当我们在shell中敲击df这条命令的时候，会看到:

root@android:/ # df
Filesystem             Size   Used   Free   Blksize
/dev                  446.8M  36.0K  446.8M   4096
/mnt/secure           446.8M  0.0 K  446.8M   4096
/mnt/asec             446.8M  0.0 K  446.8M   4096
/mnt/cart0            446.8M  0.0 K  446.8M   4096
/mnt/obb              446.8M  0.0 K  446.8M   4096
/system               1.5 G  376.6M  1.1 G   1024
/data                 5.2 G  188.9M  5.0 G   4096
/cache                124.0M  16.1M  107.9M   4096
/mnt/.cci             503.9M  16.4M  487.6M   4096
/storage/sdcard0      5.2 G  188.9M  5.0 G   4096
/mnt/external_sd      7.5 G  475.0M  7.0 G   4096

其实很简单，就是利用statfs这个函数查询文件系统相关的信息，然后依次列举出来。

如果使用这个函数？

请看下文:

http://baike.baidu.com/link?url=EVV8n-l-DXfgNwYj5Lqzo0HFvYaXMYEzTBMVtuyrq0QCvpaD0Lr0RjX81L6jTE6RXplNC_cNec8tgdsDleX2pq

那么df是怎么实现的？请看源码df.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/statfs.h>

static int ok = EXIT_SUCCESS;
//根据挂载的文件的大小来计算
static void printsize(long double n)
{
    char unit = 'K';
    n /= 1024;
    if (n > 1024) {
        n /= 1024;
        unit = 'M';
    }
    if (n > 1024) {
        n /= 1024;
        unit = 'G';
    }
    printf("%-4.1Lf%c", n, unit);
}

static void df(char *s, int always) {
	//
    struct statfs st;
	//statfs函数可用来查询文件系统相关的信息。
    if (statfs(s, &st) < 0) {
        fprintf(stderr, "%s: %s\n", s, strerror(errno));
        ok = EXIT_FAILURE;
    } else {
        if (st.f_blocks == 0 && !always)
            return;        
        printf("%-20s  ", s);
        printsize((long double)st.f_blocks * (long double)st.f_bsize);
        printf("  ");
        printsize((long double)(st.f_blocks - (long double)st.f_bfree) * st.f_bsize);
        printf("  ");
        printsize((long double)st.f_bfree * (long double)st.f_bsize);
        printf("   %d\n", (int) st.f_bsize);
    }
}

int df_main(int argc, char *argv[]) {
    printf("Filesystem             Size   Used   Free   Blksize\n");
    if (argc == 1) {
        char s[2000];
		//挂载的文件都在/proc/mounts下显示
        FILE *f = fopen("/proc/mounts", "r");

        while (fgets(s, 2000, f)) {
            char *c, *e = s;

            for (c = s; *c; c++) {
                if (*c == ' ') {
                    e = c + 1;
                    break;
                }
            }

            for (c = e; *c; c++) {
                if (*c == ' ') {
                    *c = '\0';
                    break;
                }
            }

            df(e, 0);
        }

        fclose(f);
    } else {
        int i;

        for (i = 1; i < argc; i++) {
            df(argv[i], 1);
        }
    }

    exit(ok);
}