Question

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

char *readLine(FILE *inFile)  //Simply reads line in a text file till "\n"
{
    char *line = realloc(NULL, 1);
    char c;
    int i=0;
    while (!feof(inFile))
    {
        c = fgetc(inFile);
        if (ferror(inFile)) printf("Error reading");
        if (c == 10)
            {
                realloc(line,i+1);
                line[i]= 10;
                break;
            }
        realloc(line, i+1);
        line[i++] = c;
    }
    return line;
}

int main(int argc,char **argv)
{
    FILE *inFile;
    inFile = fopen("testFile","r");
    printf("%s",readLine(inFile));
    printf("%s",readLine(inFile));
    printf("%s",readLine(inFile));
    return 0;
}

If the contents of testFile is:-

abc
def
ghi

The three printf statements should show “abc” three times.. But the output is:-

abc
def
ghi

I know I am wrong in the concept somewhere. Pls help.

Answer 1

But I am passing file pointer by value. So i should get output “abc” again and again

Ah, I understand your confusion.

A file pointer only points to the actual file structure. State such as the current offset are not part of the pointer but are part of the internal structure.

Another way to think about this is that the actual object representing the file is FILE. To get pass-by-reference semantics, you pass a pointer to the object. Since you are passing by reference, each line picks up where the last one left off.