atoi() / atof() / atol() / strtol()

// Converts a string to int. Returns 0 if conversion fails.
int atoi(const char *str);

// Converts a string to double.
double atof(const char *str);

// Converts a string to long.
long atol(const char *str);

// Converts a string to long. Lets you specify a pointer to where conversion stopped and the numeric base.
// endptr: pointer to the position where conversion ended (pass NULL if not needed). base: numeric base (2–36, or 0 for auto-detection).
long strtol(const char *str, char **endptr, int base);

// Converts a string to double (with error detection support).
double strtod(const char *str, char **endptr);

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    // Convert a string to int using atoi.
    int n = atoi("42");
    printf("atoi: %d\n", n); // Prints "atoi: 42".

    // Convert a string to double using atof.
    double d = atof("3.14");
    printf("atof: %.2f\n", d); // Prints "atof: 3.14".

    // Convert a string using strtol with error detection.
    char *end;
    long val = strtol("  -123abc", &end, 10);
    printf("strtol: %ld\n", val); // Prints "strtol: -123".
    printf("remaining: %s\n", end);    // Prints "remaining: abc".

    // Convert a hexadecimal string using strtol.
    long hex_val = strtol("0xFF", NULL, 16);
    printf("0xFF = %ld\n", hex_val); // Prints "0xFF = 255".

    // With base 0, strtol auto-detects: "0x" as hex, "0" as octal.
    long auto_val = strtol("0755", NULL, 0);
    printf("0755(octal) = %ld\n", auto_val); // Prints "0755(octal) = 493".

    // Detecting a conversion failure (not possible with atoi).
    char input[] = "abc";
    end = input;
    long result = strtol(input, &end, 10);
    if (end == input) {
        printf("Conversion failed: not a number.\n"); // This line is printed.
    }

    return 0;
}

atoi() / atol() / atof() do not support error detection. When conversion fails, they simply return 0 or 0.0, making it impossible to determine whether the input was invalid. In situations where reliability matters, use strtol() or strtod() and check the conversion endpoint via endptr.

If an overflow occurs, strtol() returns LONG_MAX or LONG_MIN and sets errno to ERANGE. When handling external input such as command-line arguments, be sure to check errno as well.

For the reverse conversion (number → string), use sprintf() / snprintf().