if use open() open a file with flags O_APPEND,you may not reset the filesize with lseek()+write().

本文通过两个示例代码对比展示了在使用O_APPEND标志打开文件时，如何影响lseek()+write()组合操作来调整文件大小。当不使用O_APPEND标志时，可以通过lseek()+write()将文件指针定位到特定位置并写入数据来调整文件大小。

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1:open a file with flags O_APPEND,you may can not reset filesize with lseek()+write().

code1:

#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
        int ret=0;
        int fd=-1;
        struct stat sb;
        fd = open("test.txt",O_RDWR|O_APPEND|O_CREAT|O_TRUNC,00777);//flags with O_APPEND
        if(fd==-1)
        {
                perror("open() error:");
        }
        ret = lseek(fd,0,SEEK_END);
        if(ret==-1)
        {
                perror("lseek() error:");
        }
        printf("before lseek() to set offset of file,the end_filesize=%d\n",ret);//ret==0

        ret=lseek(fd,100,SEEK_SET);//set offset of file with lseek()
        if(ret==-1)
        {
                perror("lseek()2 error():");
        }

        ret=lseek(fd,0,SEEK_CUR);
        if(ret==-1)
        {
                perror("lseek()3 error:");
        }
        printf("after set offset with lseek(),before write 1 byte data,cur_offset off the file is %d\n",ret);//ret==100

        ret=write(fd,"",1);//write '\0' to the file.
        if(ret==-1)
        {
                perror("write() error:");
        }

        ret=lseek(fd,0,SEEK_CUR);
        if(ret==-1)

        {
                perror("lseek()3 error:");
        }
        printf("after write 1 byte data to the file,cur_offset of the file is %d\n",ret);//ret==1;

        ret=fstat(fd,&sb);
        if(ret==-1)
        {
                perror("fstat error:");
        }
        printf("filesize = %ld\n",sb.st_size);//filesize == 1;

        close(fd);

return 0;
}

1:open a file without flags O_APPEND,you can reset filesize with lseek()+write().

code2:

#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
        int ret=0;
        int fd=-1;
        struct stat sb;
        fd = open("test.txt",O_RDWR|O_CREAT|O_TRUNC,00777);//flags with O_APPEND
        if(fd==-1)
        {
                perror("open() error:");
        }
        ret = lseek(fd,0,SEEK_END);
        if(ret==-1)
        {
                perror("lseek() error:");
        }
        printf("before lseek() to set offset of file,the end_filesize=%d\n",ret);//ret==0

        ret=lseek(fd,100,SEEK_SET);//set offset of file with lseek()
        if(ret==-1)
        {
                perror("lseek()2 error():");
        }

        ret=lseek(fd,0,SEEK_CUR);
        if(ret==-1)
        {
                perror("lseek()3 error:");
        }
        printf("after set offset with lseek(),before write 1 byte data,cur_offset off the file is %d\n",ret);//ret==100

        ret=write(fd,"",1);//write '\0' to the file.
        if(ret==-1)
        {
                perror("write() error:");
        }

        ret=lseek(fd,0,SEEK_CUR);
        if(ret==-1)

        {
                perror("lseek()3 error:");
        }
        printf("after write 1 byte data to the file,cur_offset of the file is %d\n",ret);//ret==101;

        ret=fstat(fd,&sb);
        if(ret==-1)
        {
                perror("fstat error:");
        }
        printf("filesize = %ld\n",sb.st_size);//filesize == 101;

        close(fd);

return 0;
}