CODEWITHSUNDEEP
cfile-io
lacrosse1991
lacrosse1991

Reputation: 3162

where is this "(null)" coming from

I am trying to implement some code to pick out the last line of a text file, and am currently using this example, but I cannot figure how to prevent the code from printing out "(null)" when it cannot find the file, at the moment when it cannot find the file, it will print out "file cannot open at last line: no such file or directory" then it will print the null thing below, and for this particular application i would rather that it just print out the "file cannot open" part, but not print out the null part, if anyone could point me in the right direction for this I would really appreciate it, thanks! 

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

#ifndef max
#define max(a, b) ((a)>(b))? (a) : (b)
#endif

long GetFileSize(FILE *fp){
    long fsize = 0;

    fseek(fp,0,SEEK_END);
    fsize = ftell(fp);
    fseek(fp,0,SEEK_SET);//reset stream position!!

    return fsize;
}
char *lastline(char *filepath){
    FILE *fp;
    char buff[4096+1];
    int size,i;
    long fsize;
    if(NULL==(fp=fopen(filepath, "r"))){
        perror("file cannot open at lastline");
        return NULL;
    }
    fsize= -1L*GetFileSize(fp);
    if(size=fseek(fp, max(fsize, -4096L), SEEK_END)){
        perror("cannot seek");
        exit(0);
    }
    size=fread(buff, sizeof(char), 4096, fp);
    fclose(fp);
    buff[size] = '\0';
    i=size-1;
    if(buff[i]=='\n'){
        buff[i] = '\0';
    }
    while(i >=0 && buff[i] != '\n')
        --i;
    ++i;
    return strdup(&buff[i]);
}

int main(void){
    char *last;

    last = lastline("data.txt");
    printf("\"%s\"\n", last);
    free(last);
    return 0;
}

Upvotes: 0

Views: 101

Answers (3)

P.P
P.P

Reputation: 121387

You should check the return value of fread to check whether it was successful. If fread fails then errorno will be set and your variable size won't the number of chars read. Hence, your subsequent use of the variable buff:

buff[size] = '\0';
i=size-1;
if(buff[i]=='\n'){
    buff[i] = '\0';
}

could produce unexpected results. So is the strdup(buff[i]) which is returned from the function to main.

Upvotes: 1

Karoly Horvath
Karoly Horvath

Reputation: 96258

It's coming from here: printf("\"%s\"\n", last);

if (last) {
  printf("\"%s\"\n", last);
  free(last);
}

Upvotes: 1

Michiel Pater
Michiel Pater

Reputation: 23033

You could add these two lines after the line last = lastline("data.txt");. The reason NULL is printed, is because you are printing the result of the function lastline(). This function will return NULL on failure.

if( last != NULL )
    printf( "\"%s\"\n", last );

Upvotes: 1

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