How Should I read a File line-by-line in C?

Question

I'd like to read a file line-by-line. I have fgets() working okay, but am not sure what to do if a line is longer than the buffer sizes I've passed to fgets()? And furthermore, since fgets() doesn't seem to be Unicode-aware, and I want to allow UTF-8 files, it might miss line endings and read the whole file, no?

Then I thought I'd use getline(). However, I'm on Mac OS X, and while getline() is specified in /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk/usr/include/stdio.h, it's not in /usr/include/stdio, so gcc doesn't find it in the shell. And it's not particularly portable, obviously, and I'd like the library I'm developing to be generally useful.

So what's the best way to read a file line-by-line in C?

Answer 1

Based on this answer, here's what I've come up with:

#define LINE_BUF_SIZE 1024

char * getline_from(FILE *fp) {
    char * line = malloc(LINE_BUF_SIZE), * linep = line;
    size_t lenmax = LINE_BUF_SIZE, len = lenmax;
    int c;

    if(line == NULL)
        return NULL;

    for(;;) {
        c = fgetc(fp);
        if(c == EOF)
            break;

        if(--len == 0) {
            len = lenmax;
            char * linen = realloc(linep, lenmax *= 2);

            if(linen == NULL) {
                // Fail.
                free(linep);
                return NULL;
            }
            line = linen + (line - linep);
            linep = linen;
        }

        if((*line++ = c) == '\n')
            break;
    }
    *line = '\0';
    return linep;
}

To read stdin:

char *line;
while ( line = getline_from(stdin) ) {
    // do stuff
    free(line);
}

To read some other file, I first open it with fopen():

FILE *fp;
fp = fopen ( filename, "rb" );
if (!fp) {
    fprintf(stderr, "Cannot open %s: ", argv[1]);
    perror(NULL);
    exit(1);
}

char *line;
while ( line = getline_from(fp) ) {
    // do stuff
    free(line);
}

This works very nicely for me. I'd love to see an alternative that uses fgets() as suggested by @paul-tomblin, but I don't have the energy to figure it out tonight.