String manipulation code using char ** pointer gives unexpected result

Question

I was trying to implement a string parsing code for that I need substring of the given string so I deed the following:

The header file is : test.h

#ifndef header_file
#define header_file

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

int increment(char **);

#endif

The source files : main.c test.c

test.c :

Case1 :test.c

#include"test.h"

int increment(char **string){

    char *temp = *(string);
    int value;


    if(temp != NULL){

            *(string) = ++temp;
            value = 1;
    }

    else{
            value = 0;
    }

    return value;

}

Case2 :test.c

#include"test.h"

int increment(char **string){

    char *temp = *(string);
    int value;


    if(*temp != '\0'){

            *(string) = ++temp;
            value = 1;
    }

    else{
            value = 0;
    }

    return value;

}

main.c:

#include"test.h"

int main()
{
        char str[30] = "I have done form here comes.";
        char strs[50];

        char *p = str;

        memset(strs, 0, 50);

        while(increment(&p))
        {
                strcpy(strs,p);
                printf("Originally the string is : %s\n", str);
                printf("The modified string is   : %s\n", strs);
                memset(strs, 0, 50);
        }

        return 0;
}

The makefile is :

#This is the makefile.

all : run main.o test.o

run : main.o test.o
        $(CC) -g $^ -o $@

%.o : %.c
        $(CC) -g -c $^ -o $@

.PHONY : clean

clean : 
        -rm -f *.o run

But in the first case in test.c where I tried to traversed the sub-string but It is giving some garbage result. And second case works fine.

What is going wrong in test.c case 1.

thanks!!!!!!!!!!!!

Needs to be much clearer. "It is giving some garbage result": what is? make? the C-compiler? the executable? Paste us the actual garbage result or error message. Which C-compiler do you use? — smci
The condition (temp != NULL), which ultimately governs your main loop, will never be false, because the way that increment works, p will point to a char in str, which isn't NULL. Instead, you should check that the value pointed at isn't the null character: (*temp != '\0'). — M Oehm
@smci I am using GCC-4.7 compiler and the result getting after executing binary is: The modified string is : ;45:*.flac=38;5;45:*.m4a=38;5;45:*.mid=38;5;45:*.midi=38;5;45:*.mka=38;5;45:*.mp3=38; ... And much more like this. — Abhijatya Singh
@ M Oehm I have done the changes lile (*temp != '\0') and I also tried as (temp != NULL && *temp != '\0') but I get the warning: comparison between pointer and integer if(*temp != "\0"){ and result is still the garbage. — Abhijatya Singh
*.m4a? This makes no sense. Your program doesn't contain such text at all. And even if it was in str[30], I don't see code responsible for splitting words anywhere. — rr-

rr- rr- · Accepted Answer · 2015-06-13T08:26:58

The purpose was to add prefix and suffix to every word in the string so I need the string before the word , the word , and after the word. ex: prefix_I_suffix have done form here comes. I prefix_have_suffix have done form here comes.

You're doing a few things wrong.

I don't see splitting words by spaces anywhere at all, and that seems to be your main requirement.
I see, however, some insane operations on pointers. Frankly, I can't understand the logic behind them.

So let me give you examples how it can be done - without modifying any pointers at all:

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

int main ()
{
    char str[] = "I have done answer here comes.";

    const char *delimiters = " ";
    char *pch = strtok(str, delimiters);
    while (pch != NULL)
    {
        printf("prefix_%s_suffix\n", pch);
        pch = strtok(NULL, delimiters);
    }
    return 0;
}

With copying the input to intermediate array that you can modify however you want:

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

int main ()
{
    char str[] = "I have done answer here comes.";

    char **array = NULL;
    size_t array_size = 0;

    const char *delimiters = " ";
    char *pch = strtok(str, delimiters);
    while (pch != NULL)
    {
        size_t str_size = strlen(pch) + 1; //to accommodate for `\0` byte
        char *str_copy = malloc(str_size);
        if (!str_copy)
        {
            printf("No memory!");
            return 1;
        }
        memcpy(str_copy, pch, str_size);

        ++ array_size;
        array = realloc(array, sizeof(char*) * array_size);
        if (!array)
        {
            printf("No memory!");
            return 1;
        }
        array[array_size - 1] = str_copy;

        pch = strtok(NULL, delimiters);
    }

    for (size_t i = 0; i < array_size; i ++)
    {
        printf("prefix_%s_suffix\n", array[i]);
        free(array[i]);
    }
    free(array);

    return 0;
}

String manipulation code using char ** pointer gives unexpected result

1 Answers