C語言標準庫 經常使用函數說明

void *memset(void *str, int c, size_t n)

Syntax

　　void *memset(void *str, int c, size_t n)

Description:

　　　　The C library function void *memset(void *str, int c, size_t n) copies the character c (an unsigned char) to the first n characters of the 　　　string pointed to, by the argument str.

Parameters:

• str -- This is a pointer to the block of memory to fill.
• c -- This is the value to be set. The value is passed as an int, but the function fills the block of memory using the unsigned char conversion of this value.
• n -- This is the number of bytes to be set to the value.

Return Value:

　　This function returns a pointer to the memory area str.

 

void *memchr(const void *str, int c, size_t n)

Description

　　　　The C library function void *memchr(const void *str, int c, size_t n) searches for the first occurrence of the character c (an unsigned 　　char) in the first n bytes of the string pointed to, by the argument str.

Parameters

• str -- This is the pointer to the block of memory where the search is performed.

• c -- This is the value to be passed as an int, but the function performs a byte per byte search using the unsigned char conversion of this value.

• n -- This is the number of bytes to be analyzed.

Return Value

This function returns a pointer to the matching byte or NULL if the character does not occur in the given memory area.

從buf所指內存區域的前count個字節查找字符ch。當第一次遇到字符ch時中止查找。若是成功，返回指向字符ch的指針；不然返回NULL。

 

int memcmp(const void *str1, const void *str2, size_t n)

Description

　　　　The C library function int memcmp(const void *str1, const void *str2, size_t n)) compares the first n bytes of memory area str1 and 　　memory area str2.

Parameters

• str1 -- This is the pointer to a block of memory.

• str2 -- This is the pointer to a block of memory.

• n -- This is the number of bytes to be compared.

Return Value

• if Return value < 0 then it indicates str1 is less than str2.

• if Return value > 0 then it indicates str2 is less than str1.

• if Return value = 0 then it indicates str1 is equal to str2.

 

void *memmove(void *str1, const void *str2, size_t n)

Description

　　　　The C library function void *memmove(void *str1, const void *str2, size_t n) copies n characters from str2 to str1, but for 　　　　　　overlapping memory blocks, memmove() is a safer approach than memcpy().

Declaration

　　Following is the declaration for memmove() function.

Parameters

• str1 -- This is a pointer to the destination array where the content is to be copied, type-casted to a pointer of type void*.

• str2 -- This is a pointer to the source of data to be copied, type-casted to a pointer of type void*.

• n -- This is the number of bytes to be copied.

Return Value

This function returns a pointer to the destination, which is str1.