Suppose for each line of some text file you want to read the entire line into a buffer and do some processing on it.
A common approach is to choose a fixed maximum length and hope for the best with fgets, but such a program breaks if any line's length is greater than this arbitrary limit.
The program below handles all the edge cases concomitant with fgets:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* argv[0], potentially used in error messages */
const char *progname;
typedef void (*processor)(const char *s);
int for_each_line(const char *path, processor p);
/* simple processor that prints each line to the standard output */
void print(const char *s)
{
printf("%s\n", s);
}
int main(int argc, char **argv)
{
progname = argv[0];
if (argc != 2) {
fprintf(stderr, "Usage: %s file\n", progname);
return 1;
}
return for_each_line(argv[1], print) ? 0 : 1;
}
int for_each_line(const char *path, processor p)
{
FILE *f;
char *buf, *line;
size_t capacity = 80; /* reasonable guess at max length */
size_t remaining = capacity;
int success = 1;
f = fopen(path, "r");
if (!f) {
fprintf(stderr, "%s: open %s: %s\n",
progname, path, strerror(errno));
return 0;
}
line = malloc(capacity);
if (!line) {
fprintf(stderr, "%s: malloc: %s\n", progname, strerror(errno));
fclose(f);
return 0;
}
/*
* On each iteration, read into buf the rest of a line whose length
* is at most remaining. We can be certain that we have the whole
* line only when the string contains '\n', in which case we
* remove the terminator and call the processor on the entire line.
*
* Otherwise, we double line's size and try again.
*
* It may seem tempting to also test feof(f) to check whether we
* have the whole line, but in the unlucky edge case where a file
* doesn't end with '\n' and its last line is exactly remaining-1
* in length, feof(f) will not yet be true, hence the possibility
* of printing the last line outside the loop.
*/
buf = line;
line[0] = '\0';
while (fgets(buf, remaining, f)) {
char *eol = strchr(buf, '\n');
if (eol) {
*eol = '\0';
p(line);
buf = line;
remaining = capacity;
line[0] = '\0';
}
else {
size_t used = buf + remaining - line;
line = realloc(line, capacity * 2);
if (!line) {
fprintf(stderr, "%s: realloc: %s\n", progname, strerror(errno));
fclose(f);
return 0;
}
buf = line + used - 1;
capacity *= 2;
remaining = capacity - used;
}
}
if (errno) {
fprintf(stderr, "%s: fgets: %s\n", progname, strerror(errno));
success = 0;
}
else if (line[0]) {
char *eol = strchr(buf, '\n');
if (eol)
*eol = '\0';
p(line);
}
fclose(f);
free(line);
return success;
}
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