(advanced usage) C language string function

Before the order

When you learn C Operation of language string , We know how strings are defined , String output mode , It includes input and output statements of strings , for example puts,gets ,fputs etc. .
There are also string operation functions , for example strcpy,strcmp,strcat, etc. . After finally understanding the robustness of the function , To study the strncpy And other functions that can improve the robustness of the code .

Introduce

In the use of string output statements , Always can't do without NUL, This terminator .
NUL , stay C There are basically in every string of language , It exists in the string , But not part of the string . When printing related strings , such as printf Use in %s To print strings . printf Execution time , meet NUL The character stops executing .

Existing problems

• Strlen Return value problem of

Generally, when measuring the length of a string , We will use string.h In the library strlen
Function to measure . But when you see strlen When defining the function of , You will find a little different .
strlen Function prototype of

size_t strlen( char const *string );

among size_t Is an unsigned integer type , It's defined in stddef.h in .

typedef unsigned int size_t

therefore , strlen The return value of is always greater than or equal to 0 The integer number of . If you still don't understand the importance of this , Let's look at a piece of code .

char str[] = "For text";
if( strlen(str)-10 >= 0 ){
	printf("Alright.\n");
}
else{
	printf("Error.\n");
}

Judge the result of this code ,str The length of is undoubtedly 8, Then the execution output will be “Error” Well ？

Unfortunately , No . because strlen The return value type of is unsigned int Therefore, the return value can never be greater than 0. If you want to eliminate this problem , Force the type of the return value to int The type is enough .

• strcpy Replication problem of
  The function used to copy a string is usually strcpy , The function prototype is as follows ：

char *( char *des ,char const *src );

The string src Assign parameters to des , When in src Meet in NUL when , Stop assignment .

char str[] = "frank francis";
char text[] = {'t','j','\0','o','h','s','o','n'};
strcpy(str,text);
printf("%s\n",str);

What will be the result of implementation ？

because text[2] The value of is '\0’ , Therefore, when the signal is encountered during execution, it exits the function .
So for strncpy Will this function encounter the same problem ？ The answer is yes . Put... In the code above strcpy Change it to strncpy after , The resulting structure is still “tj”.

Extended to other string operation functions , and strcpy equally , for example strcat ,strcmp ,strstr You will still meet ‘\0’ Then end the execution .

So for the middle existence ’\0’ String to operate on , How to do it? ？

String memory operation

When you meet... In a string NUL Character time , Be able to NUL The following characters are processed , This is the time C A set of functions are defined in the language standard library , Make it possible to NUL Operate on the character after the character .

among memcpy It's the copy function ,memmove The function and memcpy almost , But when the copied source and target parameters may overlap , You should use memmove .

memcmp For the comparison function , Function and strcpy be similar .

memchr Find function for character , Co search length Characters .

memset Set the function for the character , take length All previous characters use ch To replace .

So let's go back to the second part of the code ：

char str[] = "frank francis";
char text[] = {'t','j','\0','o','h','s','o','n'};
//strcpy(str,text);
memcpy(str,text,strlen(text));
printf("%s\n",str);

In this way, the use of strcpy The case when . The reason is , Because memcpy It's an operation on memory . stay C++ reference About memcpy In the introduction ：

And for strcpy It is ：

Of course , Encountered in string NUL It's rare to see the situation of , Therefore use strcpy It can basically meet the daily needs .

summary

strcpy Is defined as , String assignment function , And in the “mem” Functions in the series , The types of arguments and functions are void , So you can use “mem” A series of functions to operate on different types of variables or structures .