Question

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Easiest way to get file's contents in C

I saw this post but I still feel it involves fseek, ftell etc. And I saw a few other methods where they use feof+fgetc combination but they say feof is not a good option for loop condition. And some other suggest using fgets and reading line by line which I consider as tedious.

Are there any other shorter methods to read entire file contents?

PS: By shorter I mean lines of code.

Answer 1

Either use mmap() or do it yourself.

Omitting error checking, roughly:

int fd = open(filename, O_RDONLY);
struct stat sb;
fstat(fd, &sb);
char *contents = mmap(0, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);

This is a readonly copy of the file, allocated at an address of the kernel’s choosing. If you’re on a 32-bit system, you may have to worry about files bigger than 2 GiB. I don’t think you can portably avoid using something like fstat() to get the file size. If you want to grow the file, you can specify a larger size than the file size, according to the POSIX specification.

If you DIY, then the first three lines are the same. Then you use:

char *contents = malloc(sb.st_size);
read(fd, contents, sb.st_size);

As noted before, all error checking was omitted in these examples; don’t risk it in production-ready code.

---

I went digging for some more nearly production-ready code that uses mmap() and found:

static int sqs_mapfile(const char *file)
{
    int   fd;
    void *vp;
    struct stat sb;

    if (file == 0)
        return(SQS_NOSQSFILE);
    if ((fd = open(file, O_RDONLY, 0)) < 0)
        return(SQS_OPENFAIL);
    if (fstat(fd, &sb) != 0)
    {
        close(fd);
        return(SQS_STATFAIL);
    }
    vp = mmap(0, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
    close(fd);
    if (vp == MAP_FAILED)
        return(SQS_MMAPFAIL);

The ‘SQS_xyz’ codes are specific to the program it comes from (as is the function name). This code uses a 3-argument open() call where a 2-argument version is sufficient.