CODEWITHSUNDEEP
c++cunix
user2742399
user2742399

Reputation: 71

unable to `open` the file , but `lseek` is done without error

I am working on the unix system calls. here in my code I want to open the file and perform lseek operation on that file. please look into the following code.

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

int main(void)
{

 int fd;


 fd = open("testfile.txt", O_RDONLY);
 if(fd < 0 );
   printf("problem in openning file \n");

 if(lseek(fd,0,SEEK_CUR) == -1)
   printf("cant seek\n");
 else
   printf("seek ok\n");

 exit(0);

}

my output is : 

   problem in openning file 
   seek ok

My question is :

1) why open system call is giving me the negative file descriptor ? (I have conformed that testfile.txt file is within the same directory)

2) Here I am unable to open the file(as because open() returns me negative file descriptor) , how lseek is successful without opening the file?

Upvotes: 1

Views: 303

Answers (2)

trojanfoe
trojanfoe

Reputation: 122391

Most APIs will tell you why an error occurred, and for system calls like open() that is achieved by looking at errno (and using strerror() to get a textual version of the error). Try the following (with your errors removed):

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

int main(void)
{

 int fd;


 fd = open("testfile.txt", O_RDONLY);
 if(fd < 0 ) {   // Error removed here
   printf("problem in opening file: %s\n", strerror(errno));
   return 1;
 }

 if(lseek(fd,0,SEEK_CUR) == -1)   // You probably want SEEK_SET?
   printf("cant seek: %s\n", strerror(errno));
 else
   printf("seek ok\n");

 close(fd);

 return 0;

}

Upvotes: 2

Lidong Guo
Lidong Guo

Reputation: 2857

In fact, you open the file successfully.

Just if(fd < 0 ); is wrong, you need to remove the ;

Upvotes: 6

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