can't add on to pointer pointing to first element in a file-CodePudding

text file only contain "hi".I expected it to point to the next charecter after c and print it but it's giving h↑ instead.

int main()
{
    FILE *ptr;
    char ch1;
    ptr = fopen("rough.txt", "r");
    ch1 = getc(ptr);
    char *c = &ch1;
    printf("%c", *c);
    c  ;
    printf("%c", *c);
    return 0;
}

CodePudding user response：

If you want to increment a pointer over the contents of the file, you should read more data. A simple way to do that is to use fread to read a chunk of data rather than getc, which only reads one character. For example:

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

int
main(int argc, char **argv)
{
    const char *path = argc > 1 ? argv[1] : "rough.txt";

    FILE *ifp = fopen(path, "r");
    if( ifp == NULL ){
        perror(path);
        return 1;
    }
    char buf[128];
    char *c = buf;
    size_t read_count = fread(buf, sizeof(char), sizeof buf, ifp);
    while( c < buf   read_count ){
        putchar(*c  );
    }
    return 0;
}

CodePudding user response：

ch1 is a variable on the stack, and c was initailized with address of ch1. using the post increment operator on it wont advance on the file but on the stack.

To read from the file you can create a buffer and use fread to fill it up then advance on it with the char * like you wanted to.

int main()
{
    FILE *ptr = NULL;
    char buff[DESIRED_BUFF_SIZE] = {0};
    ptr = fopen("rough.txt", "r");
    fread(buff, <amount of chars to be read>, sizeof(char), ptr);
    /* print the chars from the buffer here*/

    return 0;
}

CodePudding user response：

getc reads exactly one character from the file. Next time you call getc it will read the following character.

c ; will not read from the file. It will only increment the c pointer, after which c will point to an invalid memory location.

Don't make things more complicatd than they already are.

You want something like this:

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

int main()
{
  FILE* inputfile;
  char ch;
  ptr = fopen("rough.txt", "r");
  
  if (ptr == NULL)  // you need to check if fopen succeeded
  {
    printf("File could not be opned\n");
    return 1;
  }

  ch = getc(inputfile);   // read first character
  printf("%c", ch);

  ch = getc(inputfile);   // read second character
  printf("%c", ch);

  fclose(inputfile);      // and don't forget to close
  return 0;
}

To read the whole file you need this:

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

int main()
{
  FILE* inputfile;
  int ch;                 // must be an int so that EOF works
  inputfile = fopen("rough.txt", "r");
  
  if (inputfile == NULL)  // you need to check if fopen succeeded
  {
    printf("File could not be opned\n");
    return 1;
  }

  while ((ch = getc(inputfile)) != EOF)
  {
    printf("%c", ch);
  }

  fclose(inputfile);
  return 0;
}

The details about EOF and why ch should be an int are explained in the chapter dealing with files in your C learning material.