C Programming/stdlib.h/itoa

The itoa (integer to ASCII) function is a widespread non-standard extension to the standard C programming language. It cannot be portably used, as it is not defined in any of the C language standards; however, compilers often provide it through the header <stdlib.h> while in non-conforming mode, because it is a logical counterpart to the standard library function atoi.

void itoa(int input, char *buffer, int radix)

itoa takes the integer input value input and converts it to a number in base radix. The resulting number (a sequence of base-radix digits) is written to the output buffer buffer.

Depending on the implementation, itoa may return a pointer to the first character in buffer, or may be designed so that passing a null buffer causes the function to return the length of the string that would have been written into a valid buffer.

For converting a number to a string in base 8 (octal), 10 (decimal), or 16 (hexadecimal), a Standard-compliant alternative is to use the standard library function sprintf.

The function itoa appeared in the first edition of Kernighan and Ritchie's The C Programming Language, on page 60. The second edition of The C Programming Language ("K&R2") contains the following implementation of itoa, on page 64 [for Spanish editions go to page 47]. The book notes several issues with this implementation, including the fact that it does not correctly handle the most negative number −2^wordsize-1.^[1]

/* itoa: convert n to characters in s */
voiditoa(intn,chars[])
{
inti,sign;

if((sign=n)<0)/* record sign */
n=-n;/* make n positive */
i=0;
do{/* generate digits in reverse order */
s[i++]=n%10+'0';/* get next digit */
}while((n/=10)>0);/* delete it */
if(sign<0)
s[i++]='-';
s[i]='0円';
reverse(s);
}

The function reverse used above is implemented two pages earlier:

#include<string.h>

/* reverse: reverse string s in place */
voidreverse(chars[])
{
inti,j;
charc;

for(i=0,j=strlen(s)-1;i<j;i++,j--){
c=s[i];
s[i]=s[j];
s[j]=c;
}
}

An itoa function (and a similar function, ftoa, that converted a float to a string) was listed in the first-edition Unix manual.^[2] Unlike the versions given above, the Unix library version had an interface roughly equivalent to

void itoa(int input, void (*subr)(char))

and would invoke the callback routine subr on each character in the output string, thus eliminating the need for a buffer big enough to hold the entire string.

1. ↑ For the solution to this exercise, see "K&R2 solutions" on clc-wiki.net.
2. ↑ "Unix Programmer's Manual", November 3, 1971. Section "Library routines".

• itoa() implementations with performance tests
• lexical_cast - C++ alternative, part of the boost libraries
• modp_numtoa - C/C++ alternative for converting integers and floats to char buffers.
• Good old Integer To Ascii conversion: itoa - Another fast implementation of itoa for various datatypes, plus some boost-style wrapping in the form of boost::lexical_cast template specializations.

• Book:C Programming/stdlib.h

• Pages with deep filing