strncmp() gives a false positive

Question

I have been expirencing an issue with strncmp and getline... so a i made a little test app. strncmp gives false positives and i can't figure out what i am doing wrong.

this is the program

#include <stdio.h>

FILE *fp=NULL;
char uname[4]="test";
char *confline=NULL;
size_t conflinelength=0;
int datlength;

main()
{
    datlength=4;
    fp=fopen("test.txt","r+");
    while (getline(&confline,&conflinelength,fp)!=-1)
    {
        if (strncmp(confline,uname,(datlength-1))==0);
        {
            printf("match\n");
            return;
        }
    }
    printf("no match\n");
    fclose(fp);
}

and this is "test.txt"...

bjklas

and the program outputs

match

`char uname[4]="test";` is OK as `datlength=4`, but suggest `char uname[]="test";`. Also `printf("match\n");` should be followed by `fclose(fp);`. — chux - Reinstate Monica, Oct 15 '13 at 22:20

score 4 · Accepted Answer · answered Oct 15 '13 at 22:16

4

if (strncmp(confline,uname,(datlength-1))==0);
    printf("match\n");

is equivalent to

if (strncmp(confline,uname,(datlength-1))==0)
    ;
printf("match\n");

You need to remove the ; from the end of your if line

answered Oct 15 '13 at 22:16

simonc

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score 3 · Answer 2 · answered Oct 15 '13 at 22:16

3

if (strncmp(confline,uname,(datlength-1))==0);
                                             ^ roh-roh

answered Oct 15 '13 at 22:16

Charlie Burns

6,994
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ryyker · Answer 3 · 2013-10-15T23:19:53.417

If you would have usedchar uname[5]="test"; this line would have automatically appended the '\0', making it a C string, but with only 4 spaces allocated, it couldn't. Instead, it is an unterminated char array...

char uname[4]="test"; results in an unterminated set of chars, i.e. not a C string. i.e. "|t|e|s|t|"
This will compile, but it cannot be processed by some string functions, (strcmp, strlen,etc) resulting in run-time errors.

Suggest char uname[]="test"; this will automatically take care of appending the NULL terminator yielding a C string: "|t|e|s|t|\0|". This method is a preferred way to initialize char arrays.

score 0 · Answer 4 · answered Oct 15 '13 at 22:34

0

I guess it is your typo when you put the semicolon ; at the end of if condition instead of its statement, i.e. right after the curve bracket }

Out of interest, I would prefer using fgets because getline is not in a standard C library

answered Oct 15 '13 at 22:34

Tu Bui

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are you referring to the line _if (strncmp(confline,uname,(datlength-1))==0);_ ? – ryyker Oct 15 '13 at 22:39

strncmp() gives a false positive

4 Answers4