Parse an integer argv argument in C with strtol

Your program expects a number in ./program 42. Then somebody runs ./program 12px, omits the argument, or passes an integer that is too large. atoi leaves you guessing about what happened. strtol gives you the information needed to decide.

What is a complete program for ./program 42?

This program requires exactly one argument, accepts a decimal integer from 0 through 100000, then prints it. That range is not a property of strtol; it is the program's rule, placed after the conversion.

#include <errno.h>#include <stdio.h>#include <stdlib.h>
static int parse_count(const char *text, long *count){    char *end;    long value;
    errno = 0;    value = strtol(text, &end, 10);    if (end == text || *end != '\0' || errno == ERANGE)        return 0;    if (value < 0 || value > 100000)        return 0;    *count = value;    return 1;}
int main(int argc, char *argv[]){    long count;
    if (argc != 2) {        fprintf(stderr, "usage: %s COUNT\n", argv[0]);        return 2;    }    if (!parse_count(argv[1], &count)) {        fprintf(stderr, "invalid count: %s\n", argv[1]);        return 1;    }    printf("count=%ld\n", count);    return 0;}

$ gcc -std=c17 -Wall -Wextra -Werror parse-argv.c -o parse-argv$ ./parse-argv 42count=42$ ./parse-argv 12px; echo status=$?invalid count: 12pxstatus=1$ ./parse-argv 999999999999999999999999; echo status=$?invalid count: 999999999999999999999999status=1$ ./parse-argv -1; echo status=$?invalid count: -1status=1$ ./parse-argv; echo status=$?usage: ./parse-argv COUNTstatus=2

argc includes the program name. With ./parse-argv 42, it is 2 and argv[1] points at the string "42". When argc is 1, reading argv[1] is already a bug.

Why is atoi not enough for an argument?

atoi("0") and atoi("hello") both return 0 on common libraries. Its return value cannot tell whether the caller asked for zero or whether no conversion happened. Its manual entry also explains the overflow problem.

strtol retains the value and gives you a pointer to the first character it did not consume. With 12px, value is 12 but end points at p. Testing *end != '\0' turns that remaining text into a rejection. That check stops 12px being treated as 12.

int count = atoi(argv[1]);

char *end;long count = strtol(argv[1], &end, 10);if (end == argv[1] || *end != '\0')    return 1;

atoi cannot distinguish an overflow either. strtol returns LONG_MAX or LONG_MIN and sets errno to ERANGE instead. Set errno = 0 immediately before the call: a successful function call does not have to reset it.

In what order should you check end and errno?

The order in the program is deliberate. end == text asks whether any digit was read. errno == ERANGE asks whether the number fits in a long. *end != '\0' asks whether the whole string was a number. The final comparisons express the program's own rule, here a bounded nonnegative count.

strtol accepts leading whitespace. That is rarely a problem for a command-line argument because the shell already separates arguments. If your format must reject it, check the first character with isspace((unsigned char)text[0]) before the call.

The program chooses base 10. Passing zero asks strtol to infer a base from a prefix: 0x for hexadecimal, and other prefixes can depend on the library and standard you target. For a quantity users must type in decimal, 10 avoids that ambiguity.

strtol

Write gm_strtol, your own version of the function. The exercise makes you advance the end pointer and handle bases, the same two pieces of information this program uses to decide.

Open the exercise

Can I store the result in an int?

Yes, but only convert to int after comparing against INT_MIN and INT_MAX. strtol promises that its result fits in a long, not in every smaller type. If your program needs an unsigned value or a wider range, choose a matching function such as strtoul or strtoll, then keep the same checks.

For an answer read at the terminal, use the same method after reading one complete line. Why fgets skips a line after scanf shows that case and why it helps to separate reading from conversion.

Frequently asked questions