CODEWITHSUNDEEP
c
Martin Vegter
Martin Vegter

Reputation: 527

C: convert argv[1] to double

I need to pass a double to my program, but the following does not work:

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

double ddd;

ddd=atof(argv[1]);

printf("%f\n", ddd);

return 0;
}

If I run my program as ./myprog 3.14 it still prints 0.000000.

Can somebody please help?

Upvotes: 10

Views: 15912

Answers (3)

gthacoder
gthacoder

Reputation: 2351

In C you don't require to declare a function before you use it (in contrast with C++), and if that happens (no prototype), compiler makes some assumptions about that function. One of those assumptions is that it returns int. There's no error, atof() works, but it works incorrectly. It typically get whatever value happens to be in the register where int is supposed to be returned (it is 0 in your case, but it can be something else).

P.S. atof() and atoi() hide input errors (which you can always see by adding option -Wall to your gcc compiler call: gcc -Wall test.c), so most people prefer to use strtol() and strtod() instead.

Upvotes: 2

danfuzz
danfuzz

Reputation: 4353

As remyabel indicated, you probably neglected to #include <stdlib.h>. The reason this matters is that, without having a declaration of atof(), the C standard mandates that the return value is assumed to be int. In this case, it's not int, which means that the actual behavior you observe (getting a return value of 0) is technically unspecified.

To be clear, without the double-returning declaration, the line ddd=atof(argv[1]) is treated as a call to an int-returning function, whose result is then cast to a double. It is likely the case that the calling conventions on the particular system you're on specify that ints and doubles get returned in different registers, so the 0 is likely just to be whatever happened to be in that particular register, while the double return value is languishing, unobserved.

Upvotes: 4

user1508519
user1508519

Reputation:

My guess is you forgot to include #include <stdlib.h>. If this is the case, gcc will issue the following warning:

warning: implicit declaration of function 'atof' [-Wimplicit-function-declaration]

And give the exact output you provided: 0.000000.

Upvotes: 9

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