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Archived:Converting C strings to numbers

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Article Metadata
Tested with
Devices(s): Nokia N93
Platform(s): S60 3rd Edition
S60 3rd Edition (initial release)
Keywords: atoi(), atof(), atol(), strtod(), strtol()
Created: nymanaki (04 Apr 2008)
Last edited: lpvalente (11 Aug 2012)



The simplest way to convert a string to an integer in Open C is to use atoi(). However, atoi() does not differentiate between '0' and invalid input and it does not allow specifying which base you are working with. Other analogous functions to atoi() are atof() and atol(). These functions can be used with the variable types "double" and "long".

Functions strtod() and strtol() offer more control over the conversion process and they will not produce unexpected results on overflow during conversion. These functions make the position of the first unread character in the input string available by assigning it to the second parameter. The function strtol() has also a third parameter to indicate the base of the numeral string.

Note: In order to use this code, you need to install Open C plug-in.

This snippet can be self-signed.

MMP file

The following libraries are required:

LIBRARY libc.lib

Source file

#include <stdlib.h> //atoi, atof(), atol(), strtod(), strtol()
#include <stdio.h> //printf
int main(void)
char* int_str = "123"; /* NOTE: atoi will convert e.g. value "xyz" to '0' */
char* double_str = "123.0";
char* long_str = "123456";
int int_result = 0;
double double_result = 0.0;
long long_result = 0;
char* string_to_convert = NULL;
char* rest_of_the_string = NULL;
double double_value = 0.0;
long long_value = 0;
/* functions atoi(), atof() and atol() */
int_result = atoi(int_str);
printf("The string %s as an integer is = %d\n",int_str,int_result);
double_result = atof(double_str);
printf("The string %s as a double is = %f\n",double_str,double_result);
long_result = atol(long_str);
printf("The string %s as a long is = %ld\n",long_str,long_result);
/* functions strtod() and strtol() */
string_to_convert = "123.456RestOfTheString";
double_value = strtod(string_to_convert, &rest_of_the_string);
printf("The string = %s, number = %f, rest = %s\n",
string_to_convert, double_value, rest_of_the_string);
string_to_convert = "123456789NoMoreNumbers";
rest_of_the_string = NULL;
long_value = strtol(string_to_convert, &rest_of_the_string, 10 ); /* use base 10 */
printf("The string = %s, number = %ld, rest = %s\n",
string_to_convert, long_value, rest_of_the_string);
return 0;


Five different string-to-number conversions have been executed and are displayed as standard output.

This page was last modified on 11 August 2012, at 18:19.
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