Moving a file pointer in a function

**127.0.0.1** · 05-04-2011

I have a function that takes a file pointer and returns the next non-blank line of text (removing comments as well). However, even though the code works in main it obviously doesn't work when isolated in a function because it is unable to move the file pointer outside of its scope.

Is there a way around this using pointer pointers? Or should I start looking at other ways to solve this problem

Code:

  int Next_Line
  (
    FILE *Input,
    char *Line,
    int  Size,
    char *Comment_Prefix
  ){
    int Line_Length;
    int Prefix_Length = strlen(Comment_Prefix);
    if(Prefix_Length < 0 || Prefix_Length > 2)
      return FAILURE;
    do{
      do{
        if(feof(Input))
          return FAILURE;
        fgets(Line, Size, Input);
      }while(Line[0] == 10);
      Line_Length = strlen(Line);
      for(int i = 0;i <= Line_Length - Prefix_Length;i++)
        if((Prefix_Length == 1 && Line[i] == Comment_Prefix[0])
        || (Prefix_Length == 2 && Line[i] == Comment_Prefix[0]
        && Line[i + 1] == Comment_Prefix[1])){
          Line[i] = '\0';
          break;
        }
      Line_Length = strlen(Line) - 1;
      for(int i = 0;i <= Line_Length;i++)
        if(Line[i] != ' ' && Line[i] != '\t')
          break;
        else if(i == Line_Length - 1)
          Line[0] = '\0';
    }while(Line[0] == '\0');
    return SUCCESS;
  }

**~~CommonTater~~** · 05-04-2011

Look into the ftell() and fseek() functions... you should be able to hand the function the file at it's current position, let it do it's thing, reposition the pointer for you and then return the new position.

Code:

int FindNextLine(FILE *fp)
  { int tc;  // test character 
    while (1)
      { tc = (fgetc(fp))  // advance file pointer
         if (isalpha(tc))
           { fseek(fp,-1,SEEK_CUR); // reposition -1 spaces
              return ftell(fp); }
         else if (tc == EOF)
           return EOF; }  }

C is not a high level language... you will find it very helpful to think in smaller blobs... Each function does one function...

Call the FindNextLine() function... then use another function (say) IsComment() to tell you if you eed to find the next line again.

Code:

  do
   {  FindNextLine(fp); }
 while (IsComment(fp));

You will probably find uses for both functions in tandem and separately throughout your code.

**Salem** · 05-04-2011

There must be something else going on, because that doesn't make any sense.

This does what you expect.

Code:

$ gcc bar.c
$ cat bar.c
#include <stdio.h>
void foo ( FILE *fp );
int main (void)
{
    FILE *fp = fopen("bar.c","r");
    char    buff[100];
    foo(fp);
    fgets( buff, sizeof buff, fp);
    fputs( buff, stdout );
    fclose(fp);
    return 0;
}
void foo ( FILE *fp ) {
    char buff[100];
    fgets( buff, sizeof buff, fp);
    fputs( buff, stdout );
}
$ ./a.out 
#include <stdio.h>
void foo ( FILE *fp );

**127.0.0.1** · 05-04-2011

Originally Posted by Salem

There must be something else going on, because that doesn't make any sense

This is the code I used to call the function (the "!feof(Input)" while loop is infinitely repeated because the file pointer doesn't increment, it just prints the first line over and over):

Code:

    FILE *Input;
    char Line[256];
    char Comment_Prefix[2] = "//";
    int Line_Length;
    int Prefix_Length = strlen(Comment_Prefix);
    Input = fopen("test.txt", "r");
    if(Input != NULL){
      while(!feof(Input)){
        Next_Line(&Input, Line, sizeof(Line), Comment_Prefix);
        printf("%s", Line);
      }
      fclose(Input);
    }

And this is the test file I had it read:

Code:

TEST    TEST TEST 1



TEST 2

                           



TEST  3


        //TES TEST TEST

//TEST

Test 4
Ttest 5 // TEST




TEST

Other than that I have no idea what other factors could contribute to fgets not working properly.

**adeyblue** · 05-04-2011

Originally Posted by 127.0.0.1

Code:

        Next_Line(&Input, Line, sizeof(Line), Comment_Prefix);

Lose the ampersand. Also turn on, turn up and fix the warnings your compiler gives. It would have complained about this. Also 2, don't use feof to control loops.

**127.0.0.1** · 05-04-2011

Originally Posted by adeyblue

Lose the ampersand. Also turn on, turn up and fix the warnings your compiler gives. It would have complained about this. Also 2, don't use feof to control loops.

Sorry, I had tried to pass a file pointer pointer to the function to see if it would help (it didn't) and forgot to correct the code in the calling. It doesn't work with this code:

Code:

/************
 * Includes *
 ************/
 
  #include "../Core/String_Utilities.h"
  #include <stdio.h>
  
/**************
 * Test_Cases *
 **************/
 
  int main
  (){
    FILE *Input;
    char Line[256];
    char Comment_Prefix[2] = "//";
    int Line_Length;
    int Prefix_Length = strlen(Comment_Prefix);
    Input = fopen("test.txt", "r");
    if(Input != NULL){
      while(!feof(Input)){
        Next_Line(Input, Line, sizeof(Line), Comment_Prefix);
        printf("%s", Line);
      }
      fclose(Input);
    }
  }

Does it matter that the function Next_Line is in a header file separate from the main?

**~~CommonTater~~** · 05-04-2011

Originally Posted by 127.0.0.1

Does it matter that the function Next_Line is in a header file separate from the main?

It shouldn't.

The function should be in a .c file separate from main and prototyped in a .h file included in main.

**127.0.0.1** · 05-04-2011

Originally Posted by CommonTater

It shouldn't.

The function should be in a .c file separate from main and prototyped in a .h file included in main.

The function implementations were together in the header (just out of laziness because I had been frequently changing the prototypes and it was frustrating for debugging), but I properly separated the String_Utilities.h into String_Utilities.c, recompiled, and still got an infinite loop.

**127.0.0.1** · 05-04-2011

Code:

This does what you expect.

Salem, your solution does indeed work -- even when I separated it into another header. Which makes me further confused about my code.

**~~CommonTater~~** · 05-04-2011

Originally Posted by 127.0.0.1

The function implementations were together in the header (just out of laziness because I had been frequently changing the prototypes and it was frustrating for debugging), but I properly separated the String_Utilities.h into String_Utilities.c, recompiled, and still got an infinite loop.

This is more form than substance...
Headers (.h) should contain prototypes, typedefs, extern variables... no actual code.
Function definitions, live structs and active variables belong in C files.

Yes you can put active code in a .h file and the compiler won't care about it... but it is widely considered to be bad form because can lead to a lot of needless code duplication and errors if the header is included in a project more than once.

**Salem** · 05-04-2011

> int Prefix_Length = strlen(Comment_Prefix);
Ooops...

> char Comment_Prefix[2] = "//";
Despite appearances, this is NOT a proper C-String. It does NOT have a \0, and thus your attempt to strlen() it will fail.

The function doesn't read anything, it just returns fail immediately.

Avoid setting a size for the string, unless it is absolutely important that it occupies a specific number of bytes.
Normally, you do

Code:

char Comment_Prefix[] = "//";

Which begs the next question, you worked so hard to return success/fail, why aren't you using it?

**mike65535** · 05-04-2011

Code:

char Comment_Prefix[] = "//";

If you aren't going to ever modify this string, it might be better to either make it a const (so the compiler will tell you if there are any attempts to write to it). Some static analysis programs will tell you this. I'm no stickler.

Another option is to define it as a macro.

**127.0.0.1** · 05-04-2011

Avoid setting a size for the string, unless it is absolutely important that it occupies a specific number of bytes.

Thank you, this did the trick. I guess I was so concerned that my function library was wrong that I didn't even consider that it was my debugging code with the problem xD.

Final code:

Code:

    FILE *Input= fopen("test.txt", "r");
    char Line[256];
    char Comment_Prefix[] = "//";
    int Line_Length;
    int Prefix_Length = strlen(Comment_Prefix);
    if(Input != NULL){
      while(Next_Line(Input, Line, sizeof(Line), Comment_Prefix))
        printf("%s", Line);
      fclose(Input);
    }

**~~CommonTater~~** · 05-04-2011

Looking at this code...

Code:

      for(int i = 0;i <= Line_Length - Prefix_Length;i++)
        if((Prefix_Length == 1 && Line[i] == Comment_Prefix[0])
        || (Prefix_Length == 2 && Line[i] == Comment_Prefix[0]
        && Line[i + 1] == Comment_Prefix[1])){
          Line[i] = '\0';
          break;

... there might be a simpler way to do this...

Code:

const char Comment_Prefix[] = "//";


if (strstr(line[i],Comment_Prefix))
  break;

strstr() from the strings.h library returns a pointer to the found string or NULL if it's not found. Seems like a pretty easy way to detect your comment prefix.

**127.0.0.1** · 05-04-2011

Originally Posted by CommonTater

... there might be a simpler way to do this...

Code:

const char Comment_Prefix[] = "//";


if (strstr(line[i],Comment_Prefix))
  break;

strstr() from the strings.h library returns a pointer to the found string or NULL if it's not found. Seems like a pretty easy way to detect your comment prefix.

Your solution doesn't work because the whole string is compared for the match, I need to know the index that the match is at so I can terminate the string if there is valid text before the comment.

Also as a side note, I got rid of the !feof code and replaced it with the safer equivient (thanks for the article adeyblue).

Code:

        if(!fgets(Line, Size, Input))
          return FAILURE;
      }while(Line[0] == 10);
      Line_Length = strlen(Line);
      Line[Line_Length - 1] = '\0';
      for(int i = 0;i <= Line_Length - Prefix_Length;i++)
        if(strstr(&Line[i], Comment_Prefix)){
          Line[i] = '\0'; /* I need to know the index the comment starts here */
          break;
        }

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