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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
strcmp — compare two strings
SYNOPSIS
#include <string.h> int strcmp(const char *s1, const char *s2);
DESCRIPTION
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.1‐2017 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 unsigned char) that differ in the strings being
compared.
RETURN VALUE
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.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
Checking a Password Entry
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() 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 oldpasswd 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");
}
}
...
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
strncmp()
The Base Definitions volume of POSIX.1‐2017, <string.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .