What is wrong in this program ?

[pereges]

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
FILE *fp;
int i, n;
typedef struct student_record
{
char name[40];
int sid;
}student;

student s;
/* I presume this creates a new file if one already doesn't exist */
fp = fopen("student. dat", "ab+");
if(fp == NULL)
{
perror("file can't be opened\n!");
exit(EXIT_FAILU RE);
}
else
{

printf("How many records you want to write ?\n");
scanf("%d", &n);
for(i=0; i<n; i++)
{
printf("Enter record %d: student name student
id\n", i);

/* There seems to be some problem here and
its obvious during run time */
scanf("%s %d", s.name, &s.sid);
fwrite(&s, sizeof(s),1, fp);
}

}

/* doesn't print at all ? */
while(fread(&s, sizeof(s), 1, fp) == 1)
printf(" %s %d \n", s.name, &s.sid);

return 0;

}

+++++++++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ +++++

The o/p I get :

How many records you want to write ?
3
Enter record 0: student name student id
Nathan Reed 5
Enter record 1: student name student id
Enter record 2: student name student id

[William Pursell]

Re: What is wrong in this program ?

On 11 Apr, 07:48, pereges <Brol...@gmail. comwrote:

fp = fopen("student. dat", "ab+");
if(fp == NULL)
{
perror("file can't be opened\n!");
exit(EXIT_FAILU RE);
}

Consider the user running the program encountering
the error message:

file can't be opened
:No such file or directory

As a user, I will be really annoyed that this
message doesn't give me the pathname of the file
in question. (and slightly annoyed by the extra newline
before the ':')

Please consider writing this as:
fp = fopen( filename = "whatever", ...)
....
perror( filename );

Or, if you really feel it necessary to be
excessively verbose:

fprintf( stderr, "%s can't be opened:", filename );
perror( NULL );

(or "%s can't be opened: %s", filename, strerror( errno )
....whatever you prefer, just make sure the filename
is in the error message.)

[Eric Sosman]

Re: What is wrong in this program ?

William Pursell wrote:

[...]
Or, if you really feel it necessary to be
excessively verbose:
>
fprintf( stderr, "%s can't be opened:", filename );
perror( NULL );

Don't do it that way, because perror() reports the
error that errno indicates, and fprintf() might change
errno. You could wind up with

flatcat.dat can't be opened: not a typewriter

(See Question 12.24 in the FAQ.)

One way to deal with this is to save and restore errno's
value around the fprintf() call:

#include <errno.h>
...
{
int errno_save = errno;
fprintf(stderr, "%s can't be opened: ", filename);
errno = errno_save;
perror(NULL);
...

.... but in a case like this the advice to use

(or "%s can't be opened: %s", filename, strerror( errno )

.... seems better.

--
Eric.Sosman@sun .com