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26.5 Executing a File

This section describes the exec family of functions, for executing a file as a process image. You can use these functions to make a child process execute a new program after it has been forked.

To see the effects of exec from the point of view of the called program, see Program Basics.

The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing. They are declared in the header file unistd.h.

— Function: int execv (const char *filename, char *const argv[])

The execv function executes the file named by filename as a new process image.

The argv argument is an array of null-terminated strings that is used to provide a value for the argv argument to the main function of the program to be executed. The last element of this array must be a null pointer. By convention, the first element of this array is the file name of the program sans directory names. See Program Arguments, for full details on how programs can access these arguments.

The environment for the new process image is taken from the environ variable of the current process image; see Environment Variables, for information about environments.

— Function: int execl (const char *filename, const char *arg0, ...)

This is similar to execv, but the argv strings are specified individually instead of as an array. A null pointer must be passed as the last such argument.

— Function: int execve (const char *filename, char *const argv[], char *const env[])

This is similar to execv, but permits you to specify the environment for the new program explicitly as the env argument. This should be an array of strings in the same format as for the environ variable; see Environment Access.

— Function: int execle (const char *filename, const char *arg0, char *const env[], ...)

This is similar to execl, but permits you to specify the environment for the new program explicitly. The environment argument is passed following the null pointer that marks the last argv argument, and should be an array of strings in the same format as for the environ variable.

— Function: int execvp (const char *filename, char *const argv[])

The execvp function is similar to execv, except that it searches the directories listed in the PATH environment variable (see Standard Environment) to find the full file name of a file from filename if filename does not contain a slash.

This function is useful for executing system utility programs, because it looks for them in the places that the user has chosen. Shells use it to run the commands that users type.

— Function: int execlp (const char *filename, const char *arg0, ...)

This function is like execl, except that it performs the same file name searching as the execvp function.

The size of the argument list and environment list taken together must not be greater than ARG_MAX bytes. See General Limits. In the GNU system, the size (which compares against ARG_MAX) includes, for each string, the number of characters in the string, plus the size of a char *, plus one, rounded up to a multiple of the size of a char *. Other systems may have somewhat different rules for counting.

These functions normally don't return, since execution of a new program causes the currently executing program to go away completely. A value of -1 is returned in the event of a failure. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for these functions:

E2BIG
The combined size of the new program's argument list and environment list is larger than ARG_MAX bytes. The GNU system has no specific limit on the argument list size, so this error code cannot result, but you may get ENOMEM instead if the arguments are too big for available memory.
ENOEXEC
The specified file can't be executed because it isn't in the right format.
ENOMEM
Executing the specified file requires more storage than is available.

If execution of the new file succeeds, it updates the access time field of the file as if the file had been read. See File Times, for more details about access times of files.

The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed.

Executing a new process image completely changes the contents of memory, copying only the argument and environment strings to new locations. But many other attributes of the process are unchanged:

If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process. These concepts are discussed in detail in Process Persona.

Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image. All other signals are set to the default action in the new process image. For more information about signals, see Signal Handling.

File descriptors open in the existing process image remain open in the new process image, unless they have the FD_CLOEXEC (close-on-exec) flag set. The files that remain open inherit all attributes of the open file description from the existing process image, including file locks. File descriptors are discussed in Low-Level I/O.

Streams, by contrast, cannot survive through exec functions, because they are located in the memory of the process itself. The new process image has no streams except those it creates afresh. Each of the streams in the pre-exec process image has a descriptor inside it, and these descriptors do survive through exec (provided that they do not have FD_CLOEXEC set). The new process image can reconnect these to new streams using fdopen (see Descriptors and Streams).