sprintf和snprintf需要注意的地方

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sprintf

int sprintf ( char * str, const char * format, ... );
Write formatted data to string Composes a string with the same text that would be printed if  format was used on  printf, but instead of being printed, the content is stored as a  C string in the buffer pointed by  str.

The size of the buffer should be large enough to contain the entire resulting string (see  snprintf for a safer version). //buffer的大小必须能够包含format后形成的字符串的长度，不然会溢出。

A terminating null character is automatically appended after the content. //自动添加'\\0'

After the  format parameter, the function expects at least as many additional arguments as needed for  format.

Parameters

str
Pointer to a buffer where the resulting C-string is stored.
The buffer should be large enough to contain the resulting string.
format
C string that contains a format string that follows the same specifications as  format in  printf (see  printf for details).
...  (additional arguments)
Depending on the  format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a  format specifier in the  format string (or a pointer to a storage location, for  n).
There should be at least as many of these arguments as the number of values specified in the  format specifiers. Additional arguments are ignored by the function.

Return Value

On success, the total number of characters written is returned. This count does not include the additional null-character automatically appended at the end of the string.
On failure, a negative number is returned.
返回值是形成字符串以后，该字符串的长度（不包括自动添加的空字符），例如下面的例子 "%d plus %d is %d"形成的字符串为" 5 plus 3 is 8 "，因此返回值为13。

Example

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/* sprintf example */
#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;
Edit & Run


Output: 

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

snprintf

int snprintf ( char * s, size_t n, const char * format, ... );
Write formatted output to sized buffer Composes a string with the same text that would be printed if  format was used on  printf, but instead of being printed, the content is stored as a  C string in the buffer pointed by  s (taking  n as the maximum buffer capacity to fill).

If the resulting string would be longer than n-1 characters, the remaining characters are discarded and not stored, but counted for the value returned by the function. //snprintf的返回值是欲写入的字符串长度，而不是实际写入的字符串长度

A terminating null character is automatically appended after the content written.

After the  format parameter, the function expects at least as many additional arguments as needed for  format.

Parameters

s
Pointer to a buffer where the resulting C-string is stored.
The buffer should have a size of at least  n characters.
n
Maximum number of bytes to be used in the buffer.
The generated string has a length of at most n-1, leaving space for the additional terminating null character. //最多拷贝n-1个字符
size_t is an unsigned integral type.
format
C string that contains a format string that follows the same specifications as  format in  printf (see  printf for details).
...  (additional arguments)
Depending on the  format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a  format specifier in the  format string (or a pointer to a storage location, for  n).
There should be at least as many of these arguments as the number of values specified in the  format specifiers. Additional arguments are ignored by the function.

Return Value

The number of characters that would have been written if  n had been sufficiently large, not counting the terminating  null character.
If an encoding error occurs, a negative number is returned.
Notice that only when this returned value is non-negative and less than  n, the string has been completely written.

Example

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/* snprintf example */
#include <stdio.h>

int main ()

  char buffer [100];
  int cx;

  cx = snprintf ( buffer, 100, "The half of %d is %d", 60, 60/2 );

  if (cx>=0 && cx<100)      // check returned value

    snprintf ( buffer+cx, 100-cx, ", and the half of that is %d.", 60/2/2 );

  puts (buffer);

  return 0;
Edit & Run


Output: 
The half of 60 is 30, and the half of that is 15.

For more examples on formatting see  printf.


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