c,string,sorting,malloc,free , Freeing array of dynamic strings / lines in C


Freeing array of dynamic strings / lines in C

Question:

Tag: c,string,sorting,malloc,free

I am writing a program that is sorting the lines from the input text file. It does its job, however I get memory leaks using valgrind.

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

char* getline(FILE * infile)
{
    int size = 1024;
    char * line = (char*)malloc(size);
    int temp;

    int i=0;
    do
    {
        (temp = fgetc(infile));
        if (temp !=EOF)
            line[i++]=(char)temp;
        if (i>=size)    
        {
            size*=2;
            line = (char*)realloc(line, size);
        }

    }while (temp != '\n' && temp !=EOF);
    if (temp==EOF)
        return NULL;
    return line;
}

void print_line (char * line)
{
    printf("%s", line);
}

int myCompare (const void * a, const void * b )
{
    const char *pa = *(const char**)a;
    const char *pb = *(const char**)b;

    return strcmp(pa,pb);
}

int main (int argc, char* argv[])
{
    FILE * infile;
    FILE * outfile;

    infile = fopen(argv[1], "r");
    if (infile==NULL)
    {
        printf("Error");
        exit(3);
    }
    outfile = fopen(argv[2], "w");
    if (outfile==NULL)
    {
        printf("Error");
        exit(3);
    }
    char * line;
    char **all_lines;
    int nlines=0;

    while((line=getline(infile))!=NULL)
    {
        print_line(line);
        nlines++;
    }

    all_lines=malloc(nlines*sizeof(char*));
    rewind(infile);
    int j=0;
    printf("%d\n\n", nlines);

    while((line=getline(infile))!=NULL)
    {

        all_lines[j]=line;
        j++;
    }

    qsort(all_lines, nlines, sizeof(char*), myCompare);

    for (int i=0; i<nlines;i++)
    {
        print_line(all_lines[i]);
        fprintf(outfile, "%s", all_lines[i]);
    }

    for(int i =0; i<nlines;i++)
    {
        free(all_lines[i]);
    }

    free(all_lines);
    fclose(infile);
    fclose(outfile);
    return 0;   
}

Any ideas where they might come from? I loop over all_lines[] and free the content, then free all_lines itself.

UPDATE:

Ok, so I've done the updates you have suggested. However, now valgrind throws error for functions fprintf in my program. Here is what it says:

11 errors in context 2 of 2:

==3646== Conditional jump or move depends on uninitialised value(s)
==3646==    at 0x40BA4B1: vfprintf (vfprintf.c:1601)
==3646==    by 0x40C0F7F: printf (printf.c:35)
==3646==    by 0x80487B8: print_line (sort_lines.c:44)
==3646==    by 0x804887D: main (sort_lines.c:77)
==3646==  Uninitialised value was created by a heap allocation
==3646==    at 0x4024D12: realloc (vg_replace_malloc.c:476)
==3646==    by 0x8048766: getline (sort_lines.c:30)
==3646==    by 0x804889A: main (sort_lines.c:75)

I would like to know why it reports error on simply fprintf those lines to a text file. I've looked up that it is something concerning gcc optimalization turning fprint into fputs but I dont get this idea


Answer:

There are multiple problems in your code:

function getline:

function MyCompare:

The lines read include the '\n' at the end. Comparison may yield unexpected results: "Hello\tworld\n" will come before "Hello\n". You should strip the '\n' in getline and modify the printf formats appropriately.

function main:

Printing the lines before sort is not very useful and complicates testing.


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