您需要留出一些空间来存储输入；由于您不提前知道输入有多大，因此您必须对存储进行一些创意。
一个常见的策略是使用一个较小的、固定大小的缓冲区从输入流中读取，并使用一个动态的、可调整大小的缓冲区来存储完整的字符串，如果它最终比固定大小的缓冲区可以处理的长度更长。这样，您可以以离散的块读取任意长的输入行，然后将这些块粘贴在一起，并根据需要调整目标缓冲区的大小。
您将从控制台循环读取固定大小的块并将其存储到动态缓冲区，直到您看到换行符，此时您退出输入循环。理想情况下，您的固定大小缓冲区应该足够大以处理最合理的情况，这样您就不需要扩展动态缓冲区。
冗长（未经测试！）示例：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define INPUT_BUF_SIZE 21 // handle strings up to 20 characters in length
int main( void )
{
/**
* Set aside a fixed-size buffer to store input from the console. This
* buffer cannot be resized after it has been allocated.
*/
char inputBuf[INPUT_BUF_SIZE];
/**
* Dynamically allocate a buffer to store the final string;
* its initial size is the same as the fixed buffer. If necessary,
* this buffer may be extended with the realloc function. We're using
* calloc instead of malloc to make sure the initial array contents
* are set to 0.
*/
char *finalBuf = calloc( INPUT_BUF_SIZE, sizeof *finalBuf );
/**
* finalBufSize tracks the total size of the dynamic buffer; finalBufLen
* tracks the length of the string currently stored in the buffer.
* These are not the same thing.
*/
size_t finalBufSize = INPUT_BUF_SIZE;
size_t finalBufLen = 0; // initially no string stored in the buffer
/**
* Read from standard input into the fixed-size buffer; loop until
* we see EOF or there's an error on input.
*/
while ( fgets( inputBuf, sizeof inputBuf, stdin ) )
{
/**
* If there isn't enough space left in finalBuf to store
* the latest chunk, double the size of finalBuf. This strategy
* minimizes the number of times you have to call realloc (which
* can be expensive).
*/
if ( strlen( inputBuf ) + finalBufLen > finalBufSize )
{
/**
* *Always* assign the result of realloc to a temp variable; if the
* call fails it will return NULL, and if you overwrite the value
* of finalBuf with NULL, you'll lose your only reference to any
* previously allocated memory.
*/
char *tmp = realloc( finalBuf, finalBufSize * 2 );
if ( tmp )
{
finalBuf = tmp;
finalBufSize *= 2;
}
else
{
/**
* We can't extend the buffer anymore, so we'll exit the
* loop and work with what we have.
*/
fprintf( stderr, "Could not extend storage buffer, exiting input loop\n" );
break;
}
}
/**
* Append the input string to the target buffer.
*/
strcat( finalBuf, inputBuf );
finalBufLen = strlen( finalBuf );
/**
* Did we see a newline in the last input chunk? If so,
* remove that newline from the final string (unless you
* want to include that in your reversal) and exit
* the loop.
*/
char *newline = strchr( finalString, '\n' );
if ( newline )
{
*newline = 0; // overwrite the newline character with the string terminator
break;
}
}
此时，finalBuf 包含来自控制台的输入，您可以反转此字符串以进行输出。完成后，释放使用 free 函数分配的内存，如下所示：
free( finalBuf );
理想情况下，您应该将所有输入处理分离到它自己的函数中，但现在这已经足够说明问题了。