CODEWITHSUNDEEP
cfilefseek
Jeries Haddad
Jeries Haddad

Reputation: 89

fseek(f,0,2); How does this work without SEEK_CUR / SEEK_SET or SEEK_END?

Aren't we supposed to use SEEK_CUR / SEEK_SET or SEEK_END for whence? How does it work with just a fixed value?

Upvotes: 1

Views: 11519

Answers (5)

Soner from The Ottoman Empire
Soner from The Ottoman Empire

Reputation: 20891

In stdio.h,

#ifndef SEEK_SET
#define SEEK_SET    0   /* set file offset to offset */
#endif
#ifndef SEEK_CUR
#define SEEK_CUR    1   /* set file offset to current plus offset */
#endif
#ifndef SEEK_END
#define SEEK_END    2   /* set file offset to EOF plus offset */
#endif

Upvotes: 2

Amit Kumar
Amit Kumar

Reputation: 59

fseek() takes three parameters.

First one is the FILE * it will seek

Second is the int Record size, the value, it is expected to hop in one jump. Notice, it can be a negative value as well.

Third parameter is the offset value from where this hopping is to be done. There are three possible values for it 0,1 and 2. 0 represents beginning of the file 1 represents current location And, 2 represents the end of file.

The SEEK_END, SEEK_CUR or,the SEEK_SET are merely the Macro Expansions with values of 2,1 and 0 respectively. So when a user uses these macro Expansions rather that using the real values underneath, the pre-processors replaces every such usage with its respective value and compilation goes on as usual.

Upvotes: -1

Sergio
Sergio

Reputation: 8209

Look into your stdio.h:

#define SEEK_END    2   /* Seek from end of file.  */

So after preprocessing your fseek(f, 0, SEEK_END) becames fseek(f, 0, 2) and compiler will see 2 instead of meaningful name. As result we can avoid this name at all and put numerical value right now.

Upvotes: 2

Alexi
Alexi

Reputation: 391

Because SEEK_XXX are macros and have a certain value, in this case, SEEK_END is equal to 2, so it's the same fseek(f,0,SEEK_END) that fseek(f,0,2), but you should use always the macros. You can see this values for example in the notes of http://man7.org/linux/man-pages/man2/lseek.2.html (at the end).

Upvotes: 2

Ishay Peled
Ishay Peled

Reputation: 2868

SEEK_SET/SEEK_CUR/SEEK_END are 0/1/2 respectively, you can use the number or definition.

See definitions here: http://unix.superglobalmegacorp.com/BSD4.4/newsrc/sys/unistd.h.html

/* whence values for lseek(2) */
#define SEEK_SET    0   /* set file offset to offset */
#define SEEK_CUR    1   /* set file offset to current plus offset */
#define SEEK_END    2   /* set file offset to EOF plus offset */

Of course, it is bad practice to directly use these numbers as it may change in future implementations (not likely though)

Upvotes: 9

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