Thread: need help with strcat function

Code:

printf("%s\n", path) ;          //this print /usr/bin/
printf("%s\n", parameter[0]) ;          //this print w
strcat(path, parameter[0]) ;
printf("%s\n", path) ;          //this print wusr/bin/

the strcat function seems to overwrite the path rather than append to it
any idea why?

thanks in advance

to use strcat you have to pass two array of character parameters. Your second parameter in your strcat call seems to be a single character to me. What you trying to acheive here. If you need to append the character string parameter to path just pass parameter without the element accessor. If you wish to add the single character you cant use strcat.

maybe i add the declaration of the variables to be clear

Code:

char *parameter[100] ;
char *path ;

i want to append my parameter to my path string
but i just cannot get the desired result

my parameter is for keeping an array of pointers (each of the pointer points to a string)
my path is just a pointer to a string

this is the whole code if needed

Code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <string.h>

int main(int argc, char *argv[])
{
    /*check correct num of arguements*/
    if(argc != 2)
    {
         printf("Wrong number of arguments!") ;
         return 0 ;
    }

    FILE *f ;
    f = fopen(argv[1], "r") ;

    char *line[100] ;
    char *word ;
    int count = 0 ;
    
    while (!feof(f))
    {
        word = malloc(100) ;
        fgets(word, 10000, f) ;
        line[count] = word ;
        count++ ;
    }
    
    char *parameter[100] ;
    char *path ;
    char *token ;
    int count2 = 0 ;
    int c ;
    for (c = 0 ; c < count - 1 ; c++)
    {
        token = strtok(line[c], " \n") ;
        while (token != NULL)
        {
            if(!strcmp("setpath", token))
            {
                //printf("newpath\n") ;
                count2++ ;
                token = strtok(NULL, " \n") ;
                path = malloc(100) ;
                path = token ;
                printf("path = %s\n", path) ;
                count2++ ;
                token = strtok(NULL, " \n") ;
                continue ;
            }
            parameter[count2] = token ;            
            printf("token = %s\n", parameter[count2]) ;
            count2++ ;
            token = strtok(NULL, " \n") ;
        }
        parameter[count2] = NULL ;
        count2 = 0 ;
    }
    
    //debugging
    printf("%s\n", path) ;
    printf("%s\n", parameter[0]) ;
    strcat(path, parameter[0]) ;
    printf("%s\n", path) ;

        
    int myid = fork() ;

    if (myid == 0)
    {
        execv(path, parameter) ;
        printf("Invalid command") ;
        exit(1) ;
    }
    waitpid(myid, 0, 0) ;
    printf("\n") ;

    fclose(f) ;

    return 0 ;
}

input file contains

Code:

setpath /usr/bin/
w -s

thanks in advance again

It looks like your problem might be coming from the fact that path points to memory returned by strtok(). Try using a new buffer for strcat() instead of using path.

Originally Posted by itsme86

It looks like your problem might be coming from the fact that path points to memory returned by strtok(). Try using a new buffer for strcat() instead of using path.

how do i do that?
i am sorry because i am rather new to c and really dun understand the malloc and buffer thing

thanks for help

By declaring another character array. That's all a buffer is.

Originally Posted by itsme86

By declaring another character array. That's all a buffer is.

as in like this?

Code:

char* newpath = malloc(100) ;
newpath = strcat(path, parameter[0]) ;
printf("%s\n", newpath) ;

this gave the same...

Wow, so many mistakes

First of all, this is C, so no mixing up declarations and statements.

2. Don't try and cram all the functionality into main() - use some functions to do part of the work.

> newpath = strcat(path, parameter[0]) ;
malloc returns memory containing uninitialised data, so your cat appends to some random point in the buffer (possibly way beyond the end of it).

> word = malloc(100) ;
> fgets(word, 10000, f) ;
fgets is only safe if you don't LIE about the buffer size (that's the 2nd post I've seen like this - what's going on?).

> while (!feof(f))
Why this is bad is in the FAQ.

I suggest you try again, and just post your code for reading in an array of lines. Lets get that sorted out before you go off with the tokenising and calling execl.

Originally Posted by Salem

Wow, so many mistakes

First of all, this is C, so no mixing up declarations and statements.

2. Don't try and cram all the functionality into main() - use some functions to do part of the work.

> newpath = strcat(path, parameter[0]) ;
malloc returns memory containing uninitialised data, so your cat appends to some random point in the buffer (possibly way beyond the end of it).

> word = malloc(100) ;
> fgets(word, 10000, f) ;
fgets is only safe if you don't LIE about the buffer size (that's the 2nd post I've seen like this - what's going on?).

> while (!feof(f))
Why this is bad is in the FAQ.

I suggest you try again, and just post your code for reading in an array of lines. Lets get that sorted out before you go off with the tokenising and calling execl.

okie i have the reading of lines done
hope i am doing it right

Code:

 
    FILE *f ;
    f = fopen(argv[1], "r") ;

    char *line[100] ;
    char *word = malloc(10000) ;
    int count = 0 ;
    
    while (fgets(word, 10000, f) != NULL)
    {
        line[count] = word ;
        printf("line = %s\n", line[count]) ;
        count++ ;
        word = malloc(10000) ;
    }
    printf("count = %d\n", count) ;

Code:

printf("%s\n", path) ;          //this print /usr/bin/
printf("%s\n", parameter[0]) ;          //this print w
strcat(path, parameter[0]) ;
printf("%s\n", path) ;          //this print wusr/bin/

now i realise 1 thing
i run this code in cygwin and it will give me wusr/bin/ for the last line of code
however when i run it in windows cmd, it give me the correct output of /usr/bin/w

why is this so?

> why is this so?
Probably luck.

Do you really need to allocate 10K for every single line?

- Read into a large buffer
- allocate the exact space
- copy
- and repeat

Code:

char buff[BUFSIZ];
while ( count < 100 && fgets( buff, BUFSIZ, fp ) ) {
    if ( (line[count] = malloc( strlen(buff) + 1 )) != NULL ) {
        strcpy( line[count], buff );
        count++;
    }
}

> f = fopen(argv[1], "r") ;
Check return results.
And watch how you're mixing declarations and statements, normal C doesn't (yet) allow such things.

[QUOTE=Salem]> why is this so?
Probably luck.

cos i was thinking there is some bug with cygwin

this is the part of the code

Code:

    printf("path = %s\n", path[1]) ;
    printf("parameter = %s\n", parameter[0]) ;
    printf("parameter = %s\n", parameter[1]) ;
    printf("parameter = %s\n", parameter[2]) ;
    
    strcat(path[1], parameter[0]) ;
    printf("newpath = %s\n", path[1]) ;

but the result was
http://img169.exs.cx/img169/8896/bug2ec.jpg

if u notice, the w actually overwrite the newpath which is not at all a variable

so is it really a bug? or is it still my coding?

thanks again

1. I can't read that image - it doesn't resolve
2. 99.x% it's still your code.

Snippets don't help. If you want a yes/no answer then you need a complete program we can compile and run before we can say what is happening (or might be happening).