STRCMP(3) Library Functions Manual STRCMP(3)

strcmp — compare two strings

#include <string.h>

int strcmp(const char *s1, const char *s2);

[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The strcmp () function shall compare the string pointed to by s1 to the string pointed to by s2.

The sign of a non-zero return value shall be determined by the sign of the difference between the values of the first pair of bytes (both interpreted as type ) that differ in the strings being compared.

[CX] The strcmp () function shall not change the setting of errno on valid input.

Upon completion, strcmp () shall return an integer greater than, equal to, or less than 0, if the string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2, respectively.

No errors are defined.

The following example compares the information read from standard input to the value of the name of the user entry. If the strcmp () function returns 0 (indicating a match), a further check will be made to see if the user entered the proper old password. The crypt(3) function shall encrypt the old password entered by the user, using the value of the encrypted password in the passwd structure as the salt. If this value matches the value of the encrypted passwd in the structure, the entered password is the correct user's password. Finally, the program encrypts the new password so that it can store the information in the passwd structure.

#include <string.h>
#include <unistd.h>
#include <stdio.h>
...
int valid_change;
struct passwd *p;
char user[100];
char oldpasswd[100];
char newpasswd[100];
char savepasswd[100];
...
if (strcmp(p->pw_name, user) == 0) {
    if (strcmp(p->pw_passwd, crypt(oldpasswd, p->pw_passwd)) == 0) {
        strcpy(savepasswd, crypt(newpasswd, user));
        p->pw_passwd = savepasswd;
        valid_change = 1;
    }
    else {
        fprintf(stderr, "Old password is not valid\n");
    }
}
...

None.

None.

None.

strncmp(3)

XBD <string.h>

First released in Issue 1. Derived from Issue 1 of the SVID.

Extensions beyond the ISO C standard are marked.

Austin Group Defect 448 is applied, adding a requirement that strcmp () does not change the setting of errno on valid input.

June 14, 2024 posix.fail