Comparing command parameter with argv[] is not working

Question

I am trying to compare the parameter of command with argv[] but it's not working. Here is my code.

./a.out -d 1

In main function

int main (int argc, char * const argv[]) {

if (argv[1] == "-d")

    // call some function here

}

But this is not working... I don't know why this comparison is not working.

See my answer here: http://stackoverflow.com/questions/3303164/why-isnt-if-maya-maya-true-in-c/3303176#3303176 — Paul Nathan, Jul 29 '10 at 18:03

score 31 · Accepted Answer · answered Jul 29 '10 at 18:02

31

You can't compare strings using ==. Instead, use strcmp.

#include <string.h>

int main (int argc, char * const argv[]) {

if (strcmp(argv[1], "-d") == 0)

// call some function here

}

The reason for this is that the value of "..." is a pointer representing the location of the first character in the string, with the rest of the characters after it. When you specify "-d" in your code, it makes a whole new string in memory. Since the location of the new string and argv[1] aren't the same, == will return 0.

answered Jul 29 '10 at 18:02

Adrian

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2

You may want to mention that the reason C strings can't be compared is because it doesn't compare the contents, just the pointers. – Cristián Romo Jul 29 '10 at 18:03
1

or if you know your args are single letters (argv[1][1] =='d') – Martin Beckett Jul 29 '10 at 18:03
@Mark: No, because `argv[1][0] == '-'`. – Adrian Jul 29 '10 at 18:09
@Martin: It is just a sample input, which means it can have more than 1 letter, but your suggestion is very good. – itsaboutcode Jul 29 '10 at 18:11
1

@itsaboutcode - thats why unix commands traditionally had single letter options – Martin Beckett Jul 29 '10 at 20:48

score 14 · Answer 2 · answered Jul 29 '10 at 18:09

In C++ let std::string do the work for you:

#include <string>
int main (int argc, char * const argv[]) {

if (argv[1] == std::string("-d"))

// call some function here

}

In C you'll have to use strcmp:

if (strcmp(argv[1], "-d") == 0)

// call some function here

}

score 2 · Answer 3 · answered Jul 29 '10 at 18:02

2

You may want to use strcmp here.

answered Jul 29 '10 at 18:02

Brandon Horsley

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score -4 · Answer 4 · edited Nov 18 '11 at 00:45

-4

won't that be:

if (argv[0] == "-d")

0 not 1?

edited Nov 18 '11 at 00:45

chown

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answered Jul 29 '10 at 18:01

Viper_Sb

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18

No, because the first element of `argv` is the program name. – Adrian Jul 29 '10 at 18:02
1

no, C and C++ have char[] for character strings like this; it turns into a pointer comparison – Paul Nathan Jul 29 '10 at 18:03
1

Nop. First, you are comparing addresses, not contents. Second, argv[0] is the command path. – ninjalj Jul 29 '10 at 18:03

Comparing command parameter with argv[] is not working

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