当我使用 C 在 Linux 中编程时，我使用 open 或 creat 函数并最终表现不同

Question

//error handle
void my_err(const char* errno_string,int line){
    fprintf(stderr,"line:%d ",line);
    perror(errno_string);
    exit(1);
}

//self-definded read data function

int my_read(int fd){
    int len;
    int ret;
    int i;
    char read_buf[64];

    //get length of file and keep point of file at the srart
    if(lseek(fd,0,SEEK_END) == -1){
        my_err("lseek",__LINE__);
    }

    if((len = lseek(fd,0,SEEK_CUR)) == -1){
        my_err("lseek",__LINE__);
    }

    if(lseek(fd,0,SEEK_SET) == -1){
        my_err("lseek",__LINE__);
    }

    printf("len:%d\n",len);

    //read data
    if((ret = read(fd,read_buf,len)) < 0){
        my_err("read",__LINE__);
    }

    //print data
    for(i = 0;i<len;i++){
        printf("%c",read_buf[i]);
    }

    printf("\n");
    return ret;
}

int main()
{
    int fd;
    char write_buf[32] = "hello boy!";

    //create example2 in current directory
    if((fd = creat("example2.c",S_IRWXU)) == -1){
    // if((fd = open("example2.c",O_RDWR|O_CREAT|O_TRUNC,S_IRWXU)) == -1){
        my_err("open",__LINE__);
    }else{
        printf("craete file success\n");
    }

    //write data
    if(write(fd,write_buf,strlen(write_buf)) != strlen(write_buf)){
        my_err("write",__LINE__);
    }
    my_read(fd);

    //Spacing of presentation files
    printf("/*------------*/\n");
    if(lseek(fd,10,SEEK_END) == -1){
        my_err("lseek",__LINE__);
    }
    if(write(fd,write_buf,strlen(write_buf)) != strlen(write_buf)){
        my_err("write",__LINE__);
    }
    my_read(fd);
    close(fd);
    return 0;
}

Line 43 is this part of main

    //create example2 in current directory
    if((fd = creat("example2.c",S_IRWXU)) == -1){
        my_err("open",__LINE__);
    }else{
        printf("craete file success\n");
    }

When I use creat , I get an error line:43 read: Bad file descriptor, but I get the correct result with open . Shouldn't both functions return file descriptors? Why should creat return the wrong file descriptor

When I use creat , I get an error line:43 read: Bad file descriptor, but I get the correct result with open . Shouldn't both functions return file descriptors? Why should creat return the wrong file descriptor

Answer 1

Shouldn't both functions return file descriptors?

They should and they do.

Why should creat return the wrong file descriptor

It shouldn't and it doesn't. read fails with Bad file descriptor error, not creat .

creat opens file write-only, so you can't read from it. It's a bad file descriptor if you want to read from it.

Answer 2

The call creat(path, mode) behaves the same as the call open(path, O_CREAT | O_WRONLY | O_TRUNC, mode) . On success, the file is opened for writing only. The file descriptor passed to read needs to be open for either reading and writing or for reading only. If the file descriptor is open for writing only, calls to read with that file descriptor will fail. When read fails, the error number EBADF means that the file descriptor is not a valid file descriptor open for reading.