How to use fgets and strtol to check if the input is a valid integer?

Question

I've tried

#define BUF_SIZE 256
char msg[BUF_SIZE];
printf("input? ");

while (fgets(msg,BUF_SIZE,stdin)!=NULL){
    char *next;
    int input = strol(msg, &next,10); //use strol 
    if ((end == msg) || (*end == '\0')){ //check validity here
       printf("invalid\n");
       printf("try again\n"); //type anther value
    else{
       printf("valid\n");
}

What's wrong with my code?
Is that the correct way to check integer input?

See the yesterday's question: https://stackoverflow.com/questions/54653523/check-that-command-line-input-is-numeric — Eugene Sh., Feb 13 '19 at 20:14
And the if / else has two opening braces, but no closing ones. Is this your actual code? — M Oehm, Feb 13 '19 at 20:15
Voting to close as a typo since the function is actually `strtol`, not `strol` — Govind Parmar, Feb 13 '19 at 20:19
Please check your compiler errors. There are many mistakes here. — stark, Feb 13 '19 at 22:09

score 0 · Answer 1 · answered Feb 14 '19 at 03:39

Wrong test and function name

// if ((end == msg) || (*end == '\0')){ //check validity here
if ((end == msg) || (*end !== '\0')){ //check validity here
  printf("invalid\n");

Better to assign to long or not at all

// int input = strol(msg, &next,10);
long input = strtol(msg, &next,10);
or 
strtol(msg, &next,10);

Use errno is one wants to also check for overflow.

errno = 0;
strtol(msg, &next,10);
if (errno == ERANGE) {
  puts("Overflow");
} else    
  ...

How to use fgets and strtol to check if the input is a valid integer?

1 Answers1