Eliminating IF statement from this code - c

Question

I have this C function which basically takes the input from the user and saves it in a static char[]. Then it checks whether it is empty. If it is empty then it prints an error, else appends a file extension to it. Here's the piece of code I'm concerned about:
const char* text_entry(){
char c;
while((c=getchar())!='\n' && c!=EOF);
static char input[20];
fgets(input, sizeof(input),stdin);
input[strcspn(input,"\n")]=0;
if(strlen(input)>0){
strncat(input, ".txt",4);
return input;
}
printf("Error! no text entered!");
return NULL;
}
My concern is that I want to remove the IF statement from it, making it more DRY. How can I do that?

Answer 1

The "DRY" is in the usage of strcspn(), strlen() and strncat(). You can avoid them by
size_t l = strcspn(input, "\n");
bool ok = false;
if (l == 0) {
fprintf(stderr, "error: too short\n");
} else if (l > sizeof input - sizeof(".txt")) {
fprintf(stderr, "error: too long\n");
} else {
strcpy(input + l, ".txt");
ok = true;
}
return ok ? input : NULL;

Answer 2

This is a minimal loop version. It doescheck* if the result fits into the input buffer.
Note:I'm not saying this is the best way todo it. But it avoids multiple scans over the input. (the gain in performance is mostly futile...)
#include <stdio.h>
#include <string.h>
char* text_entry(){
int ch;
size_t pos;
static char input[20];
//this is dubious: it consumes characters upto and including the first '\n'
while((ch=getchar())!='\n' && ch!=EOF) {;}
for (pos=0; pos < sizeof input; pos++) {
ch=getchar();
if (ch =='\n' || ch==EOF) break;
input[pos] = ch;
}
// noinput
if(!pos) return NULL;
// input too large: cannot append suffix
if(pos + sizeof ".txt" > sizeof input) return NULL;
memcpy(input+pos, ".txt", sizeof ".txt" );
// fprintf(stderr, "[%zu]\n",strlen(input));
return input;
}
int main(void)
{
char *result;
while(1) {
result = text_entry();
if (!result) continue;
printf("%s\n", result);
}
return 0;
}

Answer 3

I did not shorten your code, but I think you might find this solution more resilient, using the libreadline -lreadline
Before your returned a buffer which was declared on the stack, which means that it is unallocated on return, but yet you return its address. This is a bug.
I fixed it and gave you the opportunity to take a buffer and buffer size in parameters, as well as the extension, so the caller manages the memory for you as well as his maximum string length needs.
The loop now handles maximum input size well. It might not be shorter, but it should prove resilient.
/*
** Args:
** input_buffer: a buffer that will be whiped then filled with the user input
** buffer_size: the total size of the input buffer (maximum result length)
** extension: a dotted file type extension
**
** Returns: 0 on error or the input_buffer
*/
const char* text_entry(char *buffer, size_t buffer_size, char *extension) {
int maxsize = buffer_size - strlen(extension) - 1;
char *user_input;
int user_input_len;
do {
user_input = readline("Enter a file name: ");
user_input_len = strlen(user_input);
if (user_input_len > maxsize) {
printf("The name is too long! %d characters maximum. Try again.\n", maxsize);
free(user_input);
}
} while (!user_input_len || user_input_len > maxsize);
strcpy(buffer, user_input);
free(user_input);
strcat(buffer, extension);
printf("%s\n", buffer);
return buffer;
}