List of articles

Depth analysis ：strcat & strncat
Depth analysis ：strchr & strstr
Depth analysis ：strcmp & strncmp
Depth analysis ：strcpy & strncpy
Depth analysis ：strlen & strtok
Depth analysis ：memcpy & memmove
Depth analysis ：memset & memcmp

Preface

Library function string It is also very important code that we often use , I will deeply analyze the usage and implementation principle of commonly used functions

One 、strcmp

1. Call structure

int strcmp(const char *str1, const char *str2);

2. The use of,

strcmp： hold str1 The string pointed to and str2 Compare the string pointed to

The principle, :

1. Compare ASCII Code value
2. First string > Second string , Then return to Greater than 0 The number of
3. First string = Second string , Then return to 0
4. First string < Second string , Then return to Less than 0 The number of

3. Source analysis

int Strcmp(char const *s1, char const *s2) {
    
// Compare : hold  str1  The string pointed to and  str2  Compare the string pointed to .
    assert(s1 && s2);         // Prevent passing null pointers 
    while (*s1 == *s2 && *s1 != 0 && *s2 != 0) {
           // Compare strings 
        s1++;
        s2++;
    }
    return *s1 - *s2;         // Return the comparison result 
}

Two 、strncmp

1. Call structure

int strncmp(const char *str1, const char *str2, size_t n);

2. The use of,

strncmp： hold str1 and str2 Compare , Up to before comparison n Bytes

The principle, :

1. Compare to a different character or the end of a string or num Compare all the characters
2. First string > Second string , Then return to Greater than 0 The number of
3. First string = Second string , Then return to 0
4. First string < Second string , Then return to Less than 0 The number of

3. Source analysis

int Strncmp(char const *s1, char const *s2, int len) {
    
// Compare : hold  str1  and  str2  Compare , Up to before comparison  n  Bytes 
    assert(s1 && s2);        // Prevent passing null pointers 
    while (len-- && *s1 == *s2 && *s1 != '\0' && *s2 != '\0') {
          // Compare  n  A byte string 
        s1++;
        s2++;
    }
    return *s1 - *s2;        // Return the comparison result 
}