Thread: String and character pointers

Ok im brand new to c . Been programing in delphi and stuff for a while but at the moment im just using nedit and cc in linux.

Im trying to write a program to read a file, and print out the links it has. Most of the code i have tested and works, the problem is the "p[0] = &a;" line. The idea behind it is, i want to shift the first character in the string up some after ive found a link, so it caries on from where it left off, rather than starting from the start and finding the same link each time.

Googled for a while but im totaly stumped this time!

Code:

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

void readfile(FILE *fp, char *p);
void findtag(char *p, char *start, char finish, char *result);

main(int argc, char *argv[])
{
	FILE *fp;
	char c[10000], tag[100];
    
	if ((argc != 1) && ((fp = fopen(*++argv, "r")) != NULL))
	{
        	readfile(fp,c);
        	fclose(fp);
	
		while(c != '\0')
		{
			findtag(c,"<a",'>',tag);
			printf("%s\n",tag);
			/* findtag(tag,"href=",' ',tag); */
		} 
	}
	else
		printf("Arf\n");

}

void readfile(FILE *fp, char *p)
{
	int i;

	i = 0;
	while ((p[i++] = getc(fp)) != EOF)
        	;
	p[i - 1] = '\0';
}

void findtag(char *p, char *start, char finish, char *result)
{
	char *a;
	int i;
	
	i = 0;
	a = strstr(p, start) + strlen(start);
	if (a != '\0')
	{
		while ((result[i++] = *a++) != finish)
			;
		result[i-1] = '\0';
	}
	
	p[0] = &a;
}

Why not set a pointer to the beginning of the array in main, then pass a pointer to that pointer to findtag. That way you can update the pointer yet still use it in main for your loop test.

Code:

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

void readfile(FILE *fp, char *p);
void findtag(char **p, char *start, char finish, char *result);

main(int argc, char *argv[])
{
  FILE *fp;
  char c[10000], tag[100];
  char *p;

  if ((argc != 1) && ((fp = fopen(*++argv, "r")) != NULL))
  {
    readfile(fp,c);
    fclose(fp);

    p = c;
    while(*p != '\0')
    {
      findtag(&p,"<a",'>',tag);
      printf("%s\n",tag);
      /* findtag(tag,"href=",' ',tag); */
    } 
  }
  else
    printf("Arf\n");

}

void readfile(FILE *fp, char *p)
{
  int i;

  i = 0;
  while ((p[i++] = getc(fp)) != EOF)
    ;
  p[i - 1] = '\0';
}

void findtag(char **p, char *start, char finish, char *result)
{
  char *a;
  int i;

  i = 0;
  a = strstr(*p, start) + strlen(start);
  if (a != '\0')
  {
    while ((result[i++] = *a++) != finish)
      ;
    result[i-1] = '\0';
  }

  *p = a;
}

Please note that I have not checked this thoroughly.

Ok ive looked at it for ages, improved the code and tried sticking printf's everywhere. I still cant see why it segfaults at the end!

Also now ive tried to add code to find the "'s around links and remove them but that doesnt work either!

I hope c gets easier, im really banging my head againts a wall at the moment.

Code:

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

void readfile(FILE *fp, char *p);
bool findtag(char **p, char *start, char finish, char *result);
void cleanup(char *tag);

main(int argc, char *argv[])
{
	FILE *fp;
	char c[10000], tag[100];
	char *p, *o;

	if ((argc != 1) && ((fp = fopen(*++argv, "r")) != NULL))
	{
		readfile(fp,c);
		fclose(fp);

		p = c;
		while(findtag(&p,"<a",'>',tag))
		{
			o = tag;
			findtag(&o,"href=",' ',tag);
			cleanup(tag);
			printf("%s\n",tag);
		} 	
	}
  	else
    		printf("Arf\n");

}

void readfile(FILE *fp, char *p)
{
	int i;

	i = 0;
	while ((p[i++] = getc(fp)) != EOF)
		;
	p[i - 1] = '\0';
}

bool findtag(char **p, char *start, char finish, char *result)
{
	char *a;
	int i;

	i = 0;
  
	if (a = strstr(*p, start) + strlen(start))
	{
		while ((result[i++] = *a++) != finish)
			;
		result[i-1] = '\0';
		*p = a;
		return 1;
	}
	else
		return 0;
}

void cleanup(char *tag)
{
	char *a, temp[100];
	int i;
	
	if (tag[0] == '"')
	{
		i=1;
		*a=tag[1];
		while((*a++ = tag[i++]) != '"')
			temp[i-1] = *a;
		tag = temp;
	}
}

Code:

   a = strstr(*p, start);
   if ( a != NULL )
   {
      a += strlen(start);

Looks good , i tried somthing similar except i did "a = a + strlen(start)" which gave me an error. That was the final straw and i posted here

Could you explain why you used a += ... instead of a = a + ... ? What is += ?

What did you get that warning from by the way? im doing cc in linux to compile but if theres somthing else i could use that gave me more of a clue whats wrong with my programs i might give it a go.

if ( a != NULL ) checks if its nulll doesnt it?

thanks for all the help guys , i looked closely at the code and saw you where right, i forgot to say where the pointer was actauly pointing to. I fixed that and it no longer segmentation faults.

I didnt really understand why that would be a buffer overflow though, and the cleanup function still isnt working.

Heres the code so far

Code:

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

void readfile(FILE *fp, char *p);
bool findtag(char **p, char *start, char finish, char *result);
void cleanup(char *tag);

main(int argc, char *argv[])
{
	FILE *fp;
	char c[10000], tag[100];
	char *p, *o;

	if ((argc != 1) && ((fp = fopen(*++argv, "r")) != NULL))
	{
		readfile(fp,c);
		fclose(fp);

		p = c;
		while(findtag(&p,"<a",'>',tag))
		{
			o = tag;
			findtag(&o,"href=",' ',tag);
			cleanup(tag);
			printf("%s\n",tag);
		} 	
	}
  	else
    		printf("Arf\n");

}

void readfile(FILE *fp, char *p)
{
	int i;

	i = 0;
	while ((p[i++] = getc(fp)) != EOF)
		;
	p[i - 1] = '\0';
}

bool findtag(char **p, char *start, char finish, char *result)
{
	char *a;
	int i;

	i = 0;
  	
	a = strstr(*p, start);
	if (a != NULL)
	{
		a += strlen(start);
		while ((result[i++] = *a++) != finish)
			;
		result[i-1] = '\0';
		*p = a;
		return 1;
	}
	else
		return 0;
}

void cleanup(char *tag)
{
	char *a, temp[100];
	int i;
	
	if (tag[0] == '"')
	{
		i=1;
		a=&tag[1];
		while((*a++ = tag[i++]) != '"')
			temp[i-1] = *a;
		tag = temp;
	}
}

ok i managed to bash the cleanup function into shape

Code:

void cleanup(char *tag)
{
	char *a, temp[100];
	int i;
	
	if (tag[0] == '"')
	{
		i=0;
		a=&tag[0];
		memset(temp, 0, sizeof(temp));
		while (*++a != '"')
			temp[i++] = *a;
			
		strcpy(tag, temp);
	}
}