How does `ls` know whether it's piped or printed

Question

ls normally prints like this:

1     1249  1653  23    33    4581  6202  6447  789  836  903        config.gz    kpageflags    sysrq-trigger
10    1253  1658  24    34    4582  6206  648   79   837  91         consoles     loadavg       thread-self
1003  1255  1662  251   347   4583  6207  649   791  84   923        cpuinfo      locks         timer_list
1004  1257  1667  252   35    4689  6209  6799  8    840  925        crypto       meminfo       timer_stats
1005  1261  167   26    357   48    6232  6986  80   841  998        devices      misc          uptime
1006  1263  1670  266   36    49    6242  6992  802  846  acpi       diskstats    modules       version
1008  13    1677  27    37    5     6267  6993  803  848  asound     dma          mounts        vmallocinfo

But when you pipe it eg to cat with ls /proc | cat, it prints it as if it got -1 (works the same also with grep and other commands):

1
10
1003
1004
1005
1006
1008
1010
1012
106
107
1073
108
109
1152
117
118
1247
1249
1253

My question is, how does it know it's being piped instead of printed? Or, how it happens?

Answer 1

ls and many other programs detect whether their output (or sometimes input) is attached to a terminal by calling the C function isatty(STDOUT_FILENO). If output is anything else than a terminal (like a pipe or a file), it defaults to an output format that is more program-friendly.