Minor C problem

TwistedJester

For the first project in my system software class, we need to create a program that lets you read and edit the id3 tag (v1.1) of a music file. I have everything working pretty much, except for editting the track field. In the assignment, the track takes up one byte in the file. My problem is reading in a track number someone submits, and then reading out the value from the file as an integer. For example, if I try to set the track number to 6, it reads out (using %d) as its ascii value 54. If I use %c to print it out, it works, but only for 0-9. Is there some sort of formatting I can use with the scan codes to get what I'm looking for?

Goetterdaemmerung

int atoi(buffer);
returns the integer value written in a null-terminated (ascii) buffer.

TwistedJester

That gives me a segmentation fault.

Lewisham

Then your string pointer is no longer valid, atoi is what you need.

NAME
atoi -- convert ASCII string to integer

LIBRARY
Standard C Library (libc, -lc)

SYNOPSIS
#include <stdlib.h>

int
atoi(const char *nptr);

DESCRIPTION
The atoi() function converts the initial portion of the string pointed to
by nptr to integer representation.

It is equivalent to:

(int)strtol(nptr, (char **)NULL, 10);

SEE ALSO
atof(3), atol(3), strtod(3), strtol(3), strtoul(3)

STANDARDS
The atoi() function conforms to ANSI X3.159-1989 (``ANSI C89'').

CAVEATS
atoi does no overflow checking, handles unsigned numbers poorly, and han-
dles strings containing trailing extra characters (like ``123abc'')
poorly. Careful use of strtol(3) and strtoul(3) can alleviate these
problems.

TwistedJester

Actually, the problem wasn't printing out the value, it was correctly reading it in from the command line. Thanks for the help though duders.

Janin

char string[2] = "4";
int number = string[0] - '0';
printf ("%d", number); /* Prints 4 */

Bama

That method still only works for a single digit.

Janin

Bama wrote: »

That method still only works for a single digit.

The field in the file is only one byte long, so that's not a problem.

ASimPerson

Janin wrote: »

Bama wrote: »

That method still only works for a single digit.

The field in the file is only one byte long, so that's not a problem.

Isn't it, though? The number could be anywhere from 0-255 if it's unsigned, IIRC.

Janin

ASimPerson wrote: »

Janin wrote: »

Bama wrote: »

That method still only works for a single digit.

The field in the file is only one byte long, so that's not a problem.

Isn't it, though? The number could be anywhere from 0-255 if it's unsigned, IIRC.

If it was already a number, he wouldn't have to convert from ASCII encoding.