fdopen：无效的争论

Question

I am trying to create and open a file with fopen and fdopen to write some content. Below is the code I wrote:

    char Path[100];
    int write_fd;

    snprintf(Path,100,"%s/%s","/home/user","myfile.txt");
    printf("opening file..\n");
    write_fd = open(Path, O_WRONLY | O_CREAT | O_EXCL, 0777);

    if(write_fd!=-1)
    { 
       printf(" write_fd!=-1\n");

       FILE *file_fp = fdopen(write_fd,"a+");

       if (file_fp == NULL)
       {
              printf("Could not open file.File pointer error %s\n", std::strerror(errno));
              close(write_fd);
               return 0;         
       }
       write(write_fd, "First\n", 7);
       write(write_fd, "Second\n", 8);
       write(write_fd, "Third\n", 7);
       fclose(file_fp);
   }

The file fd write_fd is created with the permissions WRONGLY which should have permission to read/write(?). But when fdopen called on the file descriptor with mode a+ , it is throwing error saying Invalid Argument.

It is successfully opened with mode a .

What exactly differs between the modes a and a+ that causes this error ?

Answer 1

The a+ mode means append and read .

Since you have initially opened the file in write-only mode ( O_WRONLY | O_CREAT | O_EXCL ), read access is not compatible with the mode of the initial descriptor.

Therefore, the call to fdopen() rightfully fails with EINVAL .