How can I safely and simply read a line of text from a file or stdin? - linux

Question

Given that fgets only sometimes includes a linebreak, and fscanf is inherently unsafe, I would like a simple alternative to read text line-by-line from a file. Is this page a good place to find such a function?

Answer 1

Yes. The following function should satisfy this requirement without creating any damaging security flaws.
/* reads from [stream] into [buffer] until terminated by
* \r, \n or EOF, or [lastnullindex] is reached. Returns
* the number of characters read excluding the terminating
* character. [lastnullindex] refers to the uppermost index
* of the [buffer] array. If an error occurs or non-text
* characters (below space ' ' or above tilde '~') are
* detected, the buffer will be emptied and 0 returned.
*/
int readline(FILE *stream, char *buffer, int lastnullindex) {
if (!stream) return 0;
if (!buffer) return 0;
if (lastnullindex < 0) return 0;
int inch = EOF;
int chi = 0;
while (chi < lastnullindex) {
inch = fgetc(stream);
if (inch == EOF || inch == '\n' || inch == '\r') {
buffer[chi] = '\0';
break;
} else if (inch >= ' ' && inch <= '~') {
buffer[chi] = (char)inch;
chi++;
} else {
buffer[0] = '\0';
return 0;
}
}
if (chi < 0 || chi > lastnullindex) {
buffer[0] = '\0';
return 0;
} else {
buffer[chi] = '\0';
return chi;
}
}