Question

In the below program write() returns -1 while writing to a file.

   #include<sys/types.h>
   #include<sys/stat.h> 
   #include<fcntl.h>   
   #include<stdio.h>   
   #include<unistd.h> 
   #include<stdlib.h>
   int main() {

   int fd_r=0,fd_w=0;
   int w_ret=100;
   fd_r = open("reader.txt", O_RDONLY);

   fd_w = open("writer.txt",O_CREAT,S_IRWXU);

   char *buf = (char *)malloc(50);

   while(read(fd_r,buf,30))
   {

          w_ret =   write(fd_w,buf,30);
          printf("%d", w_ret);
   }
}

Questions: I am unable to debug why this is happening. Correction of code and suggestions as to how to debug such issues are highly appreciated

Answer 1

I don’t believe that O_CREAT is valid by itself for the flags: try O_CREAT | O_WRONLY.

One way to debug would be checking that the fd_w file descriptor is valid when you first open it.

“The parameter flags is one of O_RDONLY, O_WRONLY or O_RDWR which request opening the file read-only, write-only or read/write, respectively, bitwise-or’d with zero or more of the following…” http://www.linuxmanpages.com/man2/open.2.php