C: Implicit declaration of function

Question

I am working on an assignment in which we are developing our own RPC client. Upon compiling my server portion, I receive warnings for the following:

implicit declaration of function 'read'
implicit declaration of function 'write'

I understand that I would typically receive this warning if I were to create a function following my main, ex:

int main() {
    doSomething();
}

void doSomething() {
    ...
}

In the above case, it should complain about the function that I created "doSomething".

Why then would my compiler complain that a system call was declared implicitly, when it appears in a function that was declared before the main? Below is the function in which the system call appears.

void Open(int connfd) {
/*Get message size*/
unsigned char temp[4] = { 0 };
int n = read(connfd, temp, 4);
if(n < 0) {/*On error*/
    perror("Read error");
    exit(1);
}/*End if*/
unsigned int msgSize = temp[0] +
    (temp[1] * 256) + 
    (temp[2] * 256 * 2) + 
    (temp[3] * 256 * 3);
printf("msgSize = %d\n", msgSize);

/*Allocate memory for message*/
char * msg = malloc(msgSize);
if(msg == NULL) {
    perror("Allocation error");
    exit(1);
}/*End if*/
msg = memset(msg, 0, msgSize);

/*Read entire message from client*/
n = read(connfd, msg, msgSize);
if(n < 0) {/*On error*/
    perror("Read error");
    exit(1);
}/*End if*/

/*Extract pathname from message - NULL terminated*/
char * pathname = malloc(strlen(msg) + 1);
if(pathname == NULL) {
    perror("Allocation error");
    exit(1);
}/*End if*/
pathname = memset(pathname, 0, strlen(msg) + 1);
pathname = memcpy(pathname, msg, strlen(msg));

/*Extract flags from message*/
int i;
for(i = 0; i < sizeof(int); i++) {
    temp[i] = msg[strlen(pathname) + 1 + i];
}/*End for i*/
unsigned int flags = temp[0] + 
    (temp[1] * 256) + 
    (temp[2] * 256 * 2) + 
    (temp[3] * 256 * 3);

/*Extract mode from message*/
for(i = 0; i < sizeof(mode_t); i++) {
    temp[i] = msg[strlen(pathname) + 1 + sizeof(int) + 1 + i];
}/*End for i*/
mode_t mode = temp[0] + 
    (temp[1] * 256) + 
    (temp[2] * 256 * 2) + 
    (temp[3] * 256 * 3);

free(msg);/*Free msg since it is no longer needed*/

/*Open pathname*/
umask(0);
int fd = open(pathname, flags, mode);

free(pathname);/*Free pathname since it is no longer needed*/

/*Prepare response*/
char * response = malloc(sizeof(int) * 2);
if(response == NULL) {
    perror("Allocation error");
    exit(1);
}/*End if*/
response = memset(response, 0, sizeof(int) * 2);

/*Build return message*/
memcpy(&response[0], &fd, sizeof(fd));
memcpy(&response[4], &errno, sizeof(fd));

/*Can't guarante socket will accept all we try to write, cope*/
int num, put;
int left = sizeof(int) * 2; put = 0;
while(left > 0) {
    if((num = write(connfd, response + put, left)) < 0) {
        perror("inet_wstream: write");
        exit(1);
    } else {
        left -= num;
        put += num;
    }/*End else*/
}/*End while*/

free(response);/*Free response since it is no longer needed*/

return;
}/*End Open*/

Answer 1

Add #include <unistd.h> include directive in your program.

read and write functions are declared in unistd.h and you need a declaration of your functions before to be able to call them.

Answer 2

Implicit function declarations are those that the compiler sees the first time used as a function call (as opposed to those where a prototype or function definition is seen first).

"System calls" are no exception to this rule, since the C Standard(s) don't make a distinction between "ordinary functions" and "system calls". You likely forgot to include the relevant header providing the prototype (unistd.h).