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Showing content from https://en.cppreference.com/w/cpp/language/../io/c/perror.html below:

std::perror - cppreference.com

void perror( const char *s );

Prints a textual description of the error code currently stored in the system variable errno to stderr.

The description is formed by concatenating the following components:

the contents of the null-terminated byte string pointed to by s, followed by ": " (unless s is a null pointer or the character pointed to by s is the null character).
implementation-defined error message string describing the error code stored in errno, followed by '\n'. The error message string is identical to the result of std::strerror(errno).

[edit] Parameters s - pointer to a null-terminated string with explanatory message [edit] Return value

(none)

[edit] Example

#include <cerrno>
#include <cmath>
#include <cstdio>
 
int main()
{
    double not_a_number = std::log(-1.0);
    if (errno == EDOM)
        std::perror("log(-1) failed");
    std::printf("%f\n", not_a_number);
}

Possible output:

log(-1) failed: Numerical argument out of domain
nan

[edit] See also macro which expands to POSIX-compatible thread-local error number variable
(macro variable)[edit] returns a text version of a given error code
(function) [edit]

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