C++ 自定义malloc实现上奇怪的操作系统依赖行为

程序的运行时间受到您获得的随机数的影响:

1. 值为niter(迭代次数！)
2. 每个循环的值为size.
3. 值为toss(无论您是否 Select free).

您混淆了rand和random个电话[为什么？].

在某些系统(例如linux/glibc)上，rand呼叫random，但其他系统则不.

此外，在linux/glibc上，rand_r是简单的线性全等生成器，rand是not rand_r(&seed)

参见:Linear Congruential Generator

因此，为了获得一致的结果，您将受到给定系统rand和random的实施的摆布！

try 将所有rand*个呼叫替换为:(例如)xrand.这是基于"标准"数字的LCG:

static unsigned int xrand_seed;

int
xrand_r(unsigned int *seed)
{
    int val = *seed;
    val = (val * 1103515245) + 12345;
    *seed = val;
    return val & 0x7FFFFFFF;
}

int
xrand(void)
{
    return xrand_r(&xrand_seed);
}

这是testalloc.c的重构版本:

1. 它使用上面的xrand实现
2. 它具有调试printf和时间戳，因此您可以查看性能如何受到影响.
3. 它允许使用-R<seed>参数来查看随机种子如何影响性能.
4. 调用程序without参数的segfault已修复.

/*
 * a simple application to exercise malloc() and free() calls
 */

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

#define MAXITERS   100000
#define MAXSIZE    1024

#if DEBUG
#define dbgprt(_fmt...) \
    do { \
        dbgstamp(); \
        printf(_fmt); \
    } while (0)
#else
#define dbgprt(_fmt...) \
    do { } while (0)
#endif

double tsczero;

double
tscgetf(void)
{
    struct timespec ts;
    double sec;

    clock_gettime(CLOCK_MONOTONIC,&ts);
    sec = ts.tv_nsec;
    sec /= 1e9;
    sec += ts.tv_sec;

    sec -= tsczero;

    return sec;
}

void
dbgstamp(void)
{
    double tsc = tscgetf();

    static unsigned long long seqno;
    printf("[%llu/%.9f] ",seqno++,tsc);
}

static unsigned int xrand_seed;

int
xrand_r(unsigned int *seed)
{
    int val = *seed;
    val = (val * 1103515245) + 12345;
    *seed = val;
    return val & 0x7FFFFFFF;
}

int
xrand(void)
{
    return xrand_r(&xrand_seed);
}

int
main(int argc, char *argv[])
{

    --argc;
    ++argv;

    tsczero = tscgetf();

    for (;  argc > 0;  --argc, ++argv) {
        char *cp = *argv;
        if (*cp != '-')
            break;

        cp += 2;
        switch (cp[-1]) {
        case 'R':
            xrand_seed = atoll(cp);
            break;
        }
    }

    dbgprt("main: R=%u\n",xrand_seed);

    double pfree;
    if (argc > 1)
        pfree = atof(argv[1]);
    else
        pfree = 0.5;
    dbgprt("main: pfree=%g\n",pfree);

    int niter = xrand() % MAXITERS;
    dbgprt("main: niter=%d\n",niter);

    for (int i = 0; i < niter; i++) {
        dbgprt("main: ENTER i=%d/%d\n",i,niter);

        int size = xrand() % MAXSIZE;
        dbgprt("main: RAND size=%d\n",size);

        void *p = malloc(size * sizeof(char));
        dbgprt("main: RET p=%p\n",p);

        double toss = (double) xrand() / RAND_MAX;
        if (toss > pfree) {
            dbgprt("main: FREE\n");
            free(p);
        }

        dbgprt("main: EXIT\n");
    }

    printf("Successful\n");

    return 0;
}