blocks|key|1044532|text|在C中，“流”是一种抽象；从程序的角度来看，它只是字节的生产者(输入流)或消费者(输出流)。它可以对应于磁盘上的文件，对应于管道，对应于您的终端，或者对应于某些其他设备，如打印机或tty。FILE类型包含有关流的信息。通常，您不会直接处理FILE对象的内容，只需将指向它的指针传递给各种I/O例程即可。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1044533|有三个标准流：stdin是指向标准输入流的指针，stdout是指向标准输出流的指针，stderr是指向标准错误输出流的指针。在交互式会话中，这三个通常指的是您的控制台，但您可以将它们重定向到其他文件或设备：|1044534|$+myprog+<+inputfile.dat+>+output.txt+2>+errors.txt|code-block|syntax|javascript|1044535|在本例中，stdin现在指向inputfile.dat，stdout指向output.txt，stderr指向errors.txt。|1044536|fprintf将格式化文本写入您指定的输出流。|1044537|printf等效于编写fprintf(stdout,+...)，并将格式化文本写入标准输出流当前指向的任何位置。|1044538|sprintf将格式化文本写入char数组，而不是流。|1044539|entityMap^0|2M|4|3B|4|0|7|5|O|6|16|6|0|0|5|5|E|D|S|6|10|A|1B|6|1J|A|0|0|7|0|0|6|B|K|0|0|7|F|4|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]|$9|16|A|17|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]|$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]|$9|1K|A|1L|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|1M|8|@$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1P|8|@$9|1Q|A|1R|B|C]|$9|1S|A|1T|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|1U|8|@$9|1V|A|1W|B|C]|$9|1X|A|1Y|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|1Z|8|@]|D|@]|E|$]]]|V|$]]

In C, a "stream" is an abstraction; from the program's perspective it is simply a producer (input stream) or consumer (output stream) of bytes. It can correspond to a file on disk, to a pipe, to your terminal, or to some other device such as a printer or tty. The <code>FILE</code> type contains information about the stream. Normally, you don't mess with a <code>FILE</code> object's contents directly, you just pass a pointer to it to the various I/O routines. 

There are three standard streams: <code>stdin</code> is a pointer to the standard input stream, <code>stdout</code> is a pointer to the standard output stream, and <code>stderr</code> is a pointer to the standard error output stream. In an interactive session, the three usually refer to your console, although you can redirect them to point to other files or devices:

<pre><code>$ myprog &lt; inputfile.dat &gt; output.txt 2&gt; errors.txt
</code></pre>

In this example, <code>stdin</code> now points to <code>inputfile.dat</code>, <code>stdout</code> points to <code>output.txt</code>, and <code>stderr</code> points to <code>errors.txt</code>. 

<code>fprintf</code> writes formatted text to the output stream you specify. 

<code>printf</code> is equivalent to writing <code>fprintf(stdout, ...)</code> and writes formatted text to wherever the standard output stream is currently pointing.

<code>sprintf</code> writes formatted text to an array of <code>char</code>, as opposed to a stream.

blocks|key|1044404|text|到标准输出流(stdout)的printf输出|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1044405|fprintf转到文件句柄(FILE*)|1044406|sprintf转到您分配的缓冲区。(char*)|1044407|entityMap^0|7|6|F|6|0|0|7|E|5|0|0|7|I|5|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|V|8|@$9|W|A|X|B|C]|$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|K|$]]

<code>printf</code> outputs to the standard output stream (<code>stdout</code>)

<code>fprintf</code> goes to a file handle (<code>FILE*</code>)

<code>sprintf</code> goes to a buffer you allocated. (<code>char*</code>)

blocks|key|829450|text|printf(...)等同于fprintf(stdout,...)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|829451|fprintf用于输出到流。|829452|sprintf(buffer,...)用于将字符串格式化为缓冲区。|829453|请注意，还有vsprintf、vfprintf和vprintf|829454|entityMap^0|0|B|E|J|0|0|7|0|0|J|0|6|8|F|8|O|7|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|M|$]]

<code>printf(...)</code> is equivalent to <code>fprintf(stdout,...)</code>.

<code>fprintf</code> is used to output to stream.

<code>sprintf(buffer,...)</code> is used to format a string to a buffer.

Note there is also <code>vsprintf</code>, <code>vfprintf</code> and <code>vprintf</code>

blocks|key|829549|text|您还可以使用vsnprintf()函数执行非常有用的操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|829550|$+cat+test.cc
#include+<exception>
#include+<stdarg.h>
#include+<stdio.h>

struct+exception_fmt+:+std::exception
{
++++exception_fmt(char+const*+fmt,+...)+__attribute__+((format(printf,2,3)));
++++char+const*+what()+const+throw()+{+return+msg_;+}
++++char+msg_[0x800];
};

exception_fmt::exception_fmt(char+const*+fmt,+...)
{
++++va_list+ap;
++++va_start(ap,+fmt);
++++vsnprintf(msg_,+sizeof+msg_,+fmt,+ap);
++++va_end(ap);
}

int+main(int+ac,+char**+av)
{
++++throw+exception_fmt("%25s:+bad+number+of+arguments+%25d",+*av,+ac);
}

$+g%2B%2B+-Wall+-o+test+test.cc

$+./test
terminate+called+after+throwing+an+instance+of+'exception_fmt'
++what():++./test:+bad+number+of+arguments+1
Aborted+(core+dumped)|code-block|syntax|javascript|829551|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can also do very useful things with vsnprintf() function:

<pre><code>$ cat test.cc
#include &lt;exception&gt;
#include &lt;stdarg.h&gt;
#include &lt;stdio.h&gt;

struct exception_fmt : std::exception
{
 exception_fmt(char const* fmt, ...) __attribute__ ((format(printf,2,3)));
 char const* what() const throw() { return msg_; }
 char msg_[0x800];
};

exception_fmt::exception_fmt(char const* fmt, ...)
{
 va_list ap;
 va_start(ap, fmt);
 vsnprintf(msg_, sizeof msg_, fmt, ap);
 va_end(ap);
}

int main(int ac, char** av)
{
 throw exception_fmt("%s: bad number of arguments %d", *av, ac);
}

$ g++ -Wall -o test test.cc

$ ./test
terminate called after throwing an instance of 'exception_fmt'
 what(): ./test: bad number of arguments 1
Aborted (core dumped)
</code></pre>

blocks|key|829672|text|fprintf这与streams相关，其中as+printf是类似于fprintf但与streams无关的语句，即fprintf与文件相关|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|829673|entityMap^0|0|7|O|6|Y|7|1L|7|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|G|$]]

<code>fprintf</code> This is related with streams where as <code>printf</code> is a statement similar to <code>fprintf</code> but not related to streams, that is <code>fprintf</code> is file related

blocks|key|829734|text|sprintf:将格式化数据写入内存中的字符串，而不是标准输出|type|unstyled|depth|inlineStyleRanges|entityRanges|data|829735|sprintf的语法是：|829736|#include+<stdio.h>
int+sprintf+(char+*string,+const+char+*format
[,item+[,item]…]);|code-block|syntax|javascript|829737|这里,|829738|字符串指向内存中要写入数据的缓冲区的指针。|829739|格式是指指向定义格式的字符串的指针。|829740|每一项都是指定要写入的数据的变量或表达式。|829741|如果操作成功，则sprintf返回的值大于或等于零，换句话说，返回写入的字符数，不计算终止空字符，如果发生错误，则返回小于零的值。|829742|printf:打印到标准输出|829743|printf的语法为：|829744|printf+format+[argument]…|829745|sprintf()和printf()之间的唯一区别是sprintf()将数据写入字符数组，而printf()将数据写入标准输出设备。|829746|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|14|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|15|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|16|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|17|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|18|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|19|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1A|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1B|8|@]|9|@]|A|$]]|$1|W|3|X|5|F|7|1C|8|@]|9|@]|A|$G|H]]|$1|Y|3|Z|5|6|7|1D|8|@]|9|@]|A|$]]|$1|10|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]]|11|$]]

sprintf: Writes formatted data to a character string in memory instead of stdout

Syntax of sprintf is:

<pre><code>#include &lt;stdio.h&gt;
int sprintf (char *string, const char *format
[,item [,item]…]);
</code></pre>

Here,

String refers to the pointer to a buffer in memory where the data is to be written.

Format refers to pointer to a character string defining the format.

Each item is a variable or expression specifying the data to write.

The value returned by sprintf is greater than or equal to zero if the operation is successful or in other words the number of characters written, not counting the terminating null character is returned and returns a value less than zero if an error occurred.

printf: Prints to stdout

Syntax for printf is:

<pre><code>printf format [argument]…
</code></pre>

The only difference between sprintf() and printf() is that sprintf() writes data into a character array, while printf() writes data to stdout, the standard output device.

Can anyone explain in simple English about the differences between <code>printf</code>, <code>fprintf</code>, and <code>sprintf</code> with examples?

What stream is it in?

I'm really confused between the three of these while reading about "File Handling in C".

Difference between fprintf, printf and sprintf?

有谁能用简单的英语举例解释一下printf、fprintf和sprintf之间的区别吗？它在什么流中？在阅读“C中的文件处理”时，我真的把这三种语言弄糊涂了。

问fprintf，printf和sprintf之间的区别？
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回答 6

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