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c linux portability printf

How to use printf to display off_t, nlink_t, size_t and other special types?

In my program, I stat the files they want and send the data over. The fields of a stat struct are all special types:

struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for file system I/O */
blkcnt_t st_blocks; /* number of 512B blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */
};

The relevant code for my question follows:

len = snprintf( statbuf, STAT_BUFFER_SIZE,
"%crwxrwxrwx %lu %u %u %lld %s %s\r\n",
S_ISDIR( filestats.st_mode ) ? 'd' : '-',
(unsigned long ) filestats.st_nlink,
filestats.st_uid,
filestats.st_gid,
(unsigned long long ) filestats.st_size,
date,
filename);

How do I print these types in a portable and efficient way? At first I did it without casts by guessing the correct format specifiers. Apart from being an annoying programming habit, this also meant my code wouldn’t work on a 32 bit system. Now with the casts it seems to work, but on how many platforms?

There isn’t a fully portable way to do it, and it is a nuisance.

C99 provides a mechanism for built-in types like size_t with the %zu notation (and there are some extra, similar qualifiers).

It also provides the <inttypes.h> header with macros such as PRIX32 to define the correct qualifier for printing a 32-bit hexadecimal constant (in this case):

printf("32-bit integer: 0x%08" PRIX32 "\n", var_of_type_int32_t);

For the system-defined types (such as those defined by POSIX), AFAIK, there is no good way to handle them. So, what I do is take a flying guess at a ‘safe’ conversion and then print accordingly, including the cast, which is what you illustrate in the question. It is frustrating, but there is no better way that I know of. In case of doubt, and using C99, then conversion to ‘unsigned long long’ is pretty good; there could be a case for using a cast to uintmax_t and PRIXMAX or equivalent.

Or, as FUZxxl reminded me, you can use the modifier j to indicate a ‘max’ integer type. For example:

printf("Maximal integer: 0x%08jX\n", (uintmax_t)var_of_type_without_format_letter);