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fork() - Unix, Linux System Call

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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 that it incurs is the time and memory required to duplicate 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 execution. On failure, a -1 will be returned in the parent’s context, no child process will be created, and errno will be set appropriately.


Error CodeDescription
EAGAIN fork() cannot allocate sufficient memory to copy the parent’s page tables and allocate a task structure for the child.
EAGAIN It was not possible to create a new process because the caller’s RLIMIT_NPROC resource limit was encountered. To exceed this limit, the process must have either the CAP_SYS_ADMIN or the CAP_SYS_RESOURCE capability.
ENOMEM fork() failed to allocate the necessary kernel structures because memory is tight.


SVr4, 4.3BSD, POSIX.1-2001.


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