read
和write
是系统调用,因此它们在用户空间中是无缓冲的。你提交的所有内容都将直接进入内核。
底层文件系统可能有内部缓冲,但这里最大的性能影响将来自于每次调用时切换到内核空间。
fread
和fwrite
是用户空间库调用,默认情况下是带缓冲的。因此,它们会将你的访问组合在一起,以使其更快(理论上)。
试试自己操作:逐字节从文件中read
,然后逐字节从中fread
。后者应该快4000倍。
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/time.h>
#include <sys/resource.h>
int main() {
struct rusage usage_start, usage_end;
getrusage(RUSAGE_SELF, &usage_start);
int fd = open("/dev/zero", O_RDONLY);
int i = 0x400 * 0x400;
char c;
while (i--)
read(fd, &c, 1);
close(fd);
getrusage(RUSAGE_SELF, &usage_end);
printf("Time used by reading 1MiB: %zu user, %zu system.\n", ((usage_end.ru_utime.tv_sec - usage_start.ru_utime.tv_sec)* 1000000) + usage_end.ru_utime.tv_usec - usage_start.ru_utime.tv_usec, ((usage_end.ru_stime.tv_sec - usage_start.ru_stime.tv_sec)* 1000000) + usage_end.ru_stime.tv_usec - usage_start.ru_stime.tv_usec);
getrusage(RUSAGE_SELF, &usage_start);
FILE * fp = fopen("/dev/zero", "r");
i = 0x400 * 0x400;
while (i--)
fread(&c, 1, 1, fp);
fclose(fp);
getrusage(RUSAGE_SELF, &usage_end);
printf("Time used by freading 1MiB: %zu user, %zu system.\n", ((usage_end.ru_utime.tv_sec - usage_start.ru_utime.tv_sec)* 1000000) + usage_end.ru_utime.tv_usec - usage_start.ru_utime.tv_usec, ((usage_end.ru_stime.tv_sec - usage_start.ru_stime.tv_sec)* 1000000) + usage_end.ru_stime.tv_usec - usage_start.ru_stime.tv_usec);
return 0;
}
我的 OS X 返回值:
Time used by reading 1MiB: 103855 user, 442698 system.
Time used by freading 1MiB: 20146 user, 256 system.
< p>
stdio
函数只是在适当的系统调用周围包装优化代码。
这是程序的strace
:
getrusage(RUSAGE_SELF, {ru_utime={0, 0}, ru_stime={0, 0}, ...}) = 0
open("/dev/zero", O_RDONLY) = 3
接下来是1048576次。
read(3, "\0", 1) = 1
以及其余的内容:
close(3) = 0
getrusage(RUSAGE_SELF, {ru_utime={0, 200000}, ru_stime={5, 460000}, ...}) = 0
这是
fopen
的一部分:
fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2aaaaaaae000
getrusage(RUSAGE_SELF, {ru_utime={0, 200000}, ru_stime={5, 460000}, ...}) = 0
// ...
open("/dev/zero", O_RDONLY) = 3
fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 5), ...}) = 0
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fffffffb050) = -1 ENOTTY (Inappropriate ioctl for device)
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2aaaaaaaf000
现在256倍:
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4096
请注意,尽管我是逐字逐句阅读的,但
stdio
库却一次获取一个页面的文件内容。
其余部分主要是释放内存:
close(3) = 0
munmap(0x2aaaaaaaf000, 4096) = 0
getrusage(RUSAGE_SELF, {ru_utime={0, 230000}, ru_stime={5, 460000}, ...}) = 0
write(1, "Time used by reading 1MiB: 20000"..., 106Time used by reading 1MiB: 200000 user, 5460000 system.
Time used by freading 1MiB: 30000 user, 0 system.
) = 106
exit_group(0) = ?