I had never realized this.I could have very well assumed subconsciously as a hard fact that I can transition between reading and writing on an existing file opened it update mode,just like that.But two questions on SO(1,2) drove me skeptical and I decided to try it out.Here's what I find:
In the first program,prog1,I work on a file source.txt which only has the line Lethal weapon.I read the first world Lethal using fscanf() and intend to write " musket" right after that expecting to get Lethal musket.But it simply fails, and I still get the original content Lethal weapon at the end of the write operation.But in that first program,if I insert the line fseek(fp,0,SEEK_CUR), the write operation works fine and I get Lethal musket.I notice that fseek(fp,0,SEEK_CUR) serves no purpose other than call fseek() just for the sake of it,as there is no net seeking at all.
But in the second program,prog2,the same scenario doesn't need thatfseek(fp,0,SEEK_CUR) statement.To be exact,in the second program,in contrast to reading to middle of file as in first program, here I read to the end of file,and then start writing there.Here the write is successful even without using fseek(fp,0,SEEK_CUR) and I get the desired content Lethal weapon sale.
Question: Why can't we transition from read to write mode in the middle of the file and what difference fseek(fp,0,SEEK_CUR) makes to get it working?And at the same time, why the same transition works without any fuss if we read to end of file and write there?Why fseek(fp,0,SEEK_CUR) is not needed in second case?How advisable is my use of fseek(fp,0,SEEK_CUR) to make the write successful in the first program?Any better alternative?
Two questions on SO seem to have addressed the same issue to some extent,but since they were based more on seeking explanation of excerpts from texts/books,the answers oriented towards them doesn't seem to address what I intend to know in precise terms.
//PROG1
#include <stdio.h>
int main ()
{
FILE *fp=fopen("D:\\source.txt","r+");
char arr[20],brr[50];
fscanf(fp,"%s",arr);
printf("String is %s\n",arr);
//fseek(fp,0,SEEK_CUR); //Doesn't work without it
fprintf(fp," musket");
rewind(fp);
fgets(brr,50,fp);
printf("New string is %s",brr);
fclose(fp);
}
Output:
1) Without fseek() -- Lethal weapon
2) With fseek() -- Lethal musket
//PROG2
#include <stdio.h>
int main ()
{
FILE *fp=fopen("D:\\source.txt","r+");
char arr[20],brr[50];
fgets(arr,20,fp);
printf("Initial line is %s\n",arr);
fprintf(fp," sale"); //writes to end of file
rewind(fp);
printf("New string is %s",fgets(brr,50,fp));
fclose(fp);
}
Output without fseek(): -- Lethal weapon sale
