根据我的gprof结果 来改进代码结果如下Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls s/call s/call name
42.79 1.96 1.96 1 1.96 3.28 func1
28.82 3.28 1.32 1 1.32 1.32 new_func1
28.38 4.58 1.30 1 1.30 1.30 func2
% the percentage of the total running time of the
time program used by this function.
cumulative a running sum of the number of seconds accounted
seconds for by this function and those listed above it.
self the number of seconds accounted for by this
seconds function alone. This is the major sort for this
listing.
calls the number of times this function was invoked, if
this function is profiled, else blank.
self the average number of milliseconds spent in this
ms/call function per call, if this function is profiled,
else blank.
total the average number of milliseconds spent in this
ms/call function and its descendents per call, if this
function is profiled, else blank.
name the name of the function. This is the minor sort
for this listing. The index shows the location of
the function in the gprof listing. If the index is
in parenthesis it shows where it would appear in
the gprof listing if it were to be printed.
Copyright (C) 2012-2022 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
Call graph (explanation follows)
granularity: each sample hit covers 4 byte(s) for 0.22% of 4.58 seconds
index % time self children called name
<spontaneous>
[1] 100.0 0.00 4.58 main [1]
1.96 1.32 1/1 func1 [2]
1.30 0.00 1/1 func2 [4]
-----------------------------------------------
1.96 1.32 1/1 main [1]
[2] 71.6 1.96 1.32 1 func1 [2]
1.32 0.00 1/1 new_func1 [3]
-----------------------------------------------
1.32 0.00 1/1 func1 [2]
[3] 28.8 1.32 0.00 1 new_func1 [3]
-----------------------------------------------
1.30 0.00 1/1 main [1]
[4] 28.4 1.30 0.00 1 func2 [4]
-----------------------------------------------
This table describes the call tree of the program, and was sorted by
the total amount of time spent in each function and its children.
Each entry in this table consists of several lines. The line with the
index number at the left hand margin lists the current function.
The lines above it list the functions that called this function,
and the lines below it list the functions this one called.
This line lists:
index A unique number given to each element of the table.
Index numbers are sorted numerically.
The index number is printed next to every function name so
it is easier to look up where the function is in the table.
% time This is the percentage of the `total' time that was spent
in this function and its children. Note that due to
different viewpoints, functions excluded by options, etc,
these numbers will NOT add up to 100%.
self This is the total amount of time spent in this function.
children This is the total amount of time propagated into this
function by its children.
called This is the number of times the function was called.
If the function called itself recursively, the number
only includes non-recursive calls, and is followed by
a `+' and the number of recursive calls.
name The name of the current function. The index number is
printed after it. If the function is a member of a
cycle, the cycle number is printed between the
function's name and the index number.
For the function's parents, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the function into this parent.
children This is the amount of time that was propagated from
the function's children into this parent.
called This is the number of times this parent called the
function `/' the total number of times the function
was called. Recursive calls to the function are not
included in the number after the `/'.
name This is the name of the parent. The parent's index
number is printed after it. If the parent is a
member of a cycle, the cycle number is printed between
the name and the index number.
If the parents of the function cannot be determined, the word
`<spontaneous>' is printed in the `name' field, and all the other
fields are blank.
For the function's children, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the child into the function.
children This is the amount of time that was propagated from the
child's children to the function.
called This is the number of times the function called
this child `/' the total number of times the child
was called. Recursive calls by the child are not
listed in the number after the `/'.
name This is the name of the child. The child's index
number is printed after it. If the child is a
member of a cycle, the cycle number is printed
between the name and the index number.
If there are any cycles (circles) in the call graph, there is an
entry for the cycle-as-a-whole. This entry shows who called the
cycle (as parents) and the members of the cycle (as children.)
The `+' recursive calls entry shows the number of function calls that
were internal to the cycle, and the calls entry for each member shows,
for that member, how many times it was called from other members of
the cycle.
Copyright (C) 2012-2022 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
Index by function name
[2] func1 [4] func2 [3] new_func1代码如下#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
typedef struct {
unsigned char r, g, b;
} Pixel;
// 前置声明
static int readPPM(const char *filename, int *width, int *height, int *max_val, Pixel **pixels);
static int writePPM(const char *filename, int width, int height, int max_val, const Pixel *pixels);
static void grayscale(Pixel *pixels, int num_pixels);
int main() {
int width, height, max_val;
Pixel *pixels = NULL;
if (!readPPM("im.ppm", &width, &height, &max_val, &pixels)) {
fprintf(stderr, "Error: Failed to read input file\n");
return EXIT_FAILURE;
}
const int num_pixels = width * height;
grayscale(pixels, num_pixels);
if (!writePPM("im-gray.ppm", width, height, max_val, pixels)) {
fprintf(stderr, "Error: Failed to write output file\n");
free(pixels);
return EXIT_FAILURE;
}
free(pixels);
printf("Conversion completed. Output: im-gray.ppm\n");
return EXIT_SUCCESS;
}
static int readPPM(const char *filename, int *width, int *height, int *max_val, Pixel **pixels) {
FILE *file = fopen(filename, "r");
if (!file) {
fprintf(stderr, "Error opening %s: %s\n", filename, strerror(errno));
return 0;
}
char magic[3];
if (fscanf(file, "%2s", magic) != 1 || strcmp(magic, "P3") != 0) {
fprintf(stderr, "Invalid PPM format. Expected P3\n");
fclose(file);
return 0;
}
// 读取图像元数据
if (fscanf(file, "%d %d %d", width, height, max_val) != 3) {
fprintf(stderr, "Invalid image header\n");
fclose(file);
return 0;
}
const int num_pixels = *width * *height;
*pixels = malloc(num_pixels * sizeof(Pixel));
if (!*pixels) {
fprintf(stderr, "Memory allocation failed for %d pixels\n", num_pixels);
fclose(file);
return 0;
}
// 批量读取像素数据
for (int i = 0; i < num_pixels; i++) {
int r, g, b;
if (fscanf(file, "%d %d %d", &r, &g, &b) != 3) {
fprintf(stderr, "Invalid pixel data at #%d\n", i);
free(*pixels);
*pixels = NULL;
fclose(file);
return 0;
}
(*pixels)[i] = (Pixel){ (unsigned char)r, (unsigned char)g, (unsigned char)b };
}
fclose(file);
return 1;
}
static int writePPM(const char *filename, int width, int height, int max_val, const Pixel *pixels) {
FILE *file = fopen(filename, "w");
if (!file) {
fprintf(stderr, "Error creating %s: %s\n", filename, strerror(errno));
return 0;
}
// 使用大缓冲区提高I/O性能
char buffer[8192];
setvbuf(file, buffer, _IOFBF, sizeof(buffer));
fprintf(file, "P3\n%d %d\n%d\n", width, height, max_val);
const int num_pixels = width * height;
for (int i = 0; i < num_pixels; i++) {
if (fprintf(file, "%d %d %d\n", pixels[i].r, pixels[i].g, pixels[i].b) < 0) {
fprintf(stderr, "Write error at pixel #%d\n", i);
fclose(file);
return 0;
}
}
fclose(file);
return 1;
}
static void grayscale(Pixel *pixels, int num_pixels) {
Pixel *current = pixels;
const Pixel *end = pixels + num_pixels;
while (current < end) {
const unsigned char gray = (current->r + current->g + current->b) / 3;
*current++ = (Pixel){ gray, gray, gray };
}
}
本文介绍了文件系统中如何使用lseek系统调用来改变文件的读写位置，包括其基本用法、返回值及错误处理等内容，并深入探讨了在设备驱动程序中llseek方法的实现细节。
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