一直有接觸snprintf,經久不用知識點又會模糊,記錄下來以便日後查看
【依賴頭文件】
#include<stdio.h>
【函數原型】
int snprintf(char *str, size_t size, const char *format, ...);
【函數參數】
str:目標字符串；size:拷貝字節數(Bytes); format:源字符串; ...格式
【函數功能】
最多從源字符串format中拷貝size字節的內容(含字符串結尾標誌'\0')到目標字符串
The functions snprintf() write at most size bytes (including the terminating null byte ('\0')) to str.
【返回值】
成功返回源串的長度(strlen, 不含'\0')
失敗返回負值
【例子及理解】
由於最多拷貝size個字節到目標字符串,那麼通常目標字符串就設置成size大小就不會有越界問題
下面將目標字符串的長度設置爲size大小,分別實驗源串長度小於,等於,大於size的情況
【情形一】:源串小於目標字符串
實際上源串爲:"123\0"，所以只將字符'1'，'2'，'3', '\0'拷到了目標串,返回值爲源串的strlen爲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】:源串等於目標字符串
源串爲"abcdefghi\0"，正好將源串拷貝到目標字符串中,返回值爲源串的strlen爲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】源串大於目標字符串
源串爲"abcdefghijklmnopq\0"，最多從源串拷貝10個字節(含\0)到目標串那麼就是拷貝源串的9個字節內容(abcdefghi)再加一個\0到目標串,目標串的結果和情形二一樣,但是ret返回值變成了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#
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