Program crashes while trying to get a string from file and access indvidual letters in the string

Question

I am creating a C program to convert all lower case to upper case and vice versa. In the C program, I am trying to get a string from a text file and access each of the characters. But my program crashes. What is the error that I am doing here? Below is my code:

 //Program to read contents from a source file and convert the lower case characters to a upper case and vice versa and print it on a target file

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

void merge(FILE *fp,int n,char *fdata,FILE *fpr)
{
    char change_case[1000];
    int loop = 0;

    while(fgets(fdata,n,fp)!=NULL)
    {
        fgets(fdata,n,fp);

        //printf("%s",fdata);

        if ((*(fdata+loop) >= 65) && (*(fdata+loop) <= 90))
        {
            printf("%c",*(fdata+loop));
            change_case[loop] = *(fdata+loop)+32;
        }
        else if ((*(fdata+loop) >= 97) && ((*fdata+loop) <= 122))
        {
            printf("%c",*(fdata+loop));
            change_case[loop] = *(fdata+loop)-32;
        }
        else
        {
            change_case[loop] = *(fdata+loop);
        }
        loop++;

        fpr = fopen("Text File 3.dat","w");

        if (fpr == NULL)
        {
            printf("Error in accessing / writing the target file"); 
        }
        else
        {
            fputs(change_case,fpr);
            printf("Line written successfully : %s\n", change_case);
        }
    }
}

int main()
{
    FILE *sfp, *tfp; //declaring a file pointer for source file and target file
    char *fdata;
    int n = 1000;
    sfp = fopen("Text File 1.txt","r");

    if (sfp == NULL)
    {
        printf("Error in reading the source file\n");
    }

    else
    {
        merge(sfp,n,fdata,tfp);
    }   

    fclose(sfp);

return 0;
}

Welcome to Stack Overflow! Please see [Why is “while ( !feof (file) )” always wrong?](http://stackoverflow.com/q/5431941/2173917) — Sourav Ghosh, Oct 01 '15 at 09:14

score 1 · Accepted Answer · edited May 23 '17 at 11:58

1

In function void merge(FILE *fp,int n,char *fdata,FILE *fpr)-

fgets(fdata,n,fp);         // you tend to write at invalid location

memory is not allocated to fdata . You need to allocate memory to it .

fdata=malloc(n);     // check its return and also remember to free it

Note - Don't use while!feof(fp) .

You can do this instead -

while(fgets(fdata,n,fp)!=NULL)
{
     // your code
}

edited May 23 '17 at 11:58

Community

1
1

answered Oct 01 '15 at 09:11

ameyCU

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@KiranCK If there is large data then you would need to allocate more memory . If updated the answer allocating more memory . – ameyCU Oct 01 '15 at 09:21
The characters in the input file are less than 100 – Kiran C K Oct 01 '15 at 09:22
@KiranCK I don't get carsh . You sure this is the code ? – ameyCU Oct 01 '15 at 09:26
@KiranCK You should not update original code in question . And please dont cast result of `malloc`. – ameyCU Oct 01 '15 at 09:28
I have edited and pasted the code with the updates you have pointed. Still it crashes. I use Code blocks with Borland C 5.5.1 as compiler – Kiran C K Oct 01 '15 at 09:34
The test file that it points to contains 27 characters on 3 lines. A line a blank line and the last line: Text file content: Hello!. This is a test file. – Kiran C K Oct 01 '15 at 09:37
@KiranCK Well , I ran it but doesn't gave me any crash . – ameyCU Oct 01 '15 at 09:40

Program crashes while trying to get a string from file and access indvidual letters in the string

1 Answers1