open() syscall sometimes fails and sometimes succeeds

user_19089131 0
Viewed 76

I'm trying to understand why this is sometimes works, and sometimes doesn't - the open and read syscalls.

//open files   
fd1=open(argv[1],"O_RDONLY");
if(fd1==-1){
   printf("open file number 1 failed\n");
   exit(0);
} 
fd2=open(argv[2],"O_RDONLY");
if(fd2==-1){
    printf("open file number 2 failed\n");
    exit(0);
}

//read first byte
r1=read(fd1, &byte1, 1);
if(r1==-1){
    printf("read 1 1 error\n");
    close(fd1);
    exit(0);
}
r2=read(fd2, &byte2, 1);
if(r2==-1){
    printf("read 2 error\n");
    close(fd2);
    close(fd1);
    exit(0);
}

The output:

~/Desktop/os$ ./comp/out 1.txt 2.txt
open file number 1 failed
~/Desktop/os$ ./comp/out 1.txt 2.txt
read 1 1 error
~/Desktop/os$ ./comp/out 1.txt 2.txt
open file number 1 failed
~/Desktop/os$ ./comp/out 1.txt 2.txt
open file number 1 failed
~/Desktop/os$ ./comp/out 1.txt 2.txt
read 1 1 error
~/Desktop/os$ ./comp/out 1.txt 2.txt
read 1 1 error

Or as image: https://i.stack.imgur.com/NkvNC.png

c
1 Answers
"O_RDONLY"

Aaaaaaaaaaaaaaaaaaaaaaaaaa

You want

O_RDONLY




What's going on here is the second argument is a (mostly bitflags) number, and you're passing a string. Due to ASLR, the second argument is randomized so it works sometimes but not other times.

user_147680
Related