Question

I am trying to read two files in my program I wrote earlier, but it always fails.

char line[BUFSIZ];
FILE *fp2=freopen("source.dat","r");

if(fp2==NULL)
printf("Problm opening: source.dat");

FILE *fp3=freopen("result.dat", "r");

if(fp3==NULL)
printf("Problm opening: result.dat");
char line2[BUFSIZ];
int len;

while( (fgets(line2, BUFSIZ, fp2) != NULL) && (fgets(line, BUFSIZ, fp3) != NULL)) {
len=strlen(line);
if( line[len - 1] == '\n' ) line[len-1] = '\0'; len=strlen(line2);
if( line2[len - 1] == '\n' ) line2[len-1] = '\0';

rename(line, line2);
}

I’m not sure why, I know my program writes the two files I want to open. It just doesn’t get past the while loop.

Answer 1

freopen takes 3 arguments viz., filename, mode and FILE Stream object. Hence, to reopen a file, it should already be open. If we invoke a freopen as the first call, the runtime may throw an exception of uninitialized access.

Modifying the code as below

    fp2 = fopen("source.dat", "r");
    fp3 = fopen("result.dat", "r");

I am able to run your code without any problem and control continues beyond the while loop. The files stored in first file are renamed to the names stored in second file, which I presume is the objective of your program.