C library macro - NULL
C standard library - <stddef.h>
description
C library macroNULL value is a null pointer constant.It can be defined as((void *) 0), 0 or 0L,depending on the compiler vendor.
statement
The following is dependent on the compiler NULL macro statement.
#define NULL ((char *)0) 或 #define NULL 0L 或 #define NULL 0
parameter
- NA
return value
- NA
Examples
The following example demonstrates NULL macro usage.
#include <stddef.h>
#include <stdio.h>
int main ()
{
FILE *fp;
fp = fopen("file.txt", "r");
if( fp != NULL )
{
printf("成功打开文件 file.txt\n");
fclose(fp);
}
fp = fopen("nofile.txt", "r");
if( fp == NULL )
{
printf("不能打开文件 nofile.txt\n");
}
return(0);
}
Assuming that the filefile.txt already exists, but nofile.txtdoes not exist. Let's compile and run the above program, which will result in the following:
成功打开文件 file.txt 不能打开文件 nofile.txt