgrind的可执行文件时，它会生成以下错误报告（堆摘要）：

==5503== Memcheck, a memory error detector
==5503== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==5503== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==5503== Command: ./disc1_q4
==5503== 
The sum of all the elements in ARR is 45.
STR1 is: HAL. STR2 is: SRIKANT.
Incrementing all letters...
Incremented!
STR1 is now: IBM. STR2 is now: TSJLBOU.
STR4 is: A string to be copied.
==5503== 
==5503== HEAP SUMMARY:
==5503==     in use at exit: 0 bytes in 0 blocks
==5503==   total heap usage: 2 allocs, 2 frees, 1,047 bytes allocated
==5503== 
==5503== All heap blocks were freed -- no leaks are possible
==5503== 
==5503== For counts of detected and suppressed errors, rerun with: -v
==5503== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

但是当我在Cloud 9 IDE（使用Ubuntu虚拟机）上运行与Valgrind相同的程序时，它会生成正确的报告：

==2359== Memcheck, a memory error detector
==2359== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==2359== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==2359== Command: ./disc1_q4
==2359== 
The sum of all the elements in ARR is 45.
STR1 is: HAL. STR2 is: SRIKANT.
Incrementing all letters...
Incremented!
STR1 is now: IBM. STR2 is now: TSJLBOU.
STR4 is: A string to be copied.
==2359== 
==2359== HEAP SUMMARY:
==2359==     in use at exit: 0 bytes in 0 blocks
==2359==   total heap usage: 1 allocs, 1 frees, 23 bytes allocated
==2359== 
==2359== All heap blocks were freed -- no leaks are possible
==2359== 
==2359== For counts of detected and suppressed errors, rerun with: -v
==2359== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

为什么Valgrind会在我的机器上产生不正确的结果？

Answer 1

我怀疑printf()在一个你认为“错误”的情况下分配内存。

你的libc的printf()实现在内部分配一些内存是不现实的;这不是错误的。

Valgrind捕获所有分配并报告它，即使您的程序本身没有分配它。你可以通过评论printf()来测试这个。同样，其他标准函数也可以进行内存分配。所以，你无法控制它，也没有“错误”。只要没有报告泄漏，您就不必担心“总分配”。

Valgrind在我的机器上产生错误的输出

1 个答案: