In a C function declaration, what does "..." as the last parameter do?

C Problem Overview

Often I see a function declared like this:

void Feeder(char *buff, ...)

what does "..." mean?

C Solutions

Solution 1 - C

it allows a variable number of arguments of unspecified type (like printf does).

you have to access them with va_start, va_arg and va_end

see <http://publications.gbdirect.co.uk/c_book/chapter9/stdarg.html> for more information

Solution 2 - C

Variadic functions

> Variadic functions are functions which may take a variable number of arguments and are declared with an ellipsis in place of the last parameter. An example of such a function is printf.

> A typical declaration is

> int check(int a, double b, ...);

> Variadic functions must have at least one named parameter, so, for instance,

> char *wrong(...);

> is not allowed in C.

Solution 3 - C

The three dots '...' are called an ellipsis. Using them in a function makes that function a variadic function. To use them in a function declaration means that the function will accept an arbitrary number of parameters after the ones already defined.

For example:

Feeder("abc");
Feeder("abc", "def");

are all valid function calls, however the following wouldn't be:

Feeder();

Solution 4 - C

variadic function (multiple parameters)

wiki

#include <stdarg.h>

double average(int count, ...)
{
    va_list ap;
    int j;
    double tot = 0;
    va_start(ap, count); //Requires the last fixed parameter (to get the address)
    for(j=0; j<count; j++)
        tot+=va_arg(ap, double); //Requires the type to cast to. Increments ap to the next argument.
    va_end(ap);
    return tot/count;
}

Solution 5 - C

It means that a http://en.wikipedia.org/wiki/Variadic_function#Variadic_functions_in_C.2C_Objective-C.2C_C.2B.2B.2C_and_D">variadic function is being declared.

Solution 6 - C

Functions with ... as last parameter are called Variadic functions (Cppreference. 2016). This ... is used to allow variable length parameters with unspecified types.

We can use variadic functions when we are not sure about the number of parameters or their types.

Example of variadic function: Let us assume we need a sum function that will return the summation of variable number of arguments. We can use a variadic function here.

#include <stdio.h>
#include <stdarg.h>

int sum(int count, ...)
{
    int total, i, temp;
    total = 0;
    va_list args;
    va_start(args, count);
    for(i=0; i<count; i++)
    {
        temp = va_arg(args, int);
        total += temp;
    }
    va_end(args);
    return total;
}

int main()
{
    int numbers[3] = {5, 10, 15};
    // Get summation of all variables of the array
    int sum_of_numbers = sum(3, numbers[0], numbers[1], numbers[2]);
    printf("Sum of the array %d\n", sum_of_numbers);
    // Get summation of last two numbers of the array
    int partial_sum_of_numbers = sum(2, numbers[1], numbers[2]);
    printf("Sum of the last two numbers of the array %d\n", partial_sum_of_numbers);
    return 0;
}

Output:

Practice problem: A simple problem to practice variadic function can be found in hackerrank practice problem here

Reference:

Cppreference. (2016, February 13). Variadic functions. Retrieved July 25, 2018, from https://en.cppreference.com/w/c/variadic

Solution 7 - C

... = three dot = three point = called: ellipsis
- means: variable number of parameters
  - compared to normal function: fixed (number of named) parameter
function with ... para is called: Variadic function

Variadic function

Definition

valid:
- int validFunctionWithNamedParameterThenEllipsis(int a, double b, ...);
invalid
- int invalidFunctionOnlyEllipsis(...);

Example

popular case:

int printf(const char *format, ...)

call:

printf("year=%d, name=%s", 2021, "crifan");

format == "year=%d, name=%s"
- named parameter
... == 2021, "crifan"
- variable number of parameter
  - here total 2 parameter
    - first: integer type 2021
    - second: string type "crifan"

How to get/calculate paremeter for Variadic function

core logic: use va_list, with va_start, va_arg, va_end

#include <stdarg.h>

void va_start(va_list ap, last_arg);
type va_arg(va_list ap, type);
void va_end(va list ap);

Example

average

#include <stdarg.h>
#include <stdio.h>

double average(int count, ...) {
    va_list ap;
    int j;
    double sum = 0;

    va_start(ap, count); /* Requires the last fixed parameter (to get the address) */
    for (j = 0; j < count; j++) {
        sum += va_arg(ap, int); /* Increments ap to the next argument. */
    }
    va_end(ap);

    return sum / count;
}

int main(int argc, char const *argv[]) {
    printf("%f\n", average(3, 1, 2, 3));
    return 0;
}

maxof

#include <stdlib.h>
#include <stdarg.h>
#include <stdio.h>

int maxof(int, ...) ;
void f(void);

main(){
        f();
        exit(EXIT_SUCCESS);
}

int maxof(int n_args, ...){
        register int i;
        int max, a;
        va_list ap;

        va_start(ap, n_args);
        max = va_arg(ap, int);
        for(i = 2; i <= n_args; i++) {
                if((a = va_arg(ap, int)) > max)
                        max = a;
        }

        va_end(ap);
        return max;
}

void f(void) {
        int i = 5;
        int j[256];
        j[42] = 24;
        printf("%d\n",maxof(3, i, j[42], 0));
}

execl

#include <stdarg.h>

#define  MAXARGS     31

/*
 * execl is called by
 * execl(file, arg1, arg2, ..., (char *)(0));
 */
int execl(const char *file, const char *args, ...)
{
    va_list ap;
    char *array[MAXARGS +1];
    int argno = 0;

    va_start(ap, args);
    while (args != 0 && argno < MAXARGS)
    {
        array[argno++] = args;
        args = va_arg(ap, const char *);
    }
    array[argno] = (char *) 0;
    va_end(ap);
    return execv(file, array);
}

My case: hook syscall()

/*==============================================================================
 Hook: syscall()
==============================================================================*/

int syscall(int, ...);

// normally max number of syscall parameter is not exceed 8
// refer: https://opensource.apple.com/source/xnu/xnu-4570.1.46/bsd/kern/syscalls.master
int MaxSupportArgNum_syscall = 16;

%hookf(int, syscall, int number, ...){
    os_log(OS_LOG_DEFAULT, "hook_syscall_stat_stat64: number=%d", number);

    // Setting up some variables to get all the parameters from syscall
    void *paraPtr, *paraList[MaxSupportArgNum_syscall];
//    char *paraPtr, *paraList[MaxSupportArgNum_syscall];
    va_list argList;
    int curParaNum = 0;

    va_start(argList, number);
    while ((paraPtr = (void *) va_arg(argList, void *))) {
    //    while ((paraPtr = (char *) va_arg(argList, char *))) {
        paraList[curParaNum] = paraPtr;
        curParaNum += 1;
        os_log(OS_LOG_DEFAULT, "hook_syscall_stat_stat64: [%d] paraPtr=%p", curParaNum, paraPtr);
    }
    va_end(argList);

//    os_log(OS_LOG_DEFAULT, "hook_syscall_stat_stat64: argList=%{public}s", argList);
    os_log(OS_LOG_DEFAULT, "hook_syscall_stat_stat64: curParaNum=%d", curParaNum);

    bool isStat = (SYS_stat == number);
    bool isStat64 = (SYS_stat64 == number);
    if (isStat || isStat64){
        char* curPath = (char *)paraList[0];
        os_log(OS_LOG_DEFAULT, "hook_syscall_stat_stat64: isStat=%{bool}d, isStat64=%{BOOL}d, curPath=%{public}s", isStat, isStat64, curPath);
        
        bool isJbPath = isJailbreakPath(curPath);
        if (isJbPath){
            os_log(OS_LOG_DEFAULT, "hook_syscall_stat_stat64: IS jailbreak path: %{public}s", curPath);
            return OPEN_FAILED;
        } else {
            os_log(OS_LOG_DEFAULT, "hook_syscall_stat_stat64: NOT jailbreak path: %{public}s", curPath);
        }
    }

//    return %orig;
//    return %orig(number, ...);
//    int retValue = %orig();

//    int retValue = callOriginSyscall(number, curParaNum, paraList);
////    int retValue = callOriginSyscall(number, curParaNum, (void *)paraList);
//    os_log(OS_LOG_DEFAULT, "hook_syscall_stat_file: retValue=%d", retValue);
//    return retValue;

    int paraNum = curParaNum;

    int syscallRetValue = -1;

    if (0 == paraNum){
        syscallRetValue = %orig(number);
    } else if (1 == paraNum){
        void* para1 = paraList[0];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p", para1);
        syscallRetValue = %orig(number, para1);
    } else if (2 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p", para1, para2);
        syscallRetValue = %orig(number, para1, para2);
    } else if (3 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        void* para3 = paraList[2];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p,para3=%p", para1, para2, para3);
        syscallRetValue = %orig(number, para1, para2, para3);
    } else if (4 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        void* para3 = paraList[2];
        void* para4 = paraList[3];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p,para3=%p,para4=%p", para1, para2, para3, para4);
        syscallRetValue = %orig(number, para1, para2, para3, para4);
    } else if (5 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        void* para3 = paraList[2];
        void* para4 = paraList[3];
        void* para5 = paraList[4];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p,para3=%p,para4=%p,para5=%p", para1, para2, para3, para4, para5);
        syscallRetValue = %orig(number, para1, para2, para3, para4, para5);
    } else if (6 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        void* para3 = paraList[2];
        void* para4 = paraList[3];
        void* para5 = paraList[4];
        void* para6 = paraList[5];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p,para3=%p,para4=%p,para5=%p,para6=%p", para1, para2, para3, para4, para5, para6);
        syscallRetValue = %orig(number, para1, para2, para3, para4, para5, para6);
    } else if (7 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        void* para3 = paraList[2];
        void* para4 = paraList[3];
        void* para5 = paraList[4];
        void* para6 = paraList[5];
        void* para7 = paraList[6];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p,para3=%p,para4=%p,para5=%p,para6=%p,para7=%p", para1, para2, para3, para4, para5, para6, para7);
        syscallRetValue = %orig(number, para1, para2, para3, para4, para5, para6, para7);
    } else if (8 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        void* para3 = paraList[2];
        void* para4 = paraList[3];
        void* para5 = paraList[4];
        void* para6 = paraList[5];
        void* para7 = paraList[6];
        void* para8 = paraList[7];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p,para3=%p,para4=%p,para5=%p,para6=%p,para7=%p,para8=%p", para1, para2, para3, para4, para5, para6, para7, para8);
        syscallRetValue = %orig(number, para1, para2, para3, para4, para5, para6, para7, para8);
    } else if (9 == paraNum){
        void* para1 = paraList[0];
        void* para2 = paraList[1];
        void* para3 = paraList[2];
        void* para4 = paraList[3];
        void* para5 = paraList[4];
        void* para6 = paraList[5];
        void* para7 = paraList[6];
        void* para8 = paraList[7];
        void* para9 = paraList[8];
        os_log(OS_LOG_DEFAULT, "hook_syscall_orig: para1=%p,para2=%p,para3=%p,para4=%p,para5=%p,para6=%p,para7=%p,para8=%p,para9=%p", para1, para2, para3, para4, para5, para6, para7, para8, para9);
        syscallRetValue = %orig(number, para1, para2, para3, para4, para5, para6, para7, para8, para9);
    }

    os_log(OS_LOG_DEFAULT, "hook_syscall_orig: syscallRetValue=%d", syscallRetValue);
    return syscallRetValue;
}

Some Note

va_start

when call va_start, last_arg is the last named parameter, NOT the first one

for: int open(const char *path, int oflag, ...);

correct
- va_start(argList, oflag);
wrong
- va_start(argList, path);
  - (XCode's clang) compiler will warning
    - Second argument to 'va_start' is not the last named parameter

va_arg

type va_arg(va_list ap, type);

for pass type, there is a special case:

when pass in :

char
unsigned char
unsigned short

but return always is:

unsigned int

-> so when code:

curPara = (mode_t) va_arg(argList, mode_t);

according:

#include <sys/stat.h>
#include <sys/types.h>

-> mode_t==unsigned short

equivalent to:

curPara = (mode_t) va_arg(argList, unsigned short);

so compiler warning:

Second argument to 'va_arg' is of promotable type 'mode_t' (aka 'unsigned short'); this va_arg has undefined behavior because arguments will be promoted to 'int'

change to:

curPara = (mode_t) va_arg(argList, unsigned int);

could avoid warning.

Content Type	Original Author	Original Content on Stackoverflow
Question	alaamh	View Question on Stackoverflow
Solution 1 - C	knittl	View Answer on Stackoverflow
Solution 2 - C	N 1.1	View Answer on Stackoverflow
Solution 3 - C	dafmetal	View Answer on Stackoverflow
Solution 4 - C	RvdK	View Answer on Stackoverflow
Solution 5 - C	Ignacio Vazquez-Abrams	View Answer on Stackoverflow
Solution 6 - C	arsho	View Answer on Stackoverflow
Solution 7 - C	crifan	View Answer on Stackoverflow

In a C function declaration, what does "..." as the last parameter do?

C Problem Overview

C Solutions

Solution 1 - C

Solution 2 - C

Solution 3 - C

Solution 4 - C

Solution 5 - C

Solution 6 - C

Solution 7 - C

Variadic function

Definition

Example

How to get/calculate paremeter for Variadic function

Example

average

maxof

execl

My case: hook syscall()

Some Note

va_start

va_arg

Do Hibernate table classes need to be Serializable?

window.focus() not working in Google Chrome

Attributions

C Problem Overview

C Solutions

Solution 1 - C

Solution 2 - C

Solution 3 - C

Solution 4 - C

Solution 5 - C

Solution 6 - C

Solution 7 - C

Variadic function

Definition

Example

How to get/calculate paremeter for Variadic function

Related definition

Example

average

maxof

execl

My case: hook syscall()

Some Note

va_start

va_arg

Related Doc

Do Hibernate table classes need to be Serializable?

window.focus() not working in Google Chrome

Attributions