Converts string to integer checking input errors.

c_string_to_int.c

/**
 * Compile: 
 * 		gcc -Wall -Wextra -ggdb c_string_to_int.c -o string_to_int
 */

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

int string_to_int(const char * string, int * retval) {
	int tmpval = 0;
	char * endptr = NULL;
	errno = 0; // Include errno.h

	tmpval = strtol(string, &endptr, 10);

	if(string == endptr) {
		return -1;
	} else if(*endptr != '\0') {
		return -2;
	} else if(errno != 0) {
		return -3;
	} else {
		*retval = tmpval;
		return 0;
	}
}

int main(int argc, char ** argv) {
	
	if(argc < 2) {
		fprintf(stderr, "Usage: %s str\n", argv[0]);
		exit(1);
	}

	const char * string = argv[1];
	int value;

	int status = string_to_int(string, &value);
	if(status == 0) {
		printf("INTEGER VALUE: %d\n", value);
	} else {
		printf("CANNOT CONVERT \"%s\" TO INTEGER VALUE, RETURN CODE: %d\n", string, status);
	}

	return 0;
}