The three types of buffering available are unbuffered, block buffered, and line buffered. When an output stream is unbuffered, information appears on the destination file or terminal as soon as written; when it is block buffered many characters are saved up and written as a block; when it is line buffered characters are saved up until a newline is output or input is read from any stream attached to a terminal device (typically stdin). The function fflush(3) may be used to force the block out early. (See fclose(3).) Normally all files are block buffered. When the first I/O operation occurs on a file, malloc(3) is called, and a buffer is obtained. If a stream refers to a terminal (as stdout normally does) it is line buffered. The standard error
stream stderr is always unbuffered by default.
这个函数应该必须在如何输出被写到该文件之前调用。一般放在main里靠前面的语句!但是setbuf有个经典的错误,man手册上也提到了,c陷阱和缺陷上也提到了
You must make sure that both buf and the space it points to still exist by the time stream is closed, which also happens at program termination. For example, the following is illegal:
The function fflush forces a write of all user-space buffered data for the given output or update stream via the stream underlying write function. The open status of the stream is unaffected. If the stream argument is NULL, fflush flushes all open output streams.
但是fflush仅仅刷新C库里的缓冲。其他的一些数据的刷新需要调用fsync或者sync!
Note that fflush() only flushes the user space buffers provided by the C library. To ensure that the data is physically stored on disk the kernel buffers must be flushed too, e.g. with sync(2) or fsync(2).
fsync()和sync()
fsync和sync最终将缓冲的数据更新到文件里。
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#include <unistd.h>
int fsync(int fd);
fsync copies all in-core parts of a file to disk, and waits until the device reports that all parts are on stable storage. It also updates metadata stat information. It does not necessarily ensure that the entry in the directory containing the file has also reached disk. For that an explicit fsync on the file descriptor of the directory is also needed.
#include <stdio.h> int p(int a,int b)
{
return 3;
}
int main()
{
printf("%x\n",p);
int a = p(2,3);
printf("%d\n",p);
int b = p(4,5);
printf("%x\n",p);
return 0;
}
#include <stdio.h> int p(int a,int b)
{
return ((a>b)?a:b);
}
int main()
{
int (*ptr)(int ,int);
ptr = (int (*)(int,int))0x411159;
int c = ptr(5,6);
printf("%d\n",c);
return 0;
}