C: Realloc invalid pointer?

Question

This is my task from school: Write a function insertString that inserts the string s2 into s1 at index n.s1 has been allocated using malloc and should be resized (the function is void again).

The program gave me a NULL on PC and when I switched to the phone the compilor said my realloc has a invalid pointer. But I really don't know what I did wrong.

Here is the code:

void insertString(char *str1, char *str2, int n){
    int lengStr2=strlen(str2);
    printf("%d %d ", lengStr2,n);
    printf("\nstr1= %s, str2: %s, n: %d ",str1,str2,n);
    str1=(char*)realloc(str1,lengStr2+n+1);
    if (str1==NULL){
        printf("Error\n");
        free(str1);
        return -1;
    }
        printf("\nstr1= %s, str2: %s, n: %d ",str1,str2,n);
    memcpy(str1+n,str2,lengStr2+1);
        printf("\nstr1= %s, str2: %s, n: %d ",str1,str2,n);
}
void testInsertString( char *str2, int n, char *expect){
    char*str1=(char*)malloc(3*sizeof(char));
    str1="hoi";
    printf("\nstr1= %s, str2: %s, n: %d ",str1,str2,n);
    insertString(str1,str2,n);
    printf("--> result:%s --> ",str1);
    (strcmp(str1,expect)==0)?printf("Success"): printf("Failure");
    free(str1);
    printf("\nIs Free\n");
}

Here the output:

str1= hoi, str2: Hallo, n: 1 5 1
str1= hoi, str2: Hallo, n: 1 Error
--> result:hoi --> Failure
Is Free

Process returned 0 (0x0)

Please if you know what I did wrong can you show me the correct version? Even so I have the problem that I can't right a program right just by reading a text, I need to see how it should be written. So I need to see the right result to learn from the mistake ^^" (that's why some stuff in school is for me really hard). Thanks in advance ^^

str1 is a string literal. It cannot be realloced. After str1=malloc(...), you need to copy data into str1 with sprintf or strcat or the like. Reassigning str1="hoi" throws away the address of the memory you allocated. It is a memory leak. — William Pursell
– William Pursell, Commented Jan 13, 2021 at 21:29
consider adding -Wwrite-strings to compiler flags; that would cause it to warn about str1="hoi". — M.M
– M.M, Commented Jan 13, 2021 at 21:37
OT: regarding: int lengStr2=strlen(str2); the function: strlen() returns a size_t (an unsigned value) not an int — user3629249
– user3629249, Commented Jan 14, 2021 at 1:58
in function: void insertString(char *str1, char *str2, int n){ this function is declared (via the void) to not return a value, However, that function contains: return -1; When compiling, always enable the warnings, then fix those warnings. ( for gcc, at a minimum use: -Wall -Wextra -Wconversion -pedantic -std=gnu11 ) Note: other compilers use different options to produce the same results — user3629249
– user3629249, Commented Jan 14, 2021 at 2:03
regarding: str1=(char*)realloc(str1,lengStr2+n+1); if (str1==NULL){ printf("Error\n"); free(str1); return -1; 1) when calling: realloc() always assign to a 'temp' variable. Otherwise, when the function fails, the original pointer is lost (resulting in a memory leak) 2) error messages should be output to stderr, not stdout Suggest using: perror( "realloc failed" ); 3) the returned type is void* which can be assigned to any pointer. Casting just clutters the code (and is error prone) — user3629249
– user3629249, Commented Jan 14, 2021 at 2:09

William Pursell · Accepted Answer · 2021-01-13 21:32:50Z

2

char *str1 = malloc(3*sizeof(char));  /* Allocate memory */
str1="hoi";       /* Discard the only reference to the allocated memory */

The above two lines are similar in spirit to:

int x = 5; x = 7;

You need to copy the string "hoi" into the newly allocated memory, and you need to allocate at least 4 bytes to hold the string "hoi". For example:

char *hoi = "hoi";
char *str1 = malloc(strlen(hoi) + 1);
if( str1 == NULL ){
    perror("malloc");
    exit(EXIT_FAILURE);
}
sprintf(str1, "%s", hoi);

answered Jan 13, 2021 at 21:32

William Pursell

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dspr · Accepted Answer · 2021-01-14 09:26:45Z

0

Here is a fixed version :

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

// Changing the signature to return the reallocated ptr would be safer
char * insertString(char *str1, char *str2, int n){
    int lengStr2=strlen(str2);

    //because str1 could differ from the received ptr after reallocation
    str1=(char*)realloc(str1,lengStr2+n+1); 
    if (str1==NULL){
        printf("Error\n");
        return NULL;
    }
    strcpy(str1+n,str2);
    return str1; // So we return the new ptr to the caller
}
void testInsertString( char *str2, int n, char *expect){
    char*str1=(char*)malloc(6*sizeof(char)); // The null termination must be counted
    strcpy(str1, "Hello");                   // You should use strCpy here instead of =
    str1 = insertString(str1,str2,n);        // Refreshes the ptr
    printf("\n--> result:%s --> ",str1);
    (strcmp(str1,expect)==0)?printf("Success"): printf("Failure");
    free(str1);
}

int main() {
    testInsertString(" World", 5, "Hello World"); // --> result:Hello World --> Success
    return 0;
}

edited Jan 14, 2021 at 9:26

answered Jan 13, 2021 at 21:52

dspr

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1 Comment

Bob__ Over a year ago

You may want to compare the second comment in this snippet with the subsequent if statement. Hint: stackoverflow.com/questions/44789295/correct-use-of-realloc

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