strncpy不能正確存儲字符C

Question

基本上，我使用strncpy截斷字符，如果它大於字符數組大小。

所以，我有以下變量和方法。

char studentName[6]; 
char colour[5]; 
char music[7]; 

strcpy(this->studentName, "null"); 
strcpy(this->colour, "null"); 
strcpy(this->music, "null"): 

void setName (char* studentName) 
{ 
strncpy(this->studentName, studentName, 6); 
this->studentName[6] = '\0'; // SET LAST TO NULL POINTER 
} 

void setColour (char* colour) 
{ 
strncpy(this->colour, colour, 5); 
this->colour[5] = '\0'; // SET LAST TO NULL POINTER 
} 

void setMusic (char* music) 
{ 
strncpy(this->music, music, 7); 
this->music[7] = '\0'; // SET LAST TO NULL POINTER 
} 

所以，如果我設置了學生的名字Jackson，它將截斷至Jackso，但是，我的colour變量將是空白，我music變量將是null。

另外，如果我嘗試...

void setName (char* studentName) 
{ 
    strncpy(this->studentName, studentName, 6); 
    this->studentName[6-1] = '\0'; // SET LAST TO NULL POINTER 
} 

void setColour (char* colour) 
{ 
strncpy(this->colour, colour, 5); 
this->colour[5-1] = '\0'; // SET LAST TO NULL POINTER 
} 

void setMusic (char* music) 
{ 
strncpy(this->music, music, 7); 
this->music[7-1] = '\0'; // SET LAST TO NULL POINTER 
} 

，我設置了學生的名字Jackson我得到這樣的Jacksonull。它將null添加到末尾

Answer 1

嘗試使用sizeof()而不是原始數字。它允許你改變數組的大小，而不用接觸你的代碼。

您首先複製sizeof(array)-1符號，然後設置索引爲sizeof(array)-1（因爲索引從0開始）到'\0'的最後一個符號。

您的代碼編輯應該是這樣的：

char studentName[6]; 
char colour[5]; 
char music[7]; 

strcpy(this->studentName, "null"); 
strcpy(this->colour, "null"); 
strcpy(this->music, "null"): 

void setName (char* studentName) 
{ 
strncpy(this->studentName, studentName, sizeof(this->studentName)-1); // Copy n-1 chars 
this->studentName[sizeof(this->studentName)-1] = '\0'; // n is set to null symbol 
} 

void setColour (char* colour) 
{ 
strncpy(this->colour, colour, sizeof(this->color)-1); 
this->colour[sizeof(this->color)-1] = '\0'; 
} 

void setMusic (char* music) 
{ 
strncpy(this->music, music, sizeof(this->music)-1); 
this->music[sizeof(this->music)-1] = '\0'; 
} 

Answer 2

這裏（假設Linux平臺上你可能有，如果你在Windows，而不是一定要取是strlcpy執行。）

#include <bsd/string.h> 

char studentName[6]; /* Good to know: Globals are initialized to zero */ 
char colour[5]; 
char music[7]; 

/* Pretty useless function since a single strlcpy call is enough */ 
static size_t setX(char *buf, size_t buflen, const char *new_val) { 
    return strlcpy(buf, new_val, buflen); 
} 

int main(int argc, char **argv) { 
    setX(studentName, 6, "Peter"); /* Please read man page and check return value */ 
} 

現在strlcpy只要長度參數> 0，就保證NUL終止。