In C, how can I find out programmatically if a process is already running on Linux/Ubuntu to avoid having it start twice? I'm looking for something similar to pidof.
You can walk the pid
entries in /proc
and check for your process in either the cmdline
file or perform a readlink
on the exe
link (The following uses the first method).
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
#include <sys/types.h>
pid_t proc_find(const char* name)
{
DIR* dir;
struct dirent* ent;
char* endptr;
char buf[512];
if (!(dir = opendir("/proc"))) {
perror("can't open /proc");
return -1;
}
while((ent = readdir(dir)) != NULL) {
/* if endptr is not a null character, the directory is not
* entirely numeric, so ignore it */
long lpid = strtol(ent->d_name, &endptr, 10);
if (*endptr != '\0') {
continue;
}
/* try to open the cmdline file */
snprintf(buf, sizeof(buf), "/proc/%ld/cmdline", lpid);
FILE* fp = fopen(buf, "r");
if (fp) {
if (fgets(buf, sizeof(buf), fp) != NULL) {
/* check the first token in the file, the program name */
char* first = strtok(buf, " ");
if (!strcmp(first, name)) {
fclose(fp);
closedir(dir);
return (pid_t)lpid;
}
}
fclose(fp);
}
}
closedir(dir);
return -1;
}
int main(int argc, char* argv[])
{
if (argc == 1) {
fprintf("usage: %s name1 name2 ...\n", argv[0]);
return 1;
}
int i;
for(int i = 1; i < argc; ++i) {
pid_t pid = proc_find(argv[i]);
if (pid == -1) {
printf("%s: not found\n", argv[i]);
} else {
printf("%s: %d\n", argv[i], pid);
}
}
return 0;
}