strcmp on a line read with fgets

Question

I'm trying to compare two strings. One stored in a file, the other retrieved from the user (stdin).

Here is a sample program:

int main()
{
    char targetName[50];
    fgets(targetName,50,stdin);

    char aName[] = "bob";
    printf("%d",strcmp(aName,targetName));

    return 0;
}

In this program, strcmp returns a value of -1 when the input is "bob". Why is this? I thought they should be equal. How can I get it so that they are?

Answer 1

strcmp is one of the few functions that has the reverse results of true and false...if the strings are equal, the result is 0, not 1 as you would think....

if (strcmp(a, b)) {
    /* Do something here as the strings are not equal */
} else {
    /* Strings are equal */
}

Speaking of fgets, there is a likelihood that there is a newline attached to the end of the string...you need to get rid of it...

+-+-+-+--+--+
|b|o|b|\n|\0|
+-+-+-+--+--+

To get rid of the newline do this. CAVEATS: Do not use "strlen(aName) - 1", because a line returned by fgets may start with the NUL character - thus the index into the buffer becomes -1:

aName[strcspn(aName, "\n")] = '\0';

+-+-+-+--+
|b|o|b|\0|
+-+-+-+--+

Now, strcmp should return 0...

Answer 2

fgets reads until it sees a newline then returns, so when you type bob, in the console, targetName contains "bob\n" which doesn't match "bob". From the fgets documenation: (bolding added)

Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or a the End-of-File is reached, whichever comes first. A newline character makes fgets stop reading, but it is considered a valid character and therefore it is included in the string copied to str. A null character is automatically appended in str after the characters read to signal the end of the C string.

You need to remove the newline from the end of targetName before you compare.

int cch = strlen(targetName);
if (cch > 1 && targetName[cch-1] == '\n')
   targetName[cch-1] = '\0';

or add the newline to your test string.

char targetName[50];
fgets(targetName,50,stdin);

char aName[] = "bob\n";
printf("%d",strcmp(aName,targetName));