Where am I accessing illegal memory? realloc(): invalid next size-CodePudding

I'm making a shell simulator for an operating systems course homework. We were asked to add a "history" command that when typed, should print a list of commands that the user entered.

I decided to implement it using a history array that allocates dynamically more memory depending on the size of the new command.

Here is my code:

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

#define BUFFER_SIZE 100

//comment

int main(void)
{
    close(2);
    dup(1);
    char command[BUFFER_SIZE];
    char* hist = NULL;
    // int counter = 0;

    while (1)
    {
        fprintf(stdout, "my-shell> ");
        memset(command, '\0', BUFFER_SIZE);
        fgets(command, BUFFER_SIZE, stdin);
        if(strncmp(command, "exit", 4) == 0)
        {
            break;
        }
        
        //alocate memory for the current position in the hist array
        if(hist == NULL){
            hist = (char*)malloc(sizeof(char)*strlen(command));
        }
        else{
            hist = realloc(hist,sizeof(char)*strlen(command));
        }
        strcat(hist,command);
        printf("the size of the boy: %d\n",(int) strlen(hist));
        // counter  = strlen(command);

        int pid = fork();

        char *argv[BUFFER_SIZE];
        char *pch;

        pch = strtok(command, " \n");

        int i;
        for(i=0; pch != NULL; i  ){
            argv[i] = pch;
            pch = strtok(NULL, " \n");
        }
        argv[i] = NULL;

        int hasAmpersand = 0;
        //check if the last entered character was '&'
        if(*argv[i-1]=='&'){
            // printf("entered &:");
            hasAmpersand = 1;
            //replace it with '\0'
            argv[i-1] = NULL;
        }
        
        if(pid == 0){ //child process execute sys call
            if(strncmp(argv[0],"history",7) == 0){
                printf("%s\n", hist);
                exit(1);
            }
            else{
                execvp(argv[0], argv);
                printf("\nbad syntax\n");
            }
        }
        else{
            if(!hasAmpersand){ //wait for child
                while(wait(NULL) != pid);
            }
        }
    }
    free(hist);

    return 0;
}

The implementation works for up to 6 commands stored in hist but it crashed with the error

realloc(): invalid next size
aborted

I was wondering what causes the issue and I'd love suggestions on fixing it. Thanks.

CodePudding user response：

The second parameter of realloc should be the new size of the entire block, not just the size of data you are "adding" to memory area.

hist = realloc(hist, currentHistSize sizeof(char)*strlen(command));

Make sure you name currentHistSize as you want.

CodePudding user response：

You need room for the trailing string terminator '\0', so add one to the allocated size:

hist = malloc(sizeof(char)*strlen(command)   1);

The argument to realloc also needs to be adjusted. Also you probably want to remove the trailing newline after calling fgets.