va_ List usage summary

Preface

stay Variable parameter function in , You can often see va_list、va_start、va_arg、va_end Use .

What is a variable parameter function ？

C In language ,printf function , Is a variable parameter function , The number of parameters passed in is uncertain , You can pass in more than one .

printf The function prototype :

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

among ..., It stands for Variable parameters , Similar to ellipsis .

Example

Here is a simple example to experience variable parameter functions .

static void va_list_test(int param_num, ...)
{
    
    int i = 0;
    va_list ap;
    va_start(ap, param_num);

    //  Get integer parameters 
    int arg1 = va_arg(ap, int);
    printf("arg1 = 0x%x\n", arg1);

    //  Get string 
    char *arg2 = va_arg(ap, char *);
    printf("arg2 = %s\n", arg2);

    //  Get integer parameters 
    int arg3 = va_arg(ap, int);
    printf("arg3 = 0x%x\n", arg3);

    va_end(ap); //  Pay attention to the need for  va_end(ap)
}

int main()
{
    
    va_list_test(2, 0x55, "arg test", 0x66);
    return 0;
}

The output is as follows :

arg1 = 0x55
arg2 = arg test
arg3 = 0x66

analysis

The above example , Separately introduced 4 Parameters , 2, 0x55, "arg test", 0x66, It can be downloaded from va_list_test() From the format of the parameter passed in by the function ：

Parameters param_num The role of

Why do I need param_num Fixed parameters of ?

• Parameters param_num, That is, from the above example Numbers 2, It's a fixed parameter . There is no practical use in the above example , For giving va_start Position the variable parameters . va_start(ap, param_num) It can be understood as , By fixing parameters param_num Come on Find the starting position of variable parameter storage , And saved it to ap variable . This with Function call procedure , The function frame is related to the stack , No discussion .

The process of obtaining parameters separately

Get the first variable parameter

• Parameters 0x55, It's an integer parameter , because va_start(ap, param_num) Parameters passed in param_num Locate the starting position of the variable parameter , So the first one is to get integer parameters 0x55, use va_arg(ap, int) To get the first variable parameter ,0x55, according to ap The location of , Read an integer value .

Get the second variable parameter

• Parameters "arg test", It's a string parameter , Due to reading 0x55 after , ap The position of has changed , It becomes the starting position of the second parameter , therefore va_arg(ap, char *) Will take the current position , Get a string value , So get "arg test".

Get the third variable parameter

• Last parameter 0x66, It's the same thing , continue va_arg(ap, int) Get one int Parameters .

A small summary

1. We need to understand the function of the first fixed parameter , namely va_start(ap, param_num), At the same time, we need to pay attention to ap The meaning of this parameter in obtaining variable parameters , It can be understood that pointer address is cheap , Throughout Point to the address of the first variable parameter that is not obtained
2. Experimental process , Try to set variable parameters 1、2、3 The acquisition order of changes , That is, the order of acquisition is determined by int 、 char *、int It is amended as follows int 、int、char *, There will be , Parameters 2 Get exception , Parameters 3 printf A segment error occurred after . therefore The acquisition order of variables cannot be wrong , Otherwise, there will be abnormalities .
3. va_start() And va_end() Need to pair up . Don't use va_end(ap) The consequences of , Guess it may lead to memory leakage . You can check by yourself .

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va_list In a common way

In most practical cases ,va_list Are all with printf() The use of functions is similar .

For example:

int va_list_test(char *fmt, ...)
{
    
    char out[1024] = {
    0};

    va_list ap;
    va_start(ap, fmt);
    vsnprintf(out, sizeof(out), fmt, ap);
    printf("%s", out);
    va_end(ap);
}

int main()
{
    
    va_list_test("output : 0x%x, %s, 0x%x\n", 0x55, "arg test", 0x66);

    return 0;
}

The output is as follows :

output : 0x55, arg test, 0x66

analysis

In the above example , Fixed parameter fmt It's actually incoming "output : 0x%x, %s, 0x%x\n", Three variable parameters , According to %x、%s、%x In the form of , adopt vsnprintf() Format input to out in . similar sprintf().

What needs attention is why it is used vsnprintf()

Let's take a look at vsnprintf Prototypes of functions , You can see the last parameter passed in , Namely va_list Variable , So use vsnprintf In the above example, it is very convenient .

int vsnprintf(char *str, size_t size, const char *format, va_list ap);

A small summary

1. Be careful vsnprintf() How to use the function .
2. va_list Experience accumulation in common situations .