strcmp results in false when strings are equal

Question

This is my code:

        if(strcmp(pch,map[i].name)==0){
            printf("Equal\n");
            return 0;
        }

pch is read from a file, map[i].name has a known size of 64. This works great for strings smaller than 64. when comparing these two strings below of size 63:

file11111111111111111111111111111111111111111111111111111111111 and

file11111111111111111111111111111111111111111111111111111111111

everything is peachy and the result as expected is equal, but when these two (of size 64) are compared:

file111111111111111111111111111111111111111111111111111111111111 and

file111111111111111111111111111111111111111111111111111111111111

the return is false. I thought of doing:

        if(strncmp(pch,map[i].name,64)==0){
            printf("Equal\n");
            return 0;
        }

And it does work for strings of exact size of 64, but for strings that are smaller the result is random. What kind of quirkiness am i dealing with here?

EDIT: this is the full code:

    char * pch;
    char tempFilesNeeded[100*64+100];
    strcpy(tempFilesNeeded,map[i].filesNeeded);
    pch = strtok(tempFilesNeeded,",");
    while (pch != NULL)
    {
        if(strcmp(pch,map[i].name)==0){
            printf("Equal\n");
            return 0;
        }

        pch = strtok (NULL, ",");
    }

unwind · Accepted Answer · 2012-11-14 13:58:54Z

9

Well, if it's

char pch[64];

then you can't have 64 visible characters in there, since the last entry is needed for the termination. If you do have "file111111111111111111111111111111111111111111111111111111111111" in that array, it's not terminated and calling strcmp() on it invokes undefined behavior.

Also, as a minor point, saying that strcmp() returns "false" is wrong, since its return is not boolean. It returns the relation between the two first differing characters; if no characters differ the strings are equal, then it returns zero.

answered Nov 14, 2012 at 13:58

unwind

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2 Comments

xiaoyi Over a year ago

@Tom "map[i].name has a known size of 64" so he's point still holds.

xiaoyi Over a year ago

@Tom You just can't put 64-char-length string in a 64-length char array.

PhiLho · Accepted Answer · 2012-11-14 13:58:42Z

5

If one or both your arrays have an exact size of 64, you are missing the final '\0' ending the string.

answered Nov 14, 2012 at 13:58

PhiLho

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