How Can I Capture stdout from a system() Command in C/C ?

Capturing stdout from a system() command in C/C

Upon executing a system command using system(), it becomes essential to capture its output for further processing. C/C offers various approaches to achieve this, with the popen() function emerging as a reliable solution.

popen() Function

The popen() function opens a pipe from the shell and launches a process by executing the specified command. It returns a stream pointer FILE *connected to the pipe, enabling read and write operations.

Syntax and Usage

The syntax of popen() is as follows:

#include <stdio.h>

FILE *popen(const char *command, const char *type);

where:

• command is the system command to be executed.
• type specifies the type of connection to the pipe: "r" for reading or "w" for writing.

To capture the stdout of a system command, use:

FILE *fp = popen(command, "r");

The FILE *fp represents a stream connected to the stdout of the command. You can then use fgetc(), fgets(), or other stream manipulation functions to read the output.

Example

char buffer[BUFSIZ];

FILE *fp = popen("ls", "r");
while (fgets(buffer, BUFSIZ, fp) != NULL) {
    printf("%s", buffer);
}
pclose(fp);

Closing the Pipe

Once you have finished reading the output, it's essential to close the pipe using pclose() to release the associated resources:

int status = pclose(fp);

Note: capturing stdout using popen() inherits any errors encountered by the external command. Consider checking the return value of pclose() to detect any errors.

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