Sprintf() messing up and not doing what I want it to do

Question

So me and my TA have been trying to figure out why this isnt working because for my assignment we have to use sprintf() and a whole new buffer to print out each line in a text file. without the sprintf() and printing it normally it works completely fine.

This is what I need

And this is what i got using sprintf()

Heres my code:

int buff = 1000;

    char *ptr = mmap (NULL,buff, PROT_WRITE | PROT_READ ,MAP_PRIVATE| MAP_ANONYMOUS,0,0);
        if(ptr == MAP_FAILED){
            printf("Mapping Failed\n");
            return 1;
        }
    char *oof = mmap (NULL,buff, PROT_WRITE | PROT_READ ,MAP_SHARED| MAP_ANONYMOUS,0,0);
        if(oof == MAP_FAILED){
            printf("Mapping Failed\n");
            return 1;
        }
    int f1 = open("file2.txt", O_RDWR);
    int fd = open("file.txt", O_RDONLY);
        if(fd < 0){
            printf("Uh oh u made an oopsie\n");
        }else{
            printf("Hey look at that it opened\n\n");
        }

        int length = read(fd,ptr,sizeof(char)*buff);
        int count = 1;

    for(int i = 0; i<length;i++){
        if(ptr[i] == '\n'){
            count++;
        }
    }

    int lengths[count];
    for(int i = 0;i<count;i++){
        lengths[i] = 0;
    }

    int m = 0;
    for(int i = 0;i<length;i++){
        lengths[m]++;
        if(ptr[i] == '\n'){
            m++;
        }
    }

    int j = 0;
    int k = 0;
    char **lineArray = (char **) malloc(sizeof(char *) *count);
    for(int i = 0; i<count; i++){
       
        
        lineArray[i] = (char *) malloc(sizeof(char)* lengths[i]);
        k = 0;
        while(ptr[j] != '\n')
        {
            lineArray[i][k] = ptr[j];
            j++;
            k++;
        }
        j++;

        sprintf(oof,"\nlineArray[%d] = %s\n",i,lineArray[i]);
        write(1,oof,sizeof(char)*buff);
        //printf("\nlineArray[%d] = %s\n",i ,lineArray[i]);
    }

I have messed around with this so much and i know its something to do with the mmap buffer stuff but i cant figure out what to do to fix it.

Answer 1

In this part:

        sprintf(oof,"\nlineArray[%d] = %s\n",i,lineArray[i]);
        write(1,oof,sizeof(char)*buff);

you are unconditionally writing buff (=1000) bytes, so it may contain data in previous lines.

You should get the length of string from the return value of sprintf and use that as the size to write:

        int stringLength = sprintf(oof,"\nlineArray[%d] = %s\n",i,lineArray[i]);
        if (stringLength > 0) write(1,oof,sizeof(char)*stringLength);

Also you have to add '\0' to terminate the string, or sprintf with %s will go reading beyond the end of the buffer and invoke undefined behavior.

        while(ptr[j] != '\n')
        {
            lineArray[i][k] = ptr[j];
            j++;
            k++;
        }
        lineArray[i][k] = '\0'; // add this to terminate the string
        j++;