Master C String‑Number Conversion Functions: atof, atoi, atol, strtod, strtol, and More

atof – Convert String to Double

Header: #include <stdlib.h> Prototype: double atof(const char *nptr); Explanation: Scans the input string, skips leading whitespace, then parses an optional sign, digits, decimal point, or exponent (e/E). Conversion stops at the first non‑numeric character or the string terminator, returning the resulting double value. It behaves like strtod(nptr, NULL).

Example:

/* Convert strings a and b to numbers and add them */
#include <stdlib.h>
int main(){
    char *a = "-100.23";
    char *b = "200e-2";
    double c = atof(a) + atof(b);
    printf("c=%.2f
", c);
}

Output:

c=-98.23

atoi – Convert String to Integer

Header: #include <stdlib.h> Prototype: int atoi(const char *nptr); Explanation: Similar to atof but returns an int. It stops conversion at the first non‑digit character. Equivalent to strtol(nptr, NULL, 10).

Example:

/* Convert strings a and b to integers and add them */
#include <stdlib.h>
int main(){
    char a[] = "-100";
    char b[] = "456";
    int c = atoi(a) + atoi(b);
    printf("c=%d
", c);
}

Output:

c=356

atol – Convert String to Long Integer

Header: #include <stdlib.h> Prototype: long atol(const char *nptr); Explanation: Parses the string as a signed long integer, stopping at the first non‑numeric character. Equivalent to strtol(nptr, NULL, 10).

Example:

/* Add two large numbers represented as strings */
#include <stdlib.h>
int main(){
    char a[] = "1000000000";
    char b[] = "234567890";
    long c = atol(a) + atol(b);
    printf("c=%ld
", c);
}

Output:

c=1234567890

gcvt – Convert Double to String with Rounding

Header: #include <stdlib.h> Prototype: char *gcvt(double number, size_t ndigits, char *buf); Explanation: Converts a double to an ASCII string with the specified number of significant digits. Unlike ecvt and fcvt, the resulting string includes a decimal point or sign when appropriate. The converted string is stored in the buffer pointed to by buf.

Example:

#include <stdlib.h>
int main(){
    double a = 123.45;
    double b = -1234.56;
    char buf[32];
    gcvt(a, 5, buf);
    printf("a value=%s
", buf);
    gcvt(b, 6, buf);
    printf("b value=%s
", buf);
}

Output:

a value=123.45
b value=-1234.56

strtod – Convert String to Double with End Pointer

Header: #include <stdlib.h> Prototype: double strtod(const char *nptr, char **endptr); Explanation: Parses a string to a double, similar to atof, but also returns a pointer to the first character not used in the conversion via endptr. Supports optional sign, decimal point, and exponent.

Example (different bases):

/* Convert strings in decimal, binary, and hexadecimal */
#include <stdlib.h>
int main(){
    char a[] = "1000000000";   // decimal
    char b[] = "1000000000";   // binary (base 2)
    char c[] = "ffff";         // hexadecimal (base 16)
    printf("a=%d
", (int)strtod(a, NULL));
    printf("b=%d
", (int)strtod(b, NULL, 2));
    printf("c=%d
", (int)strtod(c, NULL, 16));
}

Output:

a=1000000000
b=512
c=65535

strtol – Convert String to Long Integer with Base

Header: #include <stdlib.h> Prototype: long int strtol(const char *nptr, char **endptr, int base); Explanation: Converts the string to a signed long integer using the specified base (2‑36 or 0). Base 0 selects the base automatically (e.g., “0x” for hex). The function stops at the first invalid character and can return that position via endptr. Errors set errno to ERANGE if the value is out of range.

Example:

/* Convert strings in decimal, binary, and hexadecimal */
#include <stdlib.h>
int main(){
    char a[] = "1000000000";   // decimal
    char b[] = "1000000000";   // binary (base 2)
    char c[] = "ffff";         // hexadecimal (base 16)
    printf("a=%ld
", strtol(a, NULL, 10));
    printf("b=%ld
", strtol(b, NULL, 2));
    printf("c=%ld
", strtol(c, NULL, 16));
}

Output:

a=1000000000
b=512
c=65535

strtoul – Convert String to Unsigned Long Integer

Header: #include <stdlib.h> Prototype:

unsigned long int strtoul(const char *nptr, char **endptr, int base);

Explanation: Same as strtol but returns an unsigned long. Handles bases 2‑36 or 0, with overflow indicated by ERANGE in errno.

Example: identical to the strtol example, using strtoul instead.

toascii – Convert Integer to 7‑bit ASCII

Header: #include <ctype.h> Prototype: int toascii(int c); Explanation: Masks the argument to 7 bits, effectively converting it to a valid ASCII character.

Example:

#include <stdlib.h>
int main(){
    int a = 217;
    char b;
    printf("before toascii(): a=%d(%c)
", a, a);
    b = toascii(a);
    printf("after toascii(): b=%d(%c)
", b, b);
}

Output:

before toascii(): a=217()
after toascii(): b=89(Y)

tolower – Convert Uppercase Letter to Lowercase

Header: #include <ctype.h> Prototype: int tolower(int c); Explanation: If c is an uppercase alphabetic character, returns its lowercase equivalent; otherwise returns c unchanged.

Example:

/* Convert all uppercase letters in a string to lowercase */
#include <ctype.h>
int main(){
    char s[] = "aBcDeFgH12345;!#$";
    printf("before tolower(): %s
", s);
    for (int i = 0; i < sizeof(s); ++i) {
        s[i] = tolower(s[i]);
    }
    printf("after tolower(): %s
", s);
}

Output:

before tolower(): aBcDeFgH12345;!#$
after tolower(): abcdefgh12345;!#$

toupper – Convert Lowercase Letter to Uppercase

Header: #include <ctype.h> Prototype: int toupper(int c); Explanation: If c is a lowercase alphabetic character, returns its uppercase counterpart; otherwise returns c unchanged.

Example:

/* Convert all lowercase letters in a string to uppercase */
#include <ctype.h>
int main(){
    char s[] = "aBcDeFgH12345;!#$";
    printf("before toupper(): %s
", s);
    for (int i = 0; i < sizeof(s); ++i) {
        s[i] = toupper(s[i]);
    }
    printf("after toupper(): %s
", s);
}

Output:

before toupper(): aBcDeFgH12345;!#$
after toupper(): ABCDEFGH12345;!#$