Standard C IO function and kernel IO function efficiency (time) comparison

　　

Objective

Standard C provides file-related IO functions, except that the standard error output is unbuffered (the error message can be displayed as soon as possible), all terminal-related are row buffers, and the rest are fully buffered.

We can use SETBUF,SETVBUF to change the buffer type of the specified stream.

Prototype:

   void Char *buf); int Char int mode, size_t size); Successful return 0, failure not 0

The function is used at a glance, and when the parameter BUF is specified as NULL, the operating system automatically allocates the appropriate buffer for the given stream. The length is Bufsiz (no e, defined in stdio.h).

The function of buffering can reduce the number of system calls, the system call needs the overhead, and the operation of the operating system is required from the user state to the kernel state switch. The appropriate length of buffer size is important for enhancing IO efficiency.

Comparison of kernel functions and standard C functions

File size processed: 270M of text

Comparison of file copy run times

function	Sys	User	Real	Note
Fgetc,fputc	0m0.848s	0m6.232s	0m7.614s
Fgets,fputs Fastest Speed	0m1.204s	0m0.800s	0m3.709s
Read,write Fastest Speed	0m3.380s	0m0.752s	0m0.008s	BUF[1024*8]
Read,write Buffer ==1	18m49.875s	0m35.166s	19m33.293s

Comparative conclusion:

1, the standard IO library function and the kernel function is not different how much, it is not particularly necessary in the case of standard IO is not a good choice.

2, Fgets (fputs) than fgetc (FPUTC) twice times faster, visible fgets (fputs) is not implemented with FGETC (FPUTC), otherwise their efficiency should be similar, according to the UC advanced programming that book, Fgets, Fputs is implemented with memccpy, and memccpy is written by sinks, which is naturally more efficient than the C function.

Attached memccpy prototype

　　　　

void *memcpy (voidconstvoid *src, size_t n); // Copy no more than n bytes to dest from SRC, and the N character value is encountered void *memccpy (voidconstvoidint C, size_t N);

Standard C IO function and kernel IO function efficiency (time) comparison