Returning pointers to locally allocated memory

[krreks]

I know it's not "best practice" returning pointers to local variables, but in this case I think i need it.

I'm writing a method to remove some characters at the beginning and some at the end of a string and return the result as a pointer to memory allocated in the function.

I developed the function separately, it worked, but when implemented with the rest of the code it doesn't return anything... It seems that the letters is added to the var new, but when i try printing the string, nothing is returned....

char * prepareMessage(char str[]) {
    int i=0, j=0, state=0;
 
    while(str[i]==' ') { i++; } //skip whitespace
 
    while(state == 0 && str[i]!='\0') { //find the start of the message (after 'new ')
        if(str[i]=='n' && str[i+1]=='e' && str[i+2]=='w' && str[i+3]==' ') {
            state = 1;
            i += 4;
        }
        i++;
    }
 
    char * new = malloc(strlen(str)-i);
    while(str[i]!='\0') { //copy the chars to the new var
        *(new+j) = str[i++];
        j++;
    }
 
    if(*(new+j-1)=='\"') { //remove trailing " and append \0
        *(new+j-1) = '\0';
    } else { //no trailing " - something wrong, free mem and return NULL
        free(new);
        return NULL;
        debug("ERROR: NO TRAILING \"");
    }
 
    return new;
}

The str[] comes from

char inputBuffer[200] = "";
fgets(inputBuffer, 200, stdin)

Hope someone is able to point me in the right direction...

Thanks in advance!

[JosAH]

Just a few remarks:

1) your first two while loops can be replaced by a single strstr() function call.
2) your third while loop can be replaced by a single strcpy() function call.
3) why do you use two notations? i.e. s[i] and *(s+i)

kind regards,

Jos

[krreks]

Just a few remarks:

1) your first two while loops can be replaced by a single strstr() function call.
2) your third while loop can be replaced by a single strcpy() function call.
3) why do you use two notations? i.e. s[i] and *(s+i)

char * prepareMessage(char str[]) {
    int i=0;
    char * new;
 
    char * pMsgStart = strstr(str, "new ");
    if(pMsgStart!=NULL) {
        int i = (strstr(str, "new ")-&str[0])+5;
        new = malloc(strlen(str)-i);
        strcpy(new, (str+i));
        if(new[strlen(new)-1] == '"') {
            new[strlen(new)-1] = '\0';
            return new;
        } else {
            return NULL;
        }
    } else
        return NULL;
}

Then the code is more compact and uses the built in functions (smart :). But I'm still not getting proper output... I think it might be the input that's not proper :s

This is my testcase which makes it fail:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
char * prepareMessage(char str[]) {
    int i=0, j=0, state=0;
    char * new;
 
    char * pMsgStart = strstr(str, "new ");
    if(pMsgStart!=NULL) {
        int i = (strstr(str, "new ")-&str[0])+5;
        new = malloc(strlen(str)-i);
        strcpy(new, (str+i));
        if(new[strlen(new)-1] == '"') {
            new[strlen(new)-1] = '\0';
            return new;
        } else {
            return NULL;
        }
    } else
        return NULL;
}
 
int main() {
    char inputBuffer[200];
 
    do { 
        if(fgets(inputBuffer, 200, stdin)==0) {
            strcpy(inputBuffer,"exit");
        } else {
            char * new = prepareMessage(inputBuffer);
            printf("%s\n",new);
            free(new);
        }
    } while (strcmp ("Apple",inputBuffer) != 0);
 
 
    return 0;
}

K

[JosAH]

You should malloc at least one byte more on line #12; the \0 character needs to
be stored as well.

kind regards,

Jos

[JosAH]

ps. oh, and btw, the fgets() function also returns the end-of-line character(s) in
the buffer so the last character can never be a double quote in your buffer.

kind regards,

Jos

[krreks]

You should malloc at least one byte more on line #12; the \0 character needs to
be stored as well.

Ok. But it's not much use if nothing is returned... It's something with the fgets i think... If I use a plain array like this:

char inputBuffer[200] = "new \"message\"";
char * new prepareMessage(inputBuffer);
 

it works, but the value comes from fgets() like this:

char inputBuffer[200] = "";
    char * tmp = malloc(200);
 
    do { 
        if(fgets(tmp, 200, stdin)==0) {
            strcpy(inputBuffer,"exit");
        } else {
            strcpy(inputBuffer, tmp);
            char * new = prepareMessage(inputBuffer);
            printf("%s\n",new);
            free(new);
        }
    } while (strcmp ("Apple",inputBuffer) != 0);

it's not working... Why!?

[krreks]

it's not working... Why!?

I still don't know why, but this code seems to be working. Not 100% shure, but it gives me the right string every time :)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
char * prepareMessage(char str[]) {
    int i=0, j=0, state=0;
    char * new;
 
    char * pMsgStart = strstr(str, "new ");
    if(pMsgStart!=NULL) {
        int i = (strstr(str, "new ")-&str[0])+5;
        new = malloc(strlen(str)-i+1);
        strcpy(new, (str+i));
        if(new[strlen(new)-2] == '"') {
            new[strlen(new)-2] = '\0';
            return new;
        } else {
            return NULL;
        }
    } else {
        return NULL;
    }
}
 
int main() {
    char inputBuffer[200] = "";
    char * tmp = malloc(200);
 
    do { 
        if(fgets(tmp, 200, stdin)==0) {
            strcpy(inputBuffer,"exit");
        } else {
            strcpy(inputBuffer, tmp);
            char * new = prepareMessage(inputBuffer);
            printf("%s (%d chars long)\n",new, strlen(new));
            free(new);
        }
    } while (strcmp ("Apple",inputBuffer) != 0);
 
    return 0;
}
 
 

[JosAH]

it's not working... Why!?

Read my reply #5.

kind regards,

Jos