What is the correct printf specifier for printing pid_t

Question

I'm currently using a explicit cast to long and using %ld for printing pid_t, is there a specifier such as %z for size_t for pid_t?

If not what the best way of printing pid_t?

Solution 1

There's no such specifier. I think what you're doing (casting the pid_t to long and printing it with "%ld") is fine; you could use an even wider int type, but there's no implementation where pid_t is bigger than long and probably never will be.

Solution 2

With integer types lacking a matching format specifier as in the case of pid_t, yet with known sign-ness¹, cast to widest matching signed type and print.

If sign-ness is not known for other system type, cast to the widest unsigned type or alternate opinion

pid_t pid = foo();

// C99
#include <stdint.h>
printf("pid = %jd\n", (intmax_t) pid);

Or

// C99
#include <stdint.h>
#include <inttypes.h>
printf("pid = %" PRIdMAX "\n", (intmax_t) pid);

Or

// pre-C99
pid_t pid = foo();
printf("pid = %ld\n", (long) pid);

---

¹ The pid_t data type is a signed integer type which is capable of representing a process ID.

Comments

[Mad Physicist]

Nice. If you would like to customize printf with your own spec for pid_t, check out this page: stackoverflow.com/questions/9260170/…

[Bilal Syed Hussain]

Is the total number of processes related to how bits the os is? e.g can a os have more sizeof(int)?

[Jim Balter]

See stackoverflow.com/questions/1922761/… ... pid_t is usually 32 bits, regardless of the OS bit size. A system could have larger ones, but I wouldn't expect it. In any case, long will be plenty for safety.

[chux - Reinstate Monica]

Hmmm, if pid_t is usually 32 bits, regardless of the OS bit size. is true then on implementations where long is 64-bit, printing with a "%ld" will certainly cause issues.

[Jim Balter]

@chux-ReinstateMonica Please read the question: "I'm currently using a explicit cast to long and using %ld" -- that's what I said "is fine". Of course there is a problem with using %ld without guaranteeing that a long is being passed.

[Jim Balter]

"If sign-ness is not known, cast to the widest unsigned type" -- I would not advise this. Casting to uintmax_t and printing with %ju will print all negative values incorrectly, whereas casting to intmax_t and printing with %jd will only print values > INTMAX_MAX incorrectly. (Then again, situations in which one really doesn't know whether a type includes negative values are quite rare.)

[chux - Reinstate Monica]

@JimBalter Fair point. With sign-ness unknown there is the trade-off of wrongly printing negative value as large values or large values as negative. I'd expect when sign-ness unknown, it is usually more natural to use intmax_t as small values are more common, yet conversion signed integer to uintmax_t is more narrowly defined which does not lose info (hence my preference) than unsigned integer to intmax_t can. Choose your poison .

[alx]

@MadPhysicist you could also write a macro similar to PRIi32: #define PRIpid "i".