snprintf函数使用总结

一直有接触snprintf,经久不用知识点又会模糊,记录下来以便日后查看

【依赖头文件】

#include<stdio.h>

【函数原型】

int snprintf(char *str, size_t size, const char *format, ...);

【函数参数】

str:目标字符串；size:拷贝字节数(Bytes); format:源字符串; ...格式

【函数功能】

最多从源字符串format中拷贝size字节的内容(含字符串结尾标志'\0')到目标字符串

The functions snprintf() write at most size bytes (including the terminating null byte ('\0')) to str.

【返回值】

成功返回源串的长度(strlen, 不含'\0')

失败返回负值

【例子及理解】

由于最多拷贝size个字节到目标字符串,那么通常目标字符串就设置成size大小就不会有越界问题

下面将目标字符串的长度设置为size大小,分别实验源串长度小于,等于,大于size的情况

【情形一】:源串小于目标字符串

实际上源串为:"123\0"，所以只将字符'1'，'2'，'3', '\0'拷到了目标串,返回值为源串的strlen为3

  1 #include <stdio.h>
  2 #include <strings.h>
  3 #include <string.h>
  4 int main(void)
  5 {   
  6     char a[10] = {'\0'};
  7     int i = 0;
  8     int ret = 0;
  9     memset(a, 1, sizeof(a));
 10     for(i = 0; i < 10; i++)
 11     {   
 12         printf("a[%d] = %d\n",i,a[i]);
 13     }
 14     
 15     ret = snprintf(a, 10, "%d", 123);
 16     printf("ret = %d\n",ret);
 17     
 18     for(i = 0; i < 10; i++)
 19     {
 20         printf("a[%d] = %d\n",i,a[i]);
 21     }
 22     return 0;
 23 }

root@ubuntu:/shiyan/shiyan265# gcc snprintf.c 
root@ubuntu:/shiyan/shiyan265# ./a.out
a[0] = 1
a[1] = 1
a[2] = 1
a[3] = 1
a[4] = 1
a[5] = 1
a[6] = 1
a[7] = 1
a[8] = 1
a[9] = 1
ret = 3
a[0] = 49
a[1] = 50
a[2] = 51
a[3] = 0
a[4] = 1
a[5] = 1
a[6] = 1
a[7] = 1
a[8] = 1
a[9] = 1
root@ubuntu:/shiyan/shiyan265#

【情形二】:源串等于目标字符串

源串为"abcdefghi\0"，正好将源串拷贝到目标字符串中,返回值为源串的strlen为9

  1 #include <stdio.h>
  2 #include <strings.h>
  3 #include <string.h>
  4 int main(void)
  5 {   
  6     char a[10] = {'\0'};
  7     int i = 0;
  8     int ret = 0;
  9     memset(a, 1, sizeof(a));
 10     for(i = 0; i < 10; i++)
 11     {   
 12         printf("a[%d] = %d\n",i,a[i]);
 13     }
 14     
 15     //ret = snprintf(a, 10, "%d", 123);
 16     ret = snprintf(a, 10, "%s","abcdefghi");
 17     printf("ret = %d\n",ret);
 18     
 19     for(i = 0; i < 10; i++)
 20     {   
 21         printf("a[%d] = %d\n",i,a[i]);
 22     }
 23     return 0;

root@ubuntu:/shiyan/shiyan265# gcc snprintf.c 
root@ubuntu:/shiyan/shiyan265# ./a.out
a[0] = 1
a[1] = 1
a[2] = 1
a[3] = 1
a[4] = 1
a[5] = 1
a[6] = 1
a[7] = 1
a[8] = 1
a[9] = 1
ret = 9
a[0] = 97
a[1] = 98
a[2] = 99
a[3] = 100
a[4] = 101
a[5] = 102
a[6] = 103
a[7] = 104
a[8] = 105
a[9] = 0
root@ubuntu:/shiyan/shiyan265#

【情形三】源串大于目标字符串

源串为"abcdefghijklmnopq\0"，最多从源串拷贝10个字节(含\0)到目标串那么就是拷贝源串的9个字节内容(abcdefghi)再加一个\0到目标串,目标串的结果和情形二一样,但是ret返回值变成了17,即strlen("abcdefghijklmnopq")

  1 #include <stdio.h>
  2 #include <strings.h>
  3 #include <string.h>
  4 int main(void)
  5 {
  6     char a[10] = {'\0'};
  7     int i = 0;
  8     int ret = 0;
  9     memset(a, 1, sizeof(a));
 10     for(i = 0; i < 10; i++)
 11     {
 12         printf("a[%d] = %d\n",i,a[i]);
 13     }
 14 
 15     //ret = snprintf(a, 10, "%d", 123);
 16     //ret = snprintf(a, 10, "%s","abcdefghi");
 17     ret = snprintf(a, 10, "%s", "abcdefghijklmnopq");
 18     printf("ret = %d\n",ret);
 19 
 20     for(i = 0; i < 10; i++)
 21     {
 22         printf("a[%d] = %d\n",i,a[i]);
 23     }
 24     return 0;
 25 }

root@ubuntu:/shiyan/shiyan265# gcc snprintf.c 
root@ubuntu:/shiyan/shiyan265# ./a.out
a[0] = 1
a[1] = 1
a[2] = 1
a[3] = 1
a[4] = 1
a[5] = 1
a[6] = 1
a[7] = 1
a[8] = 1
a[9] = 1
ret = 17
a[0] = 97
a[1] = 98
a[2] = 99
a[3] = 100
a[4] = 101
a[5] = 102
a[6] = 103
a[7] = 104
a[8] = 105
a[9] = 0
root@ubuntu:/shiyan/shiyan265#

snprintf函数使用总结

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