I/O操作

内存分配

静态和全局变量程序启动时自动静态分配内存，函数参数和变量在构建时自动分配，退出时自动释放，当内存不固定时会动态分配
使用malloc动态分配内存

/* Include standard C library header. */
#include< stdlib.h>
...
/* 分配一个16长度的int指针数组 */
int* dynamicIntArray = (int*) malloc(sizeof(int) * 16);
if (NULL == dynamicIntArray) {
/* 内存不够分配 */
...
} else {
/* Use the memory through the integer pointer. */
*dynamicIntArray = 0;
dynamicIntArray[8] = 8;
...
/* 释放内存. */
free(dynamicIntArray);
dynamicIntArray = NULL;
}

sizeof获取数据类型需要的内存大小
动态修改分配好的内存

int* newDynamicIntArray = (int*) realloc(
dynamicIntArray, sizeof(int) * 32);
if (NULL == newDynamicIntArray) {
/* Unable to reallocate enough memory. */
...
} else {
/* Update the memory pointer. */
dynamicIntArray = newDynamicIntArray;
...

C++中动态分配内存，指定数据类型

int* dynamicInt = new int;
if (NULL == dynamicInt) {
/* Unable to allocate enough memory. */
...
} else {
/* Use the allocated memory. */
*dynamicInt = 0;
...
}

delete dynamicInt;
dynamicInt = 0;

int* dynamicIntArray = new int[16];
if (NULL == dynamicIntArray) {
/* Unable to allocate enough memory. */
...
} else {
/* Use the allocated memory. */
dynamicIntArray[8] = 8;
...
}

delete[] dynamicIntArray;
dynamicIntArray = 0;

标准I/O

Low-level I/O 精细的数据处理
Stream I/O 缓存I/O控制，用于处理流
stdin标准输入：应用程序的标准输入流
stdout标准输出：应用程序的标准输出流
stderr标准错误：应用程序标准错误流

打开一个文件流

#include
...
FILE* stream = fopen("/data/data/com.example.hellojni/test.txt", "w");
if (NULL == stream)
{
/* File could not be opened for writing. */
}
else
{
/* Use the stream. */
/* Close the stream. */
}

FILE* fopen(const char* filename, const char* opentype);第二个参数opentype
r：打开现有文件为只读。
w：打开文件只写。如果该文件已经存在，它被截断至零长度。
a：打开追加模式的文件。文件内容被保存下来，并附加到文件的末尾。如果该文件不存在，则打开一个新的文件
r+：打开在读写模式下的文件。
w+：打开在读写模式下的文件。如果该文件已经存在，它被截断零长度。
a+：打开文件进行读取和追加。初始文件位置被设置为开始用于读取和向用于附加该文件的结束。

写入流

size_t fwrite(const void* data, size_t size, size_t count, FILE* stream);
从流stream中读取count个数的char写入
如成功则返回count的个数

写入字符串到流中,写入失败返回EOF

/* Writing character sequence to stream. */
if (EOF == fputs("hello\n", stream))
{
/* Error occured while writing to the stream. */
}

写入单个字符，写入成功返回字符本身，否则返回EOF

char c = 'c';
/* Writing a single character to stream. */
if (c ! = fputc(c, stream))
{
/* Error occured while writing character to string.
}

写入格式化数据到流
int fprintf(FILE* stream, const char* format, …);

  %d，%i：格式化整数参数作为符号的十进制。
%u：格式化无符号整数为无符号小数。
%o：格式化的无符号整数参数为八进制。
%x：格式化的无符号整数参数为十六进制。
%c：格式化整数参数为单个字符。
%f：格式化双精度参数为浮点数。
%e：格式固定格式的双精度参数。
%s：打印指定的空值终止的数组。
%p：打印给定指针的内存地址。
%%：写一个％字符。

例如

/* Writes the formatted data. */
if (0>fprintf(stream, "The %s is %d.", "number", 2))
{
/* Error occurred while writing formatted data. */
}

成功返回写入流字符串，当应用程序的正常终止。当一个换行符是写在行缓冲的情况下。当缓冲区已满。
当流被关闭。下会失败
调用int fflush(FILE* stream);刷新缓冲区

char data[] = { 'h', 'e', 'l', 'l', 'o', '\n' };
size_t count = sizeof(data) / sizeof(data[0]);
/* Write data to stream. */
fwrite(data, sizeof(char), count, stream);
/* Flush the output buffer. */
if (EOF == fflush(stream))
{
/* Error occured while flushing the buffer. */
}

fputs(“abcd”,stream);写入abcd到流中

读取流

size_t fread(void* data, size_t size, size_t count, FILE* stream);
读取字符，从stream中读取4位char到buffer中，成功则返回count

char buffer[5];
size_t count = 4;
/* Read 4 characters from the stream. */
if (count ! = fread(buffer, sizeof(char), count, stream))
{
/* Error occured while reading from the stream. */
}
else
{
/* Null terminate. */
buffer[4] = NULL;
/* Output buffer. */
MY_LOG_INFO("read: %s", buffer);
}

读取字符串,读取最多1024个字符到buffer中

char buffer[1024];
/* Read newline terminated character sequence from the stream. */
if (NULL == fgets(buffer, 1024, stream))
{
/* Error occured while reading the stream. */
}
else
{
MY_LOG_INFO("read: %s", buffer);

fgetc方法可读取一个简单无符号类型字符

unsigned char ch;
int result;
/* Read a single character from the stream. */
result = fgetc(stream);
if (EOF == result)
{
/* Error occured while reading from the stream. */
}
else
{
/* Get the actual character. */
ch = (unsigned char) result;
}

读取格式化字符 int fscanf(FILE* stream, const char* format, …);

char s[5];
int i;
/* Stream has "The number is 2" */
/* Reads the formatted data. */
if (2 != fscanf(stream, "The %s is %d", s, &i))
{
/* Error occured while reading formatted data. */
}

成功返回读取的个数，失败返回EOF，s为number i为2

判断是否到文件流结尾int feof(FILE* stream);到达结尾则返回一个非0值

char buffer[1024];
/* Until the end of the file. */
while (0 == feof(stream))
{
/* Read and output string. */
fgets(buffer, 1024, stream);
MY_LOG_INFO("read: %s", buffer);

指针流偏移fseek函数 fseek(stream, -4, SEEK_CUR);
SEEK_SET：偏移量是相对于流的开头。
SEEK_CUR：偏移量是相对于当前位置。
SEEK_END：偏移是相对于流的末尾。

先写入abcd，然后从当前位置向前偏移4位覆盖写入efgh

/* Write to the stream. */
fputs("abcd", stream);
/* Rewind for 4 bytes. */
fseek(stream, -4, SEEK_CUR);
/* Overwrite abcd with efgh. */
fputs("efgh", stream);

返回0则成功，否则返回非零

检查流是否在之前发生过错误，如有则返回非零值

if (0 != ferror(stream))
{
/* Error occured on the previous request. */
}

关闭流

if (0 != fclose(stream))
{
/* Error occured while closing the stream. */
}

执行shell命令

include

int result;
= system("mkdir /data/data/com.example.hellojni/temp");

打开一个流指针
FILE popen(const char command, const char* type);
关闭 pclose

#include
...
FILE* stream;
/* Opening a read-only channel to ls command. */
stream = popen("ls", "r");
if (NULL == stream)
{
MY_LOG_ERROR("Unable to execute the command.");
}
else
{
char buffer[1024];
int status;
/* Read each line from command output. */
while (NULL ! = fgets(buffer, 1024, stream))
{
MY_LOG_INFO("read: %s", buffer);
}
/* Close the channel and get the status. */
status = pclose(stream);
MY_LOG_INFO("process exited with status %d", status);
}

获取系统属性

int __system_property_get(const char* name, char* value);
通过名字获取一个属性，ro.product.model为当前系统属性，获取值为如HM-2A

include

char value[PROP_VALUE_MAX];
/* Gets the product model system property. */
if (0 == __system_property_get("ro.product.model", value))
{
/* System property is not found or it has an empty value. */
} {
MY_LOG_INFO("product model: %s", value);

找一个系统值__system_property_find

const prop_info* property;
/* Gets the product model system property. */
property = __system_property_find("ro.product.model");
if (NULL == property)
{
/* System property is not found. */
}
else
{
char name[PROP_NAME_MAX];
char value[PROP_VALUE_MAX];
/* Get the system property name and value. */
if (0 == __system_property_read(property, name, value))
{
MY_LOG_INFO("%s is empty.");
}
else
{
MY_LOG_INFO("%s: %s", name, value);
}
}

__system_property_read函数为读取值，name数组为可选参数，如果有，则读取的值不大于name数组长度，value为读取返回的值，读取的值不大于value数组的长度

用户和组

uid_t uid;
/* Get the application user ID. */
uid = getuid();
MY_LOG_INFO("Application User ID is %u", uid);

char* username;
/* Get the application user name. */
username = getlogin();
MY_LOG_INFO("Application user name is %s", username);