此为《深入理解计算机系统》一书上的示例程序，matvec 函数分配了内存，但是未初始化内存的值，因此 y 指向的内存的值是不确定的。在此段小程序中问题是显而易见的，在大型软件系统中可以能借助与 Valgrind 来检测
#include <stdlib.h>
#include <stdio.h>
#define COUNT 3
int* matvec(int A[][COUNT], int* x, int n) {
int i, j;
int* y = (int*)malloc(n * sizeof(int));
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
y[i] += A[i][j] * x[j];
}
}
return y;
}
void main() {
int a[COUNT][COUNT] = {
{0, 1, 2},
{2, 3, 4},
{4, 5, 6}
};
int x[3] = {1, 2, 3};
int* y = matvec(a, x, 3);
for (int i = 0; i < COUNT; i++) {
printf("%d ", y[i]);
}
free(y);
}
让我们试试使用 Valgrind，注意 Valgrind 只能检测出使用了未初始化的内存，并且对外部环境造成影响的情况。如果把第 26 行的 去掉 ，则 Valgrind 不会检测出任何问题
bzhu@bzhu-Macmini:~/src$ gcc -g 9.11.2.c && valgrind --tool=memcheck --leak-check=full --track-origins=yes ./a.out
==31526== Memcheck, a memory error detector
==31526== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==31526== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==31526== Command: ./a.out
==31526==
==31526== Conditional jump or move depends on uninitialised value(s)
==31526== at 0x48D3AD8: __vfprintf_internal (vfprintf-internal.c:1687)
==31526== by 0x48BDEBE: printf (printf.c:33)
==31526== by 0x109334: main (9.11.2.c:26)
==31526== Uninitialised value was created by a heap allocation
==31526== at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==31526== by 0x1091D0: matvec (9.11.2.c:7)
==31526== by 0x1092FE: main (9.11.2.c:24)
==31526==
==31526== Use of uninitialised value of size 8
==31526== at 0x48B781B: _itoa_word (_itoa.c:179)
==31526== by 0x48D36F4: __vfprintf_internal (vfprintf-internal.c:1687)
==31526== by 0x48BDEBE: printf (printf.c:33)
==31526== by 0x109334: main (9.11.2.c:26)
==31526== Uninitialised value was created by a heap allocation
==31526== at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==31526== by 0x1091D0: matvec (9.11.2.c:7)
==31526== by 0x1092FE: main (9.11.2.c:24)
==31526==
==31526== Conditional jump or move depends on uninitialised value(s)
==31526== at 0x48B782D: _itoa_word (_itoa.c:179)
==31526== by 0x48D36F4: __vfprintf_internal (vfprintf-internal.c:1687)
==31526== by 0x48BDEBE: printf (printf.c:33)
==31526== by 0x109334: main (9.11.2.c:26)
==31526== Uninitialised value was created by a heap allocation
==31526== at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==31526== by 0x1091D0: matvec (9.11.2.c:7)
==31526== by 0x1092FE: main (9.11.2.c:24)
==31526==
==31526== Conditional jump or move depends on uninitialised value(s)
==31526== at 0x48D43A8: __vfprintf_internal (vfprintf-internal.c:1687)
==31526== by 0x48BDEBE: printf (printf.c:33)
==31526== by 0x109334: main (9.11.2.c:26)
==31526== Uninitialised value was created by a heap allocation
==31526== at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==31526== by 0x1091D0: matvec (9.11.2.c:7)
==31526== by 0x1092FE: main (9.11.2.c:24)
==31526==
==31526== Conditional jump or move depends on uninitialised value(s)
==31526== at 0x48D386E: __vfprintf_internal (vfprintf-internal.c:1687)
==31526== by 0x48BDEBE: printf (printf.c:33)
==31526== by 0x109334: main (9.11.2.c:26)
==31526== Uninitialised value was created by a heap allocation
==31526== at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==31526== by 0x1091D0: matvec (9.11.2.c:7)
==31526== by 0x1092FE: main (9.11.2.c:24)
==31526==
8 20 32 ==31526==
==31526== HEAP SUMMARY:
==31526== in use at exit: 0 bytes in 0 blocks
==31526== total heap usage: 2 allocs, 2 frees, 1,036 bytes allocated
==31526==
==31526== All heap blocks were freed -- no leaks are possible
==31526==
==31526== For lists of detected and suppressed errors, rerun with: -s
==31526== ERROR SUMMARY: 19 errors from 5 contexts (suppressed: 0 from 0)
另外，最简单的情况，也是读取了未初始化的内存。未初始化的内存可以来自与堆上，也可以来自于普通变量（栈上）。
int main()
{
int x;
printf ("x = %d\n", x);
}
更详细描述，请参考Valgrind 手册