C中的字符串流

Question

print2fp(const void *buffer, size_t size, FILE *stream) {

 if(fwrite(buffer, 1, size, stream) != size)
  return -1;

 return 0;
}

如何将数据写入字符串流而不是文件流？

Answer 1

posix 2008标准中有一个非常简洁的功能：open_memstream（）。你这样使用它：

char* buffer = NULL;
size_t bufferSize = 0;
FILE* myStream = open_memstream(&buffer, &bufferSize);

fprintf(myStream, "You can output anything to myStream, just as you can with stdout.\n");
myComplexPrintFunction(myStream);    //Append something of completely unknown size.

fclose(myStream);    //This will set buffer and bufferSize.
printf("I can do anything with the resulting string now. It is: \"%s\"\n", buffer);
free(buffer);

Answer 2

使用sprintf：http://www.cplusplus.com/reference/cstdio/sprintf/

这是参考的一个例子：

#include <stdio.h>

int main ()
{
  char buffer [50];
  int n, a=5, b=3;
  n = sprintf(buffer, "%d plus %d is %d", a, b, a+b);
  printf("[%s] is a string %d chars long\n", buffer, n);
  return 0;
}

输出：

[5 plus 3 is 8] is a string 13 chars long

根据评论中的建议进行更新：

使用snprintf因为它更安全（它可以防止缓冲区溢出攻击）并且更加便携。

#include <stdio.h>

int main ()
{
  int sizeOfBuffer = 50;
  char buffer[sizeOfBuffer];
  int n, a = 5, b = 3;
  n = snprintf(buffer, sizeOfBuffer, "%d plus %d is %d", a, b, a+b);
  printf ("[%s] is a string %d chars long\n", buffer, n);
  return 0;
}

请注意，snprintf的第二个参数实际上是要使用的最大允许大小，因此您可以将其设置为比sizeOfBuffer更低的值，但是对于您的情况，它将是不必要的。 snprintf只写sizeOfBuffer-1字符并使用最后一个字节作为终止字符。

以下是snprintf文档的链接：http://www.cplusplus.com/reference/cstdio/snprintf/