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RedHat 9 (Linux i386) - man page for fork (redhat section 2)

FORK(2) 						     Linux Programmer's Manual							   FORK(2)

fork - create a child process
#include <sys/types.h> #include <unistd.h> pid_t fork(void);
fork creates a child process that differs from the parent process only in its PID and PPID, and in the fact that resource utilizations are set to 0. File locks and pending signals are not inherited. Under Linux, fork is implemented using copy-on-write pages, so the only penalty incurred by fork is the time and memory required to dupli- cate the parent's page tables, and to create a unique task structure for the child.
On success, the PID of the child process is returned in the parent's thread of execution, and a 0 is returned in the child's thread of exe- cution. On failure, a -1 will be returned in the parent's context, no child process will be created, and errno will be set appropriately.
EAGAIN fork cannot allocate sufficient memory to copy the parent's page tables and allocate a task structure for the child. ENOMEM fork failed to allocate the necessary kernel structures because memory is tight.
The fork call conforms to SVr4, SVID, POSIX, X/OPEN, BSD 4.3.
clone(2), execve(2), vfork(2), wait(2) Linux 1.2.9 1995-06-10 FORK(2)