have a doubt regarding files

[raghuveerd]

I am trying to write some text in to a file through the program. And then trying to print that text on to the screen. My program is running but with two warnings.
I couldn't understand and overcome those two warning messages
say:" Possible use of 'str' before definition in function main "
Could someone explain what does it mean

#include<stdio.h>

int main()
{

FILE *fp,*fs;
char *str;

fp=fopen("veer.txt","w");
fprintf(fp,"hi Welcome to Cprogramming.");
fclose(fp);

fs=fopen("veer.txt","r");
fgets(str,100,fs);
printf("&#37;s",str);
fclose(fs);

}

[robwhit]

str doesn't have any space associated with it.

[hk_mp5kpdw]

As a pointer, str contains an address to somewhere in memory where you are trying to store data into that you are reading from the file. Since it is not initialized to anything, this address stored in str is whatever random value happens to be there. When you read data from the file into this random address you could experience many strange and wonderful (not) things. The fact that it might work is just luck. If you happen to read into some critical area your program could just as easily crash. You need to allocate space for your pointer (using malloc perhaps) or use a fixed array of some set size (100 it seems).

Either:

char *str;
str = malloc(100);
...
fgets(str,100,fs);
printf("%s",str);
...
free(str);

Or:

char str[100];
...
fgets(str,100,fs);
printf("%s",str);

[abachler]

#include<stdio.h>
 
int main()
{
 
FILE *fp,*fs;
char *str;
 
str = (char*)malloc(101); 
str[100] = 0; // makes it zero terminated 

fp=fopen("veer.txt","w");
fprintf(fp,"hi Welcome to Cprogramming.");
fclose(fp);
 
fs=fopen("veer.txt","r");
fgets(str,100,fs);
printf("&#37;s",str);
fclose(fs);
 
free(str);
}

there that fixes the problem, hope we get an A

[vart]

fgets puts the null char into the string - so you do not need an extra char and explicitely setting it to null, fgets will do it for you even if the string is longer than the buffer

[iMalc]

have a doubt regarding files

It's "Question" not "Doubt".
"Doubt" is a statement about yourself only.
A "Question" is something you ask other people.

Please learn the difference, and tell everyone else you know to learn it too!

[zacs7]

Don't cast malloc (see the FAQ), also check the return value ensuring your malloc succeeded (returns a NULL pointer if it failed).

char * str = NULL;

/* blah */
if((str = malloc(size)) == NULL)
{
    perror("Malloc failed");
    return 1;
}

/* blah */

free(str);
str = NULL;